No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Formation of a G-Quartet-Fe Complex and Modulation of Electronic and Magnetic Properties of the Fe Center
Abstract
Although the G-quartet structure has been extensively investigated due to its biological importance, the formation mechanism, in particular, the necessity of metal centers, of an isolated G-quartet on solid surfaces remains ambiguous. Here, by using scanning tunneling microscopy under well-controlled ultra-high-vacuum conditions and density functional theory calculations we have been able to clarify that besides the intraquartet hydrogen bonding a metal center is mandatory for the formation of an isolated G-quartet. Furthermore, by subtly perturbing the local coordination bonding schemes within the formed G-quartet complex via local nanoscale scanning tunneling microscopy manipulations, we succeed in modulating the d orbitals and the accompanying magnetic properties of the metal center. Our results demonstrate the feasibility of forming an isolated G-quartet complex on a solid surface and that the strategy of modulating electronic and magnetic properties of the metal center can be extended to other related systems such as molecular spintronics.
... For instance, the presence of metal ions is not necessary for the fabrication of xanthine-quartets (XQs) 18 or the tetrad form of lipophilic phenolic guanosine derivatives, 20 but is required to form GQ and hypoxanthine-quartet (HX-Q) networks. 19,21,22 Although the cooperative effect, responsible for Watson-Crick base pairing, strengthens the hydrogen bonds in a GQ relative to those in isolated G dimers, 23,24 recent reports show that this effect is not sufficient to initiate a stable GQ from its molecular building blocks. 18,19,21,22 A pristine form of GQ is electrostatically unfavorable as four oxygen atoms are located towards the central cage; thus, incorporation of a monovalent cation, either K + or Na + , 15,22,25 or post-deposition of transition metal adatoms onto guanine molecules and annealing were required for GQ self-assembly. ...
... 19,21,22 Although the cooperative effect, responsible for Watson-Crick base pairing, strengthens the hydrogen bonds in a GQ relative to those in isolated G dimers, 23,24 recent reports show that this effect is not sufficient to initiate a stable GQ from its molecular building blocks. 18,19,21,22 A pristine form of GQ is electrostatically unfavorable as four oxygen atoms are located towards the central cage; thus, incorporation of a monovalent cation, either K + or Na + , 15,22,25 or post-deposition of transition metal adatoms onto guanine molecules and annealing were required for GQ self-assembly. 18,21 The geometry of a stabilized initial seed is an additional crucial factor in the single-step growth of the molecular network at RT, where the nucleation can proceed from either a at terrace or an edge region. ...
... 18,19,21,22 A pristine form of GQ is electrostatically unfavorable as four oxygen atoms are located towards the central cage; thus, incorporation of a monovalent cation, either K + or Na + , 15,22,25 or post-deposition of transition metal adatoms onto guanine molecules and annealing were required for GQ self-assembly. 18,21 The geometry of a stabilized initial seed is an additional crucial factor in the single-step growth of the molecular network at RT, where the nucleation can proceed from either a at terrace or an edge region. To block surface diffusion towards undesirable nucleation sites, a heterogeneous admixture is employed. ...
Guanine-quadruplex, consisting of several stacked guanine-quartets (GQs), has emerged as an important category of novel molecular targets with applications from nanoelectronic devices to anticancer drugs. Incorporation of metal cations into GQ structure is utilized to form stable G-quadruplexes, while cation-free GQ network has been challenging. Here we report the room temperature (RT) molecular self-assembly of extended pristine GQ networks on Au(111) surface. An implanted molybdenum atom within Au(111) surface is used to nucleate and stabilize the cation-free GQ network. Additionally, decoration of Au(111) surface with 7-armchair graphene nanoribbons (7-AGNRs) enhances the GQ domain size by suppressed influence of the disordered phase nucleated from Au step edges. Scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations confirm GQ networks’ formation and unravel the nucleation and growth mechanism. Our work, utilizing the hetero-atom doped substrate, provides a facile approach to enhance the stability and domain size of the GQ self-assembly, which would be applicable for other molecular structures.
... For example, the presence of metal atoms is not necessitated for the fabrication of X-quartets 14 but is required to form GQ and HX-Q networks. 13,21,22 A pristine form of GQ is electrostatically unfavourable as four oxygen atoms are located towards the central cage. Thus it needs to be stabilized by the presence of a caged cation, 22 either K + or Na + cations; 19,22,23 or otherwise a new GQ-M complex (or supramolecular structure) is produced after introducing transition metal adatoms to guanine molecular source. ...
... Thus it needs to be stabilized by the presence of a caged cation, 22 either K + or Na + cations; 19,22,23 or otherwise a new GQ-M complex (or supramolecular structure) is produced after introducing transition metal adatoms to guanine molecular source. 14,21 In this regard, the cooperative effect of hydrogen bonds, responsible for Watson-Crick base pairing, is also unable to stabilize the metal-free GQ network on the pristine Au(111) surface in addition to resonance-assisted hydrogen bonding (RAHB) effect, even though considered presumably, 12 but revisited and corrected afterward. 13,14,21,24 Also, the geometry of a stabilized initial seed is an additional crucial factor in forming and stabilizing the molecular network, where the nucleation can proceed from either a flat terrace or edge region. ...
... 14,21 In this regard, the cooperative effect of hydrogen bonds, responsible for Watson-Crick base pairing, is also unable to stabilize the metal-free GQ network on the pristine Au(111) surface in addition to resonance-assisted hydrogen bonding (RAHB) effect, even though considered presumably, 12 but revisited and corrected afterward. 13,14,21,24 Also, the geometry of a stabilized initial seed is an additional crucial factor in forming and stabilizing the molecular network, where the nucleation can proceed from either a flat terrace or edge region. To block surface diffusion towards undesirable nucleation sites, a heterogeneous admixture is employed. ...
Guanine-quadruplex, consisting of several stacked guanine-quartets (GQs), has emerged as an important category of novel molecular targets with applications from nanoelectronic devices to anticancer drugs. Incorporation of metal cations into GQ structure is utilized to form stable G-quadruplexes, while no other passage has been reported yet. Here we report the room temperature (RT) molecular self-assembly of extensive metal-free GQ networks on Au(111) surface. Surface defect induced by an implanted molybdenum atom within Au(111) surface is used to nucleate and stabilize the cation-free GQ network. Additionally, the decorated Au(111) surface with 7-armchair graphene nanoribbons (7-AGNRs) results in more extensive GQ networks by curing the disordered phase nucleated from Au step edges spatially and chemically. Scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations confirm GQ networks' formation and unravel the nucleation and growth mechanism. This method stimulates cation-free G-quartet network formation at RT and can lead to stabilizing new emerging molecular self-assembly.
... [1][2][3][4][5][6] Structuralt ransformationo fm etal-organic structures, which relies on reorganization of componentsb yb reakage and reformation of bondstoachievechange in constitution, [7][8][9] has opened the way to adaptive and evolutive chemistry.S uch highly sophisticated structures have also attracted broadi nterest in the surfacec hemistry community as solid surfaces provide versatile platforms for regulating and arraying metal-organic nanostructures. [4-6, 10, 11] Numerous on-surface investigations have demonstrated that coordination chemistry is an efficient strategy for forming various static metal-organic structures, such as well-defineds urface-supportedn etworks, [12][13][14][15] chains, [16][17][18] and clusters, [19][20][21][22] where usually only one kind of coordination binding site [12,13,[15][16][17][19][20][21][22] or two differents ites with similarc oordinationm ode [14,18] are involved. Moreover, some studies have also shown that it is possible to induce structuralt ransformationsb yr egulation of metal/molecule stoichiometric ratios and/ort emperature. ...
... [1][2][3][4][5][6] Structuralt ransformationo fm etal-organic structures, which relies on reorganization of componentsb yb reakage and reformation of bondstoachievechange in constitution, [7][8][9] has opened the way to adaptive and evolutive chemistry.S uch highly sophisticated structures have also attracted broadi nterest in the surfacec hemistry community as solid surfaces provide versatile platforms for regulating and arraying metal-organic nanostructures. [4-6, 10, 11] Numerous on-surface investigations have demonstrated that coordination chemistry is an efficient strategy for forming various static metal-organic structures, such as well-defineds urface-supportedn etworks, [12][13][14][15] chains, [16][17][18] and clusters, [19][20][21][22] where usually only one kind of coordination binding site [12,13,[15][16][17][19][20][21][22] or two differents ites with similarc oordinationm ode [14,18] are involved. Moreover, some studies have also shown that it is possible to induce structuralt ransformationsb yr egulation of metal/molecule stoichiometric ratios and/ort emperature. ...
