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Transient Absorption Study of Peridinin and Peridinin−Chlorophyll a−Protein after Two-Photon Excitation†
Abstract
Two-photon excited pump−probe spectroscopy was carried out on the peridinin−chlorophyll a−protein complex (PCP) and on the peridinin molecule in methanol solution. Our data are consistent with earlier two-photon fluorescence excitation spectra1,2 and lead to the conclusion that, for peridinin in methanol solution, a separate two-photon-allowed transition exists just to the red of the strongly allowed S0 − S2(Bu+) band. In the PCP complex, two-photon excitation at 1150 nm can also be interpreted as preparing a new state. In both cases, the new state is most likely the S1(Ag-) state. It is not yet possible to conclude experimentally whether peridinin possesses a single low lying state with solvent dependent properties or two distinct states, the Ag- and the charge transfer states.
... A characteristic feature of PCP is that intramolecular charge transfer (ICT) precedes the associated energy transfer from Per to Chl a [13,41]. The energy transfer arising from this channel is supposed to be slower than the one that involves a locally excited state [42]. We thus need to analyze the ICT character of the states to allow for an assignment of our proposed energy channels to experiment. ...
... Note that the proposed model does not invoke an IC between 1B u and 2A g , a scenario that has so far not been considered. The main difference to the commonly accepted models is that we expect 1B u to act directly as an energy donor to Chl a, as already indicated by several experiments [13,39,42]. For such an "S 2 pathway", the orientation of the DFT/MRCI transition dipoles (Table 4) appears to be more beneficial for non-adiabatic coupling between 1B u and Q y than for energy transfer from 1B u to hot Q x states of Chl a [13,42]. ...
... The main difference to the commonly accepted models is that we expect 1B u to act directly as an energy donor to Chl a, as already indicated by several experiments [13,39,42]. For such an "S 2 pathway", the orientation of the DFT/MRCI transition dipoles (Table 4) appears to be more beneficial for non-adiabatic coupling between 1B u and Q y than for energy transfer from 1B u to hot Q x states of Chl a [13,42]. Furthermore, our calculations indicate that fast geometry relaxation of the 1B u state of Per induces ICT character and lowers the energy sufficiently to facilitate direct energy transfer to the Q y state of Chl a. Relaxation of 2A g , Fig. 4 (left side), will presumably reach a different minimum (not computed) that might even be below the 1B u ICT minimum. ...
We present a computationally derived energy transfer model for the peridinin-chlorophyll a-protein (PCP), which invokes vibrational relaxation in the two lowest singlet excited states rather than internal conversion between them. The model allows an understanding of the photoinduced processes without assuming further electronic states or a dependence of the 2Ag state character on the vibrational sub-state. We report molecular dynamics simulations (CHARMM22 force field) and quantum mechanics/molecular mechanics (QM/MM) calculations on PCP. In the latter, the QM region containing a single peridinin (Per) chromophore or a Per-Chl a (chlorophyll a) pair is treated by density functional theory (DFT, CAM-B3LYP) for geometries and by DFT-based multireference configuration interaction (DFT/MRCI) for excitation energies. The calculations show that Per has a bright, green light absorbing 2Ag state, in addition to the blue light absorbing 1Bu state found in other carotenoids. Both states undergo a strong energy lowering upon relaxation, leading to emission in the red, while absorbing in the blue or green. The orientation of their transition dipole moments indicates that both states are capable of excited-state energy transfer to Chl a, without preference for either 1Bu or 2Ag as donor state. We propose that the commonly postulated partial intramolecular charge transfer (ICT) character of a donating Per state can be assigned to the relaxed 1Bu state, which takes on ICT character. By assuming that both 1Bu and 2Ag are able to donate to the Chl a Q band, one can explain why different chlorophyll species in PCP exhibit different acceptor capabilities.
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... These studies provided basic information about energy transfer pathways and rates in MFPCP. The lifetime of the S 1 /ICT state in MFPCP is 2.7 ± 0.3 ps (Zigmantas et al. 2002;Krueger et al. 2001;Linden et al. 2004;Ilagan et al. 2006a;van Stokkum et al. 2009), which is significantly shorter than in any solvent (Bautista et al. 1999b). Since the intrinsic peridinin lifetime in MFPCP is not known, it had to be calculated using the overall energy transfer efficiency of ~88 % obtained from fluorescence excitation experiments (Bautista et al. 1999a). ...