... The reason for choosing such as ystem is that:1 )the 9eG molecule contains two neighboring coordinationb inding sites (i.e.,O 6a nd N7 sites, see Scheme1,w hile the N3 site is potentially sterically hindered);2)from the experimentalf indings [15] and also theoretical calculations, we conclude that the Fe atom preferentially coordinates with the O6 site. [19] It thus may allow us to construct am odel system for investigating the coordination prioritya nd diversity,a nd more importantly,t or ealizet he reversible structuralt ransformations on the surfaceu nder UHV conditionsb yv irtue of intrinsic dynamicc haracteristics of coordination bonds.H erein, from the combination of high-resolution scanning tunneling microscopy (STM)a nd density functional theory (DFT)c alculations, we show that:1 )byd eposition of Fe atoms on a9 eG-precovered Au(111)s urfacea tr oom temperature (RT) in ac ontrolled stepwise dosage, we achievet he sequential formation of various surface nanostructures composed of metal-organic G 4 Fe 1 ,h eterochiral G 3 Fe 1 ,G 4 Fe 2 ,G 3 Fe 3 and homochiral G 3 Fe 1 motifs (Scheme 1) as elementary building blocks;2 )these processes are reversible by controlled deposition of additional 9eG on ac ertain structure-covered surface at RT;3 )such as ystem thus demonstratesr eversible structural transformations on as olid surface, in which controllable formation of multiple metal-organic motifsb yu se of different coordination binding modes can be dually responsive to metal atoms and molecules, is achieved under UHV conditions. The key to making these interconversionss uccessful is the reversiblec haracteristico fc oordination bonds together with coordinationp riority and diversity. ...
Structural transformation of metal-organic nanostructures holds great promise for structural diversity and flexibility and opens the way towards an adaptive and evolutive chemistry owing to the dynamic characteristic of coordination bonds. It is thus generally interesting and also challenging to develop systems showing reversible structural transformations which involve multiple metal-organic motifs on surfaces. Here, we have successfully constructed a system that presents structural transformations on a solid surface, in which controllable formation of multiple metal-organic nanostructures (with different coordination binding modes by use of distinct binding sites) dually responsive to both metal atoms and molecules is achieved at room temperature (RT) under ultrahigh vacuum (UHV) conditions. The key to making these interconversions successful is the intrinsic dynamic characteristic of coordination bonds together with the coordination priority and diversity.
... [18] The effect of methylation on the intermolecular interactions between G molecules was recently studied by Wang and coworkers. [19] The STM analysis, corroborated by density functional theory (DFT), revealed that Recently, functionalization of N(9) position of G molecules was employed by Xu and co-workers [20] to steer the self-assembly of G into ribbon-like architectures under UHV conditions. While it was considered that G molecules self-assemble into G4-based 2D networks on Au(111), the authors demonstrated that N(9)-ethylguanine (G3) once deposited on gold surface forms G3-ribbons A (Figure 4b). ...
... b-c) Reproduced with permission. [20] 2014, American Chemical Society. Reproduced with permission. ...
... H-bonding pattern a Self-assembled motif b Substrate c Guanine (G) N(2)-H···O(6) and N(1)-H···N (7) Ribbon A Au(111) [20] , HOPG [21], [23], [26] N(1)-H···O(6) and N(2)-H···N(3) Ribbon B HOPG [32] N(2)-H···N (7) and N(1)-H···O(6) c Quartet Au(111) [16], [17], [20] , HOPG [30] Cytosine Xanthine (X) N(1)-H···O (2) and N(7)-H···O(6) Pentamer I d Au(111) [57] N(1)-H···O(6) and N(7)-H···O (2) Pentamer II d Au(111) [57] N(1)-H···O (2) and N(7)-H···O(6) Ribbon HOPG [58] Isocytosine (iC) N(1)-H···O(4), N(2)-H···O(4) and N(2)-H···N(3) Ribbon HOPG [60] ...
The self-assembly of small organic molecules interacting via non-covalent forces is a viable approach towards the construction of highly ordered nanostructured materials. Among various molecular components, natural and unnatural nucleobases can undergo non-covalent self-association to form supramolecular architectures with ad hoc structural motifs. Such structures, when decorated with appropriate electrically/optically active units, can be used as scaffolds to locate such units in pre-determined positions in 2D on a surface, thereby paving the way towards a wide range of applications, e.g., in optoelectronics. This review discusses some of the basic concepts of the supramolecular engineering of natural and unnatural nucleobases and derivatives thereof as well as self-assembly processes on conductive solid substrates, as investigated by scanning tunnelling microscopy in ultra-high vacuum and at the solid/liquid interface. By unravelling the structure and dynamics of these self-assembled architectures with a sub-nanometer resolution, a greater control over the formation of increasingly sophisticated functional systems is achieved. The ability to understand and predict how nucleobases interact, both among themselves as well as with other molecules, is extremely important, since it provides access to ever more complex DNA- and RNA-based nanostructures and nanomaterials as key components in nanomechanical devices.
... Fe in that study) is mandatory for the formation of an isolated G-quartet-Fe complex. [16] As mentioned above, in this work we focus on the interactions between 9eG molecules and different salts on a solid surface. We first added Scheme 1. Schematic illustration of the formation of a G ribbon structure and a G-quartet-M complex on Au (111), where M represents Na, K or Ca. Figure 1. ...
... [15] Very recently, an isolated G-quartet-Fe complex was also achieved on an Au(111) surface under well-controlled UHV conditions. [16] Note that in these cases, the metal atoms were brought onto the surfaces to facilitate the formation of the G-quartet structures and stabilize them. To mimic solution chemistry, it would be of utmost interest to directly add salts onto solid surfaces, especially under UHV conditions, to explore the feasibility and universality of the formation of G-quartet complexes in a solventless environment. ...
... In this study, we have selected the 9-ethylguanine molecule (shortened as 9eG, see Scheme 1) as a guanine (G) analogue because: 1) the possible G/9H to G/7H tautomerization process can be inhibited by modifying the ethyl group to the N9 site; [17] 2) the 9eG molecule itself forms a G-ribbon structure (see upper panel of Scheme 1 and Figure S1 of the Supporting Information) rather than a G-quartet structure. [16] With respect to the salts, it was recently reported that NaCl and LiCl thin films grown on solid surfaces can directly interact with organic molecules by providing cations and may then form ionic selfassemblies, [18][19][20][21] which is quite different from their traditional application as insulating layers. [22][23][24][25] Inspired by this, herein, NaCl, KBr and CaCl 2 (typical alkali and alkaline earth salts) [6-9, 11, 12] are transferred onto an Au(111) surface to facilitate the subsequent investigation on the formation of possible Gquartet-M complexes (where M denotes Na, K, and Ca, respectively) in such a solventless UHV environment. ...
Template cations have been extensively employed in the formation, stabilization and regulation of structural polymorphism of G-quadruplex structures in vitro. However, the direct addition of salts onto solid surfaces, especially under ultra-high-vacuum (UHV) conditions, to explore the feasibility and universality of the formation of G-quartet complexes in a solventless environment has not been reported. By combining UHV-STM imaging and DFT calculations, we have shown that three different G-quartet-M (M: Na/K/Ca) complexes can be obtained on Au(111) using alkali and alkaline earth salts as reactants. We have also identified the driving forces (intra-quartet hydrogen bonding and electrostatic ionic bonding) for the formation of these complexes and quantified the interactions involved. Our results demonstrate a novel route to fabricate G-quartet-related complexes on solid surfaces, providing an alternative feasible way to bring metal elements to surfaces for constructing metal-organic systems.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
... The direct identification of intermolecular interactions and the delicate tailoring of structural motifs with molecular precision on solid surfaces have recently attracted considerable interest because of the prospect for artificial design of functional molecular nanostructures and nanodevices. Scanning tunneling microscopy (STM), especially under ultrahigh vacuum (UHV) conditions, has proven to be a powerful method for the precise manipulation of single molecules to trigger various single-molecule behaviors, such as have also been extended to larger molecular structural motifs with great progress in the following aspects: 1) moving clusters, [21] chains, [22][23][24][25] and patches; [26] 2) dissociating dimers, clusters, and complexes by breaking hydrogen bonds, [27] coordination bonds, [28][29][30] and carbon-metal bonds, [31] respectively; 3) constructing structural motifs by forming new bonds ranging from hydrogen bonds, [32] coordination bonds, [30,33] to robust covalent bonds; [20,34,35] and 4) probing different hierarchical interactions [36] and identifying hydrogen-bonding configurations [27] and covalent bonding sites. [37] Most of the previous studies mentioned above mainly exhibited either the dissociation or construction of various structural motifs. ...
... Scanning tunneling microscopy (STM), especially under ultrahigh vacuum (UHV) conditions, has proven to be a powerful method for the precise manipulation of single molecules to trigger various single-molecule behaviors, such as translation, [1][2][3] rotation, [3][4][5][6][7][8][9] flipping, [10] cis-trans isomerization, [11][12][13] tautomerization, [14][15][16] dehydrogenation, [17][18][19] and dehalogenation. [20] Not limited to single molecules, STM manipulations have also been extended to larger molecular structural motifs with great progress in the following aspects: 1) moving clusters, [21] chains, [22][23][24][25] and patches; [26] 2) dissociating dimers, clusters, and complexes by breaking hydrogen bonds, [27] coordination bonds, [28][29][30] and carbon-metal bonds, [31] respectively; 3) constructing structural motifs by forming new bonds ranging from hydrogen bonds, [32] coordination bonds, [30,33] to robust covalent bonds; [20,34,35] and 4) probing different hierarchical interactions [36] and identifying hydrogen-bonding configurations [27] and covalent bonding sites. [37] Most of the previous studies mentioned above mainly exhibited either the dissociation or construction of various structural motifs. ...
... The key to making this process successful is the hydrogen bonding, which is comparatively facile to be broken to achieve controllable scission, and on the other hand, the directional characteristic makes seamless stitching feasible. Furthermore, as previously reported, [17,28,42] STM manipulations have been employed on organic molecules to either change the two-dimensional self-assembly or break a peripheral C À H bond of a single molecule, and even to directly perturb single coordination bonds to modulate the electronic properties of the single metal centers. Inspired by these reports, we speculated that the transformation from an isolated elementary metal-organic motif into a higher-level one and vice versa might have an influence on the electronic properties of the metal centers as well. ...
Scanning tunneling microscopy (STM) manipulation techniques have proven to be a powerful method for advanced nanofabrication of artificial molecular architectures on surfaces. With increasing complexity of the studied systems, STM manipulations are then extended to more complicated structural motifs. Previously, the dissociation and construction of various motifs have been achieved, but only in a single direction. In this report, the controllable scission and seamless stitching of metal-organic clusters have been successfully achieved through STM manipulations. The system presented here includes two sorts of hierarchical interactions where coordination bonds hold the metal-organic elementary motifs while hydrogen bonds among elementary motifs are directly involved in bond breakage and re-formation. The key to making this reversible switching successful is the hydrogen bonding, which is comparatively facile to be broken for controllable scission, and, on the other hand, the directional characteristic of hydrogen bonding makes precise stitching feasible.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
... [2,3] Owing to the advantage of dynamic characteristics of non-covalent interactions,the transformation of supramolecular nanostructures with constitutional diversity and adaptability would be of fundamental importance for potential applications in molecular switching devices. [4,5] Among others, by virtue of coordination selectivity and diversity with respect to different metals, [6,7] various static metal-organic structures can be fabricated on the surface,a nd furthermore these structures could be responsive to metal/molecule stoichiometric ratios,c overage,a nd/or temperatures resulting in structural transformations via dynamic coordination chemistry. [8][9][10][11][12][13] In addition to the abovementioned intrinsic regulation factors (that is,c onstituent molecules and metals), the introduction of at hird agent may offer another train of thought to obtain structural transformations. ...
... 4:1) on Au(111) at room temperature (RT) and further annealing at 390 Kf or 10 min results in the formation of ar homboid network structure as shown in Figure 1a.F rom the close-up STM image (Figure 1b), we identify that the network structure is composed of two kinds of elementary motifs (that is,the trimeric and the dimeric ones as indicated by the corresponding contours). Tw oe nantiomers of the trimeric motif are also observed as indicated by green and blue trimeric contours where the individual molecular chiralities are denoted by Ra nd L. As experienced with coordination schemes between DNAb ases and transition metals, [7,9,[26][27][28] after extensive structural search, we assign this trimeric motif to aG 3 Ni 1 metal-organic structure as highlighted in Figure 1c.F rom the DFT-optimized model superimposed on the corresponding STM image,w ed istinguish that it is formed by three Gm olecules (with different chiralities) coordinating with one Ni atom via one N7 site and two O6 sites,and the intermolecular NH···O and NH···N hydrogen bonds further stabilize the structure.T he dimeric structure (depicted by the white contour in Figure 1b)i s assigned to ah ydrogen-bonded dimer according to the morphology and the well-established NH···O hydrogen bonds involved. These two elementary structural motifs are also linked together via hydrogen bonds. ...
The structural transformation of supramolecular nanostructures with constitutional diversity and adaptability by dynamic coordination chemistry would be of fundamental importance for potential applications in molecular switching devices. The role of halogen doping in the formation of elementary metal–organic motifs on surfaces has not been reported. Now, the 9-ethylguanine molecule (G) and Ni atom, as a model system, are used for the structural transformation and stabilization of metal–organic motifs induced by iodine doping on Au(111). The iodine atoms are homogeneously located at particular hydrogen-rich locations enclosed by G molecules by electrostatic interactions, which would be the key for such an unexpected stabilizing effect. The generality and robustness of this approach are demonstrated in different metal–organic systems (G/Fe) and also by chlorine and bromine.
... [2,3] Owing to the advantage of dynamic characteristics of non-covalent interactions, the transformation of supramolecular nanostructures with constitutional diversity and adaptability would be of fundamental importance for potential applications in molecular switching devices. [4,5] Among others, by virtue of coordination selectivity and diversity with respect to different metals, [6,7] various static metal-organic structures can be fabricated on the surface, and furthermore these structures could be responsive to metal/molecule stoichiometric ratios, coverage, and/or temperatures resulting in structural transformations via dynamic coordination chemistry. [8][9][10][11][12][13] In addition to the abovementioned intrinsic regulation factors (that is, constituent molecules and metals), the introduction of a third agent may offer another train of thought to obtain structural transformations. ...
... From the close-up STM image (Figure 1 b), we identify that the network structure is composed of two kinds of elementary motifs (that is, the trimeric and the dimeric ones as indicated by the corresponding contours). Two enantiomers of the trimeric motif are also observed as indicated by green and blue trimeric contours where the individual molecular chiralities are denoted by R and L. As experienced with coordination schemes between DNA bases and transition metals, [7,9,[26][27][28] after extensive structural search, we assign this trimeric motif to a G 3 Ni 1 metal-organic structure as highlighted in Figure 1 c. From the DFT-optimized model superimposed on the corresponding STM image, we distinguish that it is formed by three G molecules (with different chiralities) coordinating with one Ni atom via one N7 site and two O6 sites, and the intermolecular NH···O and NH···N hydrogen bonds further stabilize the structure. ...
The structural transformation of supramolecular nanostructures with constitutional diversity and adaptability by dynamic coordination chemistry would be of fundamental importance for potential applications in molecular switching devices. The role of halogen doping in the formation of elementary metal–organic motifs on surfaces has not been reported. Now, the 9-ethylguanine molecule (G) and Ni atom, as a model system, are used for the structural transformation and stabilization of metal–organic motifs induced by iodine doping on Au(111). The iodine atoms are homogeneously located at particular hydrogen-rich locations enclosed by G molecules by electrostatic interactions, which would be the key for such an unexpected stabilizing effect. The generality and robustness of this approach are demonstrated in different metal–organic systems (G/Fe) and also by chlorine and bromine.
... 在Au(111)表面可以与单个铁原子 [79] 、钠原子 [80] 形成 四聚体, 与两个钾原子形成分子网格结构 [75] , 与氯化钠 反应后, 由氯原子和钠原子在不同位置与分子相互作用 形成的分子链状结构 [81] . 图 10 G分子、镍原子、碘原子形成的金属-有机分子网格结构 [78] . ...
... For example, Flemming et al. had systematically studied the coexistence phenomenon of homo-chiral and hetero-chiral nanostructures of adenine [32], the co-adsorption behavior of guanine-cytosine [30], and the novel quartet-shape of adenine-thymine binary assembly system [33] by using ambient STM technique. Moreover, they had investigated the coordination networks of guanine-potassium (G-K) [34] and guanine-ferrum (G-quartet-Fe) [35] by employing UHV-STM technique. All these works suggest that, melamine and nucleic-acid bases can both act as desirable component during the fabrication of biological materials and the design of molecular devices. ...
The co-adsorption behavior of nucleic-acid base (thymine; cytosine) and melamine was investigated by scanning tunneling microscopy (STM) technique at liquid/solid (1-octanol/graphite) interface. STM characterization results indicate that phase separation happened after dropping the mixed solution of thymine-melamine onto highly oriented pyrolytic graphite (HOPG) surface, while the hetero-component cluster-like structure was observed when cytosine-melamine binary assembly system is used. From the viewpoints of non-covalent interactions calculated by using density functional theory (DFT) method, the formation mechanisms of these assembled structures were explored in detail. This work will supply a methodology to design the supramolecular assembled structures and the hetero-component materials composed by biological and chemical compound.
Electronic supplementary material
The online version of this article (doi:10.1186/s11671-016-1767-0) contains supplementary material, which is available to authorized users.
Advanced fabrication of surface metal–organic complexes with specific coordination configuration and metal centers will facilitate to exploit novel nanomaterials with attractive electronic/magnetic properties. The precise on‐surface synthesis provides an appealing strategy for in situ construction of complex organic ligands from simple precursors autonomously. In this paper, distinct organic ligands with stereo‐specific conformation are separately synthesized through the well‐known dehalogenative coupling. More interestingly, the exo‐bent ligands promote the mono‐iron chelated complexes with the Fe center significantly decoupled from the surface and of high spin, while the endo‐bent ligands lead to bi‐iron chelated ones instead with ferromagnetic properties.
Using a simple solution-based drop-casting method, we demonstrate formation of large area (several tens of μm) uniform ultra-thin film of metal-coordinated electron rich ligand, based on modified adenine, on a highly oriented pyrolytic graphite surface (HOPG). On the contrary ligand alone crystallized into small domains (a few hundreds of nm maximum) on surface. We show that a delicate balance of coordination bonds and weak H-bonding leads to this unusual uniform growth as supported by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The mesoscopic uniformity of film was understood using X-ray photo electron spectroscopy (XPS), while the microscopic pattern of ultra-thin films was corroborated with density functional calculations. Two dimensional film of electron rich molecules are of importance in thin film based electronic applications and therefore their economic processing is also of significance.
Precise “bottom-up” synthesis by surface-assisted chemical reaction provides advanced approaches for the fabrication of two-dimensional (2D) robust covalent nanostructures, which are bound to contain defects due to the irreversible properties of covalent bonds. The further introduction of metal-organic coordination interactions possessing self-correction abilities and yielding comparable stabilities would be a strategically advanced supplement for the construction of well-ordered complex covalent networks. In this report, the on-surface synthesis of Au-coordinated bipyrenes and polypyrenes are achieved on Au(111) by a combination of triggering surface-assisted dehalogenative coupling and capture of Au adatoms. The resulting nanostructures are characterized using scanning tunneling microscope (STM) and X-ray photoelectron spectroscopy (XPS) analysis, and further verified by density functional theory (DFT) calculations. Quinone group functionality present in the pyrene precursor molecules is the key by which the formation of O-Au-O coordination bonds is facilitated in turn forcing the expulsion of bromine atoms from the thus formed two-dimensional nanostructures.
Out of the quest to decipher the nature of self-assembly and underlying thermodynamics for the formation of nucleobase quartets on 2D materials, molecular dynamics simulations and free energy calculations are performed for three nucleobases namely guanine, xanthine and hypoxanthine. Graphene, hexagonal boron nitride (h-BN) and black phosphorene (black Pn) are shown to maintain all the quartet structures over wide ranges of temperatures and their thermal stability follow the order: hypoxanthine > guanine > xanthine. Disruption of the structural integrity for quartets is studied at various temperatures employing a careful hydrogen bond analyses protocol. Decay profile for a quartet on different surfaces decreases as: h-BN > graphene > black Pn. The thermal stability of a single quartet is governed by the free energy gained through quartet formation (while the decay rates in an assembly are influenced by inter-quartet interactions during collision and . h2D-C2N, a novel 2D porous material is demonstrated to disrupt all types of quartets due to extensive hydrogen bonding with the polar surface. It is revealed that, 2D materials which stabilize nucleobases through π-π stacking and have small corrugation in the free energy landscape for in-plane molecular translation are able to stabilize quartets while presence of localized electron density and hydrogen bond donor sites along with large free energy barriers for translocation lead to quartet disruption.
The construction of two-dimensional (2D) self-assembled nanostructures has been one of the considerably interesting areas of on-surface chemistry in the past few decades, which is benefited from the rapid development and improvement of scanning probe microscopy techniques. In this research field, many attempts have been made to the controllable fabrication of well-ordered and multifunctional surface nanostructures, which attracted interest because of the prospect for artificial design of functional molecular nanodevices. DNA and RNA are considered to be programmable self-assembly systems and it is possible to use their base sequences to encode instructions for assembly in a predetermined fashion at the nanometer scale. As important constituents of nucleic acids, nucleobases, with intrinsic functional groups for hydrogen bonding, coordination bonding, and electrostatic interactions, can be employed as a potential system for the versatile construction of various biomolecular nanostructures, which may be used to structure the self-assembly of DNA-based artificial molecular constructions and play the important role in the novel biosensors based on surface functionalization. In this article, we will review the recent progresses of on-surface self-assembly of nucleobases and their derivatives together with different reactants (e.g., metals, halogens, salts and water), and as a result, various 2D surface nanostructures are summarized.
The transformation effects of metal ions and temperature on the DNA bases guanine (G) metal-organic coordination motifs in water have been investigated by scanning tunneling microcopy (STM). The G molecules form an ordered hydrogen-bonded structure at the water- highly oriented pyrolytic graphite (HOPG) interface. The STM observations reveal that the canonical G/9H form can be transformed into the G/(3H, 7H) tautomer by increasing the temperature of the G solution to 38.6oC. Moreover, metal ions bind with G molecules to form G4Fe13+, G3Fe32+ and the heterochiral intermixed G4Na1+ metal-organic networks after the introduction of the alkali-metal ions in cellular environment.
We use pyrene-4,5,9,10-tetraketone molecules with substituents of varying bulkiness in the 2,7 positions to probe the generality and versatility of the previously reported on-surface coordination of two pyrene-4,5,9,10-tetraketone with a single metal atom, leading to one-dimensional coordination polymers. Three different low index surfaces of group 11 metals (Cu, Ag and Au) are used to provide both the support and the metal atoms for metal organic coordination. By real space visualisation with single molecule resolution employing scanning tunnelling microscopy we investigate the molecular self-assembly and show how this can be substantiated with the formation of metal-organic linear and cyclic oligomers, depending on the employed substrate.
From the interplay of high-resolution scanning tunneling microscopy imaging/manipulations and density functional theory calculations, we display the hierarchical formation of supramolecular networks by codeposition of 9eG molecules and Fe atoms on Au(111) based on the flexible coordination bonds (the adaptability and versatility in the coordination modes). In the first step, homochiral islands composed of homochiral G4Fe2 motifs are formed; and then in the second step, thermal treatment results in the transformation to the porous networks composed of heterochiral G4Fe2 motifs with the ratio of the components constant. In-situ STM manipulations and the coexistence of some other heterochiral G4Fe2 motifs and clusters also show the flexibility of the coordination bonds involved. These studies may provide fundamental understanding in the regulations of multilevel supramolecular structures and shed light on the formation of designed supramolecular nanostructures.
Despite the fact that DNA bases have been well-studied on surface, on-surface synthesis of one-dimensional DNA analog through in-situ reactions is still an interesting topic to be investigated. Herein, from the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we have delicately designed a halogenated derivative of adenine as precursor to realize the combination of DNA bases and Ullmann reaction, and then successfully synthesized adenine oligomers on Au(111) via Ullmann coupling. This model system provides a possible bottom-up strategy of fabricating adenine oligomers on surface which may further give access to man-made DNA strands with multiple bases.
From the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we show the first real-space evidence of the formation of GCGC tetrad on the...
Here we applied ionic interactions as the driving force to fabricate well-ordered bicomponent assemblies by using two porphyrin ions equipped with opposite-charged groups. Two kinds of bimolecular chessboard structures were...
Supramolecular coordination chemistry is currently a very popular topic in metallorganic chemistry and has a very large impact on a broad field of applications. More importantly, the invention of STM has opened new doorways to study these concepts on surfaces. This review summarizes the recent progress on surface-confined metallosupramolecular engineering based on the supramolecular coordination chemistry, with the aid of STM. At the beginning, a discussion of metalloids, alkali metals, and alkaline earth metal-based metallosupramolecular engineering is conducted. Next, transition metal-based coordination chemistry on surfaces is discussed. Then, polygonal, double- and triple-decker structures based on rare-earth-metal coordination chemistry are presented. Based on these supramolecular structures, the dynamics of coordination as well as the formed supramolecules are discussed. In the end, the coordination chemistry, including stability of coordination bonds, organic molecules, and gas molecule adsorption is described. Throughout this review, the coordination structures, dynamics and reactivity have been emphasized, which are important current and future research themes.
Through high-resolution UHV-STM imaging and DFT calculations, we find that a di-carbonitrile molecule unexpectedly prefers to form a hydrogen-bonded nanostructure rather than coordination bonded ones on both Cu(110) and Cu(100) at room temperature. These findings highlight the importance of the surface symmetry and molecule–surface interactions in controllable fabrication of metal–ligand coordination nanostructures.
Metal–organic coordination structures are materials comprising reticular metal centers and organic linkers in which the two constituents bind with each other via metal–ligand coordination interaction. The underlying chemistry is more than a century old but has attracted tremendous attention in the last two decades owing to the rapidly development of metal–organic (or porous coordination) frameworks. These metal-coordination materials exhibit extraordinarily versatile topologies and many potential applications. Since 2002, this traditionally three-dimensional chemistry has been extended to two-dimensional space, that is, to synthesize metal–organic coordination structures directly on solid surfaces. This endeavor has made possible a wide range of so-called surface-confined metal–organic networks (SMONs) whose topology, composition, property and function can be tailored by applying the principle of rational design. The coordination chemistry manifests unique characteristics at the surfaces, and in turn the surfaces provide additional control for design structures and properties that are inaccessible in three-dimensional space.
In this review, our goal is to comprehensively cover the progress made in the last 15 years in this rapidly developing field. The review summarizes (1) the experimental and theoretical techniques used in this field including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, density functional theory, and Monte Carlo and kinetic Monte Carlo simulation; (2) molecular ligands, metal atoms, substrates, and coordination motifs utilized for synthesizing SMON; (3) representative SMON structures with different topologies ranging from finite-size discrete clusters to one-dimensional chains, two-dimensional periodical frameworks and random networks; and (4) the properties and potential applications of SMONs. We conclude the review with some perspectives.
Polygonal supramolecular architectures of a Pt(II) complex including trimers, tetramers, pentamers and hexamers were self-assembled via hydrogen bonding between isocytosine moieties; their structure at the solid/liquid interface was unravelled by in situ scanning tunneling microscopy imaging. Density functional theory calculations provided in-depth insight into thermodynamics of their formation by exploring the different energy contributions attributed to the molecular self-assembly and adsorption processes.
Self-assembly provides an effective approach for the fabrication of supramolecular complexes or heterojunction materials, which have unique properties and potential applications in many fields. In this study, the self-assembled structures of stearic acid (SA) and nucleic acid base, guanine (G), are formed at the liquid-solid interface. Two main configurations, namely SA-G-SA and SA-G-G-SA, are observed and the intermolecular recognition mechanism between G and SA is proposed from the hydrogen-bonding point of view.
Water is vital for life as a solvent, and specifically, it has been well established that DNA molecules are hydrated in vivo and water has been found to be responsible for the presence of some non-canonical DNA base tautomers. Theoretical investigations have shown that the existence of water could significantly influence the relative stability of different DNA base tautomers and reduce the energy barrier of tautomeric conversions, and thus promote the formation of some rare base tautomers. In this work, we report the real-space experimental evidence of rare base tautomers. From the high-resolution scanning tunneling microscopy (STM) imaging we surprisingly find the formation of the rare guanine tautomer, i.e. G/(3H,7H) form, on the Au(111) surface by delicately introducing water into the system. The key to the formation of this rare tautomer is proposed to be the "water bridge" that largely reduces the energy barriers of intramolecular proton-transfer processes as revealed by extensive density functional theory (DFT) calculations. The real-space experimental evidence and the proposed mechanism make a step forward towards the fundamental understanding of water-assisted base tautomerization processes.
Lanthanide-based metal-organic compounds and architectures are promising systems for sensing, heterogeneous catalysis, photoluminescence, and magnetism. Herein, the fabrication of interfacial 2D lanthanide-carboxylate networks is introduced. This study combines low- and variable-temperature scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) experiments, and density functional theory (DFT) calculations addressing their design and electronic properties. The bonding of ditopic linear linkers to Gd centers on a Cu(111) surface gives rise to extended nanoporous grids, comprising mononuclear nodes featuring eightfold lateral coordination. XPS and DFT elucidate the nature of the bond, indicating ionic characteristics, which is also manifest in appreciable thermal stability. This study introduces a new generation of robust low-dimensional metallosupramolecular systems incorporating the functionalities of the f-block elements.
Although tautomerization may directly affect the chemical or biological properties of molecules, real-space investigation on the tautomeric behaviors of organic molecules in a larger area of molecular networks has been scarcely reported. In this paper, we choose guanine (G) molecule as a model system. From the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we have successfully achieved the tautomeric recognition, separation and interconversion of G molecular networks (formed by two tautomeric forms G/9H and G/7H) with the aid of NaCl on the Au(111) surface in ultra-high vacuum (UHV) conditions. Our results may serve as a prototypical system to provide important insights into tautomerization related issues, which should be intriguing to biochemistry, pharmaceutics and other related fields.
Fractals are ubiquitous in nature as in the forms of snowflakes, clouds, lighting, leaves and trees, etc. The formation of fractals is of utmost interest to artists and scientists. Efforts in forming molecular fractals have mainly been devoted in synthetic chemistry, and few concentrations have been focused on surfaces albeit the rapid development in surface self-assembly. Herein, by a careful design of the molecular precursor we have successfully constructed the metal-organic Sierpiński triangles on Au(111) via on-surface coordination chemistry, which is demonstrated by the interplay of high-resolution STM imaging and DFT calculations. The coordination Sierpiński triangles show high stabilities as evidenced by room temperature STM imaging, and could suffer a thermal treatment up to 450 K. The successful fabrication of metal-organic Sierpiński triangles provides an alternative method for constructing molecular fractals, and may give implications for constructing other novel and complicated surface patterns.
Clear anti-smudge coatings with a thickness of up to tens of micrometers have been prepared through a graft-copolymer-based approach from commercial precursors. The coatings repel water, diiodomethane, hexadecane, ink, and an artificial fingerprint liquid. In addition, they can be readily applied onto different substrates using different coating methods. These coatings could find applications in protecting hand-held electronic devices from fingerprints, windows from stains, and buildings from graffiti.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Following extensive evidence for the formation of four-stranded DNA G-quadruplex structures in vitro, DNA G-quadruplexes have been observed within human cells. Although chemically distinct, RNA can also fold in vitro into G-quadruplex structures that are highly stable because of the 2'-hydroxyl group. However, RNA G-quadruplexes have not yet been reported in cells. Here, we demonstrate the visualization of RNA G-quadruplex structures within the cytoplasm of human cells using a G-quadruplex structure-specific antibody. We also demonstrate that small molecules that bind to G-quadruplexes in vitro can trap endogenous RNA G-quadruplexes when applied to cells. Furthermore, a small molecule that exhibits a preference for RNA G-quadruplexes rather than DNA G-quadruplexes in biophysical experiments also shows the same selectivity within a cellular context. Our findings provide substantive evidence for RNA G-quadruplex formation in the human transcriptome, and corroborate the selectivity and application of stabilizing ligands that target G-quadruplexes within a cellular context.
The Kondo effect caused by the adsorption of iron phthalocyanine (FePc) on Au(111) was investigated by the combination of density functional theory and a numerical renormalization group calculation with scanning tunneling microscopy. We found that a novel Kondo effect is realized for a single FePc molecule on Au(111) by tuning the symmetry of the ligand field through the local coordination to the substrate. For FePc in the on top configuration where fourfold symmetry around the Fe2+ ion is held, the orbital degrees of freedom survive, resulting in the spin+orbital SU(4) Kondo effect. In contrast, the reduced symmetry in the bridge configuration freezes the orbital degrees of freedom, leading to the spin SU(2) Kondo effect. These results provide a novel example to manipulate the many-body phenomena by tuning the local symmetry.
We present the design and performance of a high-pressure scanning tunneling microscope (HP-STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures (1×10-10-1000 mbar) on a routine basis. The HP-STM is integrated in a gold-plated high-pressure cell with a volume of only ~0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gasses is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric H2 pressures where impurity-related structures were observed.
The atomic-scale identification of the G4K1 structural motif is achieved using an interplay of STM imaging and DFT calculations. Its high stability is found to be caused by the delicate balance between hydrogen bonding and metal-ligand interaction, which is of utmost relevance to model interactions of the G-quadruplex with cations in vivo.
Four-stranded G-quadruplex nucleic acid structures are of great interest as their high thermodynamic stability under near-physiological conditions suggests that they could form in cells. Here we report the generation and application of an engineered, structure-specific antibody employed to quantitatively visualize DNA G-quadruplex structures in human cells. We show explicitly that G-quadruplex formation in DNA is modulated during cell-cycle progression and that endogenous G-quadruplex DNA structures can be stabilized by a small-molecule ligand. Together these findings provide substantive evidence for the formation of G-quadruplex structures in the genome of mammalian cells and corroborate the application of stabilizing ligands in a cellular context to target G-quadruplexes and intervene with their function.
Pt deposited onto a Ge(001) surface gives rise to the spontaneous formation of atomic nanowires on a mixed Pt-Ge surface after high-temperature annealing. We study possible structures of the mixed surface and the nanowires by total energy (density functional theory) calculations. Experimental scanning-tunneling microscopy images are compared to the calculated local densities of states. On the basis of this comparison and the stability of the structures, we conclude that the formation of nanowires is driven by an increased concentration of Pt atoms in the Ge surface layers. Surprisingly, the atomic nanowires consist of Ge instead of Pt atoms.
In addition to the canonical double helix, DNA can fold into various other inter- and intramolecular secondary structures. Although many such structures were long thought to be in vitro artefacts, bioinformatics demonstrates that DNA sequences capable of forming these structures are conserved throughout evolution, suggesting the existence of non-B-form DNA in vivo. In addition, genes whose products promote formation or resolution of these structures are found in diverse organisms, and a growing body of work suggests that the resolution of DNA secondary structures is critical for genome integrity. This Review focuses on emerging evidence relating to the characteristics of G-quadruplex structures and the possible influence of such structures on genomic stability and cellular processes, such as transcription.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
STM study of the self-assembly at the solid-liquid interface of substituted guanines exposing in the N(9)-position alkyl side chains with different lengths revealed the formation of distinct crystalline nanopatterns.
Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical or quantum behaviour. Individual atoms, however, are difficult to arrange in regular patterns. Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment. Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5 nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic anisotropy in supramolecular layers akin to that used in metallic thin films.
We report single-molecule level STM observations of chiral complexes generated by the assembly of achiral components at a metal surface. Following co-deposition of iron atoms and 1,3,5-tricarboxylic benzoic acid (trimesic acid, TMA) on Cu(100) in ultrahigh vaccum, TMA molecules react with the metal centers, and metal-ligand interactions stabilize R and S chiral complexes which are clearly distinguished by STM.
We report that the Kondo effect exerted by a magnetic ion depends on its chemical environment. A cobalt phthalocyanine molecule adsorbed on an Au111 surface exhibited no Kondo effect. Cutting away eight hydrogen atoms from the molecule with voltage pulses from a scanning tunneling microscope tip allowed the four orbitals of this molecule to chemically bond to the gold substrate. The localized spin was recovered in this artificial molecular structure, and a clear Kondo resonance was observed near the Fermi surface. We attribute the high Kondo temperature (more than 200 kelvin) to the small on-site Coulomb repulsion and the large half-width of the hybridized d-level.
We report the manipulation of a Kondo resonance originating from the spin-electron interactions between a two-dimensional molecular assembly of TBrPP-Co molecules and a Cu(111) surface at 4.6 K. By manipulating nearest-neighbor molecules with a scanning tunneling microscope tip we are able to tune the spin-electron coupling of the center molecule inside a hexagonal molecular assembly in a controlled step-by-step manner. The Kondo temperature increases from 105 to 170 K with decreasing the number of nearest neighbor molecules from six to zero. The scattering of surface electrons by the molecules located at edges of the molecular layer reduces the spin-electron coupling strength for the molecules inside the layer. Investigations of different molecular arrangements indicate that the observed Kondo resonance is independent on the molecular lattice.
Guanine-rich nucleic acid sequences can adopt noncanonical four-stranded secondary structures called guanine (G)-quadruplexes. Bioinformatics analysis suggests that G-quadruplex motifs are prevalent in genomes, which raises the need to elucidate their function. There is now evidence for the existence of DNA G-quadruplexes at telomeres with associated biological function. A recent hypothesis supports the notion that gene promoter elements contain DNA G-quadruplex motifs that control gene expression at the transcriptional level. We discovered a highly conserved, thermodynamically stable RNA G-quadruplex in the 5' untranslated region (UTR) of the gene transcript of the human NRAS proto-oncogene. Using a cell-free translation system coupled to a reporter gene assay, we have demonstrated that this NRAS RNA G-quadruplex modulates translation. This is the first example of translational repression by an RNA G-quadruplex. Bioinformatics analysis has revealed 2,922 other 5' UTR RNA G-quadruplex elements in the human genome. We propose that RNA G-quadruplexes in the 5' UTR modulate gene expression at the translational level.
Metal-organic coordination interactions are prime candidates for the formation of self-assembled, nanometer-scale periodic networks with room-temperature structural stability. We present X-ray photoelectron spectroscopy measurements of such networks at the Cu(100) surface which provide clear evidence for genuine metal-organic coordination. This is evident as binding energy shifts in the O 1s and Fe 3p photoelectron peaks, corresponding to O and Fe atoms involved in the coordination. Our results provide the first clear evidence for charge-transfer coordination in metal-organic networks at surfaces and demonstrate a well-defined oxidation state for the coordinated Fe ions.
The recent development of the “scanning tunneling microscope” (STM) by Binnig et al. [8.1–5] has made possible the direct real-space imaging of surface topography. In this technique, a metal tip is scanned along the surface while ad justing its height to maintain constant vacuum tunneling current. The result is essentially a contour map of the surface. This contribution reviews the the ory [8.6–8] of STM, with illustrative examples. Because the microscopic structure of the tip is unknown, the tip wave functions are modeled as s-wave functions in the present approach [8.6, 7]. This approximation works best for small effective tip size. The tunneling current is found to be proportional to the surface local density of states (at the Fermi level), evaluated at the position of the tip. The effective resolution is roughly [2Å(R+d)]1/2, where R is the effective tip radius and d is the gap distance. When applied to the 2x1 and 3x1 reconstructions of the Au(l10) surface, the theory gives excellent agreement with experiment [8.4] if a 9 Å tip radius is assumed. For dealing with more complex or aperiodic surfaces, a crude but convenient calculational technique based on atom charge superposition is introduced; it reproduces the Au(l10) results reasonably well. This method is used to test the structure-sensitivity of STM. The Au(l10) image is found to be rather insensitive to the position of atoms beyond the first atomic layer.
It was found previously [Small2009, 5, 1952 and Angew. Chem., Int. Ed.2005, 44, 2270] that guanine molecules when assembled in a vacuum on the Au(111) surface at room temperature form a hydrogen-bonded network consisting of guanine quartets of the same chirality; this was supported by ground-state density functional theory (DFT) gas-phase calculations. In this Article, we re-examine this system and show that many more (almost equally stable) both homo- and heterochiral structures are possible; however, the homochiral structure observed experimentally becomes definitely the most favorable only if the vibrational contribution to the free energy is accounted for. Interaction with the gold surface, considered within a continuum model, is found to be of minor importance for the comparison of different structures stabilities. We also discuss in some detail the phase transition to another heterochiral guanine phase found upon annealing as reported previously [Small2009, 5, 1952]. We find that, within the current precision of DFT, neither vibrational contribution to free energy nor the finite size effects of guanine islands on the surface can explain the transition observed. A new explanation is proposed on the basis of a liquid-to-solid transition whereby during annealing freely diffusing molecular quartets mix up sufficiently to form on average a heterochiral structure, which upon cooling becomes kinetically trapped.
Coordination bonding between para-quarterphenyl-dicarbonitrile linkers and gadolinium on Ag(111) has been exploited to construct pentameric mononuclear supramolecules, consisting of a rare-earth center surrounded by five molecular linkers. By employing a scanning tunneling microscope tip, a manipulation protocol was developed to position the individual pentamers at the surface. In addition, the tip was used to extract and replace individual linkers yielding tetrameric, pentameric, nonameric and dodecameric metallosupramolecular arrangements. These results open new avenues towards advanced nanofabrication methods and rare-earth nanochemistry by combining the versatility of metal-ligand interactions and atomistic manipulation capabilities.
You can't top the CopperTop: Tetramolecular G-quadruplexes modified with terminal pyridine ligands exhibit metal-triggered stabilization as monitored by thermal denaturation studies, circular dichroism, and nondenaturing gel electrophoresis. Formation of the square-planar Cu(II) (pyridine)4 complex was confirmed by EPR measurements. The metal complexation is fully reversible by removal of the transition metal with ethylenediaminetetraacetic acid (edta).
We report single-molecule level STM observations of chiral complexes generated by the assembly of achiral components at a metal surface. Following co-deposition of iron atoms and 1,3,5-tricarboxylic benzoic acid (trimesic acid, TMA) on Cu(100) in ultrahigh vaccum, TMA molecules react with the metal centers, and metal-ligand interactions stabilize R and S chiral complexes which are clearly distinguished by STM.
A new lipophilic guanosine carrying a porphyrin chromophore on
the ribose moiety has been prepared: evidence is reported for the formation
of a supramolecular complex based on G-quartets and containing an array of
eight porphyrins.
Synthetic lethality is a genetic concept in which cell death is induced by the combination of mutations in two sensitive genes, while mutation of either gene alone is not sufficient to affect cell survival. Synthetic lethality can be also achieved "chemically" by combination of drug-like molecules targeting distinct but cooperative pathways. Previously, we reported that the small molecule pyridostatin (PDS) stabilizes G-quadruplexes in cells and elicits a DNA damage response (DDR) by causing the formation of DNA double strand breaks (DSB). We hypothesize that cell death mediated by ligand-induced G-quadruplex stabilization could be potentiated in cells deficient in DNA damage repair genes. Here, we demonstrated that PDS acts synergistically both with NU7441, an inhibitor of the DNA-PK kinase crucial for non-homologous end joining (NHEJ) repair of DNA DSBs, and BRCA2-deficient cells that are genetically impaired in homologous recombination-mediated DSB repair (HR). G-quadruplex targeting ligands have potential as cancer therapeutic agents, acting synergistically with inhibition or mutation of the DNA damage repair machinery.
Don't be dim! By combining the technique with DFT calculations, STM manipulation was extended to the probing of intermolecular hydrogen-bonding configurations in self-assembled nanostructures. It was also possible to convert one configuration into another in a controlled fashion through the careful manipulation of a particular structural unit.
Surface-confined molecular networks can serve as templates to steer the adsorption and organization of secondary ligands, metal atoms, and clusters. Here, the incorporation of Ni atoms and clusters into open two-dimensional robust metal−organic templates self-assembled from butadiyne dibenzoic acid molecules and Fe atoms on Au(111) and Ag(100) surfaces is investigated by scanning tunneling microscopy. The metal substrate plays a crucial role in the interaction of Ni atoms with the metal−organic host networks. On Ag(100) the metal−organic template steers the growth of Ni clusters underneath the network pattern near the central butadiyne moiety. In contrast, on Au(111) Ni interacts preferentially with the benzene rings forming size-limited clusters inside the network cavities. Thereby, on both surfaces Ni clusters consisting of a few atoms with both high areal density and thermal stability up to 450 K are realized. The Ni-functionalized networks enable the coordination of additional molecules into the open structures demonstrating the utilization of selective interactions for the assembly of multicomponent architectures at different organizational stages.
We report a scanning tunneling microscopy (STM) investigation of the self-assembly of two-dimensional metal-organic coordination networks comprised of Mn or Ni atoms (M) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on a Ag(100) surface. The growth is dominated by the coupling of the cyano ligands to the substrate, and this interaction is reduced by lateral coordination of the molecules to metal adatoms. Initially mononuclear M-TCNQ(4) complexes form at room temperature, which hierarchically assemble into reticulated M-TCNQ(2) networks at sufficiently high metal concentrations. M-TCNQ(1) networks with fully Coordinated molecules can be obtained at elevated substrate temperatures and sufficient metal concentration. A commensurate a and an incommensurate beta phase can be distinguished that are unrelated to the antedecent M-TCNQ(4) complexes. The coordination of the cyano groups to metal adatoms alters the balance between lateral interactions and coupling of the molecules to the substrate. This effect is accompanied by distinct changes in the apparent heights of metal centers in the STM data indicating changes in their electronic configuration.
By means of STM, direct evidence on the sub-nanometer scale of a dynamer operating at surfaces has been provided. Octadecyl guanine (G) was reversibly interconverted at the solid–liquid interface between two highly ordered supramolecular motifs, that is, hydrogenbonded ribbons and G4-based architectures, upon subsequent addition of [2.2.2]cryptand, potassium picrate (K+(pic)-), and trifluoromethanesulfonic acid. The visualization of such supramolecular interconversion at the solid–liquid interface opens new avenues towards understanding the mechanism of formation and function of complex nucleobase architectures such as DNA or RNA. Furthermore, the in situ reversible assembly and reassembly between two highly ordered supramolecular structures at the surfaces represents the first step towards the generation of nanopatterned responsive architectures.
Contrary to what was assumed previously, self-assembled G-quartets may be obtained even in the absence of a templating alkali metal cation. This observation leads to the suggestion that this structural motif, which has been implicated in a variety of biological functions, might be more widespread than believed. Specifically, it is found that guanosine derivative 1 exists as a tetrameric ensemble both in chlorinated organic solvents and in the solid state (see picture). Ar = p-Me2N-C6H4, Rib = ribose.
Scanning tunnelling microscopy (STM) has proved to be a fascinating and powerful technique in the field of surface science. The fact that sets the STM apart from most other surface sensitive techniques is its ability to resolve the structure of surfaces on an atomic scale, that is atom-by-atom, and furthermore its ability to study the dynamics of surface processes. This article presents a survey of recent STM studies of well characterized single crystal metal surfaces under ultra-high vacuum conditions. It particularly addresses STM investigations of clean metal surfaces, adsorbates on metal surfaces, adsorbate-induced restructuring of metal surfaces, chemical reactions on metal surfaces, metal-on-metal growth and finally studies of electron confinement and quantum size effects on metal surfaces.
Programmed: A combination of chemical synthesis and scanning tunneling microscopy has demonstrated high selectivity in the formation of coordination nano-architectures from instructed metal-ligand mixtures under surface-confined conditions. Processing of the molecular information, here the coordination preferences of the two ligands and two metals, lead to the formation of two different regular networks.
By means of the (1)H chemical shifts and the proton-proton proximities as identified in (1)H double-quantum (DQ) combined rotation and multiple-pulse spectroscopy (CRAMPS) solid-state NMR correlation spectra, ribbon-like and quartet-like self-assembly can be identified for guanosine derivatives without isotopic labeling for which it was not possible to obtain single crystals suitable for diffraction. Specifically, characteristic spectral fingerprints are observed for dG(C10)(2) and dG(C3)(2) derivatives, for which quartet-like and ribbon-like self-assembly has been unambiguously identified by (15)N refocused INADEQUATE spectra in a previous study of (15)N-labeled derivatives (Pham, T. N.; et al. J. Am. Chem. Soc.2005, 127, 16018). The NH (1)H chemical shift is observed to be higher (13-15 ppm) for ribbon-like self-assembly as compared to 10-11 ppm for a quartet-like arrangement, corresponding to a change from NH···N to NH···O intermolecular hydrogen bonding. The order of the two NH(2)(1)H chemical shifts is also inverted, with the NH(2) proton closest in space to the NH proton having a higher or lower (1)H chemical shift than that of the other NH(2) proton for ribbon-like as opposed to quartet-like self-assembly. For the dG(C3)(2) derivative for which a single-crystal diffraction structure is available, the distinct resonances and DQ peaks are assigned by means of gauge-including projector-augmented wave (GIPAW) chemical shift calculations. In addition, (14)N-(1)H correlation spectra obtained at 850 MHz under fast (60 kHz) magic-angle spinning (MAS) confirm the assignment of the NH and NH(2) chemical shifts for the dG(C3)(2) derivative and allow longer range through-space N···H proximities to be identified, notably to the N7 nitrogens on the opposite hydrogen-bonding face.
We show that the cooperative reinforcement between hydrogen bonds in guanine quartets is not caused by resonance-assisted hydrogen bonding (RAHB). This follows from extensive computational analyses of guanine quartets (G(4)) and xanthine quartets (X(4)) based on dispersion-corrected density functional theory (DFT-D). Our investigations cover the situation of quartets in the gas phase, in aqueous solution as well as in telomere-like stacks. A new mechanism for cooperativity between hydrogen bonds in guanine quartets emerges from our quantitative Kohn-Sham molecular orbital (MO) and corresponding energy decomposition analyses (EDA). Our analyses reveal that the intriguing cooperativity originates from the charge separation that goes with donor-acceptor orbital interactions in the σ-electron system, and not from the strengthening caused by resonance in the π-electron system. The cooperativity mechanism proposed here is argued to apply, beyond the present model systems, also to other hydrogen bonds that show cooperativity effects.
In a stable relationship: Watson-Crick hydrogen bonding plays a key role in stabilizing the highly ordered supramolecular porous structure formed by co-deposition of biomimetically modified nucleobases cytosine and guanine onto a Au(111) surface under ultrahigh vacuum conditions. A combination of high-resolution STM imaging and density functional theory has been used to determine the structure of the network (see picture).
In this study, through the choice of the well-known G-K biological coordination system, bioligand-alkali metal coordination has for the first time been brought onto an inert Au(111) surface. Using the interplay between high-resolution scanning tunneling microscopy and density functional theory calculations, we show that the mobile G molecules on Au(111) can effectively coordinate with the K atoms, resulting in a metallosupramolecular porous network that is stabilized by a delicate balance between hydrogen bonding and metal-organic coordination.
Several oligo(p-phenylene-vinylene) oligomers capped with a guanosine or a guanine moiety have been prepared via a palladium-catalyzed cross-coupling reaction. Their self-assembly, in both the absence and presence of alkaline salts, has been studied by means of different techniques in solution (NMR, MS, UV-vis, CD, fluorescence), solid state (X-ray diffraction), and on surfaces (STM, AFM). When no salt is added, these pi-conjugated molecules self-associate in a mixture of hydrogen-bonded oligomers, among which the G-quartet structure may be predominant if the steric hindrance around the guanine base becomes important. In contrast, in the presence of sodium or potassium salts, well-defined assemblies of eight functional molecules (8mers) can be formed selectively and quantitatively. In these assemblies, the pi-conjugated oligomers are maintained in a chirally tilted (J-type) stacking arrangement, which is manifested by negative Cotton effects, small bathochromic absorption and emission shifts, and fluorescence enhancements. Furthermore, these self-assembled organic nanostructures, approximately 1.5-2.0 nm high and 8.5 nm wide, exhibit an extraordinary stability to temperature or concentration changes in apolar media, and they can be transferred and imaged over solid substrates as individual nanoparticles, showing no significant dissociation or further aggregation.
Roberto Otero is acknowledged for stimulating discussion. We acknowledge the financial support from the Danish Ministry for Science, Technology, and Innovation for the iNANO Center, from the Danish Research Councils, and from the Carlsberg Foundation. H.G acknowledges the Marie Curie-Intra-European Fellowship (MEIF-CT-2004-010038). We would also like to acknowledge the computer time on the HPCx supercomputer provided via the Materials Chemistry Consortium. R.E.A.K. is also grateful to the EPSRC for financial support (grant GR/P01427/01).
We present a theory for tunneling between a real surface and a model probe tip, applicable to the recently developed ‘‘scanning tunneling microscope.’’ The tunneling current is found to be proportional to the local density of states of the surface, at the position of the tip. The effective lateral resolution is related to the tip radius R and the vacuum gap distance d approximately as [(2 Å)(R+d)]1/2. The theory is applied to the 2×1 and 3×1 reconstructions of Au(110); results for the respective corrugation amplitudes and for the gap distance are all in excellent agreement with experimental results of Binnig et al. if a 9-Å tip radius is assumed. In addition, a convenient approximate calculational method based on atom superposition is tested; it gives reasonable agreement with the self-consistent calculation and with experiment for Au(110). This method is used to test the structure sensitivity of the microscope. We conclude that for the Au(110) measurements the experimental ‘‘image’’ is relatively insensitive to the positions of atoms beyond the first atomic layer. Finally, tunneling to semiconductor surfaces is considered. Calculations for GaAs(110) illustrate interesting qualitative differences from tunneling to metal surfaces.
We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Molecular self-assembly is central to many processes in both biology and supramolecular chemistry. The G-quartet, a hydrogen-bonded macrocycle formed by cation-templated assembly of guanosine, was first identified in 1962 as the basis for the aggregation of 5'-guanosine monophosphate. We now know that many nucleosides, oligonucleotides, and synthetic derivatives form a rich array of functional G-quartets. The G-quartet surfaces in areas ranging from structural biology and medicinal chemistry to supramolecular chemistry and nanotechnology. This Review integrates and summarizes knowledge gained from these different areas, with emphasis on G-quartet structure, function, and molecular recognition.
(Figure Presented) String quartets: Guanine molecules self-assemble into a hydrogen-bonded network of G quartets upon deposition under ultrahigh vacuum conditions onto Au(111) surfaces as shown by STM (see picture). The resilience of the H-bonded network (stable up to 400 K) is explained through a cooperative increase in the strength of H-bonds between guanine molecules in a quartet.
Line them up: Metal-organic chains (see scanning tunneling microscopy image) have been created in situ by self-organized growth at a metal surface under ultrahigh vacuum. These 1D arrangements of metal centers (Fe, Cu), regularly spaced by organic linkers such as trimesitylic acid, open new possibilities for the study of low-dimensional magnetism. (Figure Presented).
15N solid-state NMR refocused INADEQUATE spectra of two lipophilic deoxyguanosine derivatives unambiguously identify different intermolecular hydrogen-bonding arrangements that are indicative of either guanine ribbon or quartet self-assembly. The observation of guanine quartet formation in the absence of metal ions is a further example that challenges the accepted dogma that quartet formation requires metal ions.
A new density functional (DF) of the generalized gradient approximation (GGA) type for general chemistry applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C(6) x R(-6). A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common density functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on standard thermochemical benchmark sets, for 40 noncovalently bound complexes, including large stacked aromatic molecules and group II element clusters, and for the computation of molecular geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for standard functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean absolute deviation of only 3.8 kcal mol(-1). The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the average CCSD(T) accuracy. The basic strategy in the development to restrict the density functional description to shorter electron correlation lengths scales and to describe situations with medium to large interatomic distances by damped C(6) x R(-6) terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chemical method for large systems where dispersion forces are of general importance.
We report solid-state 17O NMR determination of the 17O NMR tensors for the keto carbonyl oxygen (O6) of guanine in two 17O-enriched guanosine derivatives: [6-17O]guanosine (G1) and 2',3',5'-O-triacetyl-[6-17O]guanosine (G2). In G1.2H2O, guanosine molecules form hydrogen-bonded G-ribbons where the guanine bases are linked by O6...H-N2 and N7...H-N7 hydrogen bonds in a zigzag fashion. In addition, the keto carbonyl oxygen O6 is also weakly hydrogen-bonded to two water molecules of hydration. The experimental 17O NMR tensors determined for the two independent molecules in the asymmetric unit of G1.2H2O are: Molecule A, CQ=7.8+/-0.1 MHz, etaQ=0.45+/-0.05, deltaiso=263+/-2, delta11=460+/-5, delta22=360+/-5, delta33=-30+/-5 ppm; Molecule B, CQ=7.7+/-0.1 MHz, etaQ=0.55+/-0.05, deltaiso=250+/-2, delta11=440+/-5, delta22=340+/-5, delta33=-30+/-5 ppm. In G1/K+ gel, guanosine molecules form extensively stacking G-quartets. In each G-quartet, four guanine bases are linked together by four pairs of O6...H-N1 and N7...H-N2 hydrogen bonds in a cyclic fashion. In addition, each O6 atom is simultaneously coordinated to two K+ ions. For G1/K+ gel, the experimental 17O NMR tensors are: CQ=7.2+/-0.1 MHz, etaQ=0.68+/-0.05, deltaiso=232+/-2, delta11=400+/-5, delta22=300+/-5, delta33=-20+/-5 ppm. In the presence of divalent cations such as Sr2+, Ba2+, and Pb2+, G2 molecules form discrete octamers containing two stacking G-quartets and a central metal ion, that is, (G2)4-M2+-(G2)4. In this case, each O6 atom of the G-quartet is coordinated to only one metal ion. For G2/M2+ octamers, the experimental 17O NMR parameters are: Sr2+, CQ=6.8+/-0.1 MHz, etaQ=1.00+/-0.05, deltaiso=232+/-2 ppm; Ba2+, CQ=7.0+/-0.1 MHz, etaQ=0.68+/-0.05, deltaiso=232+/-2 ppm; Pb2+, CQ=7.2+/-0.1 MHz, etaQ=1.00+/-0.05, deltaiso=232+/-2 ppm. We also perform extensive quantum chemical calculations for the 17O NMR tensors in both G-ribbons and G-quartets. Our results demonstrate that the 17O chemical shift tensor and quadrupole coupling tensor are very sensitive to the presence of hydrogen bonding and ion-carbonyl interactions. Furthermore, the effect from ion-carbonyl interactions is several times stronger than that from hydrogen-bonding interactions. Our results establish a basis for using solid-state 17O NMR as a probe in the study of ion binding in G-quadruplex DNA and ion channel proteins.
We describe a general multinuclear (1H, 23Na, 87Rb) NMR approach for direct detection of alkali metal ions bound to G-quadruplex DNA. This study is motivated by our recent discovery that alkali metal ions (Na+, K+, Rb+) tightly bound to G-quadruplex DNA are actually "NMR visible" in solution (Wong, A.; Ida, R.; Wu, G. Biochem. Biophys. Res. Commun. 2005, 337, 363). Here solution and solid-state NMR methods are developed for studying ion binding to the classic G-quadruplex structures formed by three DNA oligomers: d(TG4T), d(G4T3G4), and d(G4T4G4). The present study yields the following major findings. (1) Alkali metal ions tightly bound to G-quadruplex DNA can be directly observed by NMR in solution. (2) Competitive ion binding to the G-quadruplex channel site can be directly monitored by simultaneous NMR detection of the two competing ions. (3) Na+ ions are found to locate in the diagonal T4 loop region of the G-quadruplex formed by two strands of d(G4T4G4). This is the first time that direct NMR evidence has been found for alkali metal ion binding to the diagonal T4 loop in solution. We propose that the loop Na+ ion is located above the terminal G-quartet, coordinating to four guanine O6 atoms from the terminal G-quartet and one O2 atom from a loop thymine base and one water molecule. This Na+ ion coordination is supported by quantum chemical calculations on 23Na chemical shifts. Variable-temperature 23Na NMR results have revealed that the channel and loop Na+ ions in d(G4T4G4) exhibit very different ion mobilities. The loop Na+ ions have a residence lifetime of 220 micros at 15 degrees C, whereas the residence lifetime of Na+ ions residing inside the G-quadruplex channel is 2 orders of magnitude longer. (4) We have found direct 23Na NMR evidence that mixed K+ and Na+ ions occupy the d(G4T4G4) G-quadruplex channel when both Na+ and K+ ions are present in solution. (5) The high spectral resolution observed in this study is unprecedented in solution 23Na NMR studies of biological macromolecules. Our results strongly suggest that multinuclear NMR is a viable technique for studying ion binding to G-quadruplex DNA.