... Since the intrinsic peridinin lifetime in MFPCP is not known, it had to be calculated using the overall energy transfer efficiency of ~88 % obtained from fluorescence excitation experiments (Bautista et al. 1999a). Based on experimental evidence that the S 2 route accounts for 25-50 % of total energy transfer (Krueger et al. 2001;Zigmantas et al. 2002;Linden et al. 2004), the S 1 /ICT lifetime of peridinin yields 15-20 ps, categorizing the protein environment of peridinins in MFPCP to be of moderate polarity. ...
... Krueger et al. (2001) estimated the efficiency of the S 2 pathway to be 25-50 % based on global analysis of transient absorption data, while a lower limit of 25 % was obtained from analysis of the Chl a kinetics at 670 nm (Zigmantas et al. 2002). Later, fluorescence up-conversion experiments on peridinin in methanol and in MFPCP yielded S 2 lifetimes of 130 and 66 fs, respectively, resulting in ~50 % efficiency of the S 2 pathway (Linden et al. 2004). This fluctuation of values demonstrates the complications in determining the efficiency of the S 2 channel, whichbased on calculations -was not predicted to exist (Damjanovic et al. 2000). ...
An important component of the photosynthetic apparatus is a light-harvesting system that captures light energy and transfers it efficiently to the reaction center. Depending on environmental conditions, photosynthetic antennas have adopted various strategies for this function. The water soluble antenna complex of dinoflagellates, peridinin–chlorophyll a protein (PCP), represents a unique light-harvesting strategy because, unlike other antenna systems which have a preponderance of chlorophyll, the carotenoid peridinin serves in PCP as the major light-harvesting pigment. The key structural feature of peridinin is a conjugated carbonyl group which makes the spectroscopic properties of peridinin very sensitive to its local environment. This property is a crucial factor for maintaining the high efficiency of energy transfer between peridinin and Chl a in PCP. PCP is also amenable to site-directed mutagenesis and reconstitution with different pigments, allowing to study effects of both pigment and amino acid exchange on energy transfer pathways within the complex. Since high resolution structures of native, reconstituted and mutated PCP complexes are now available, this knowledge provides an ideal platform to relate structural motifs to energy transfer pathways and efficiencies in PCP. This Chapter summarizes results of structural and spectroscopic investigations of PCP and related proteins, emphasizing the specific light-harvesting strategy developed by dinoflagellates.
... 2) Carotenoids play dual crucial roles in light-harvesting and photoprotection in plant and bacterial photosynthesis, and their ultrafast relaxation kinetics have been extensively studied to understand the mechanism of these physiological functions. 3) Recent novel techniques, such as coherent control [4][5][6][7][8][9] and multi-photon excitation spectroscopies, [9][10][11][12][13][14][15][16] have shown a new orientation for clarifying the complex ultrafast dynamics of carotenoids. Coherent control spectroscopy made possible to excite and probe a specific vibrational mode in a complex molecular system. ...
... For example, two-photon excitation measurements have been applied to the study of carotenoids in order to determine the vibronic structure and observe the dynamics of the dark S 1 state for those free in solution or bound to pigment-protein complexes without interference from other excited states, such as the optically allowed S 2 state. [9][10][11][12][13][14][15][16] In the present work, we address the ultrafast nonlinear optical responses and multi-photon excitation processes of all-trans--carotene in solution. Several investigations have shown the time-resolved excited-state dynamics of carotenoids followed by the direct two-photon excitation up to the S 1 state. ...
... However, the results were not spectrally resolved. 9,14,16) In this study, we performed the femtosecond pump-probe dispersive (spectrally resolved) spectroscopy. The multichannel detection system provided us with more useful information on excited states. ...
The ultrafast nonlinear optical responses of all-trans-beta-carotene have been investigated by femtosecond pump-probe spectroscopy under a nonresonant excitation condition to the optically allowed S2 (11Bu+) state. Instantaneous signals were ascribed to coherent nonlinear optical effects in a three-level electronic system in beta-carotene without any convolution of the population of excited states. The temporal responses of the coherent signals were identified with an instrumental response function of the system. At a longer delay (>1 ps), transient signals due to the dark S1 (21Ag-) state induced by two-photon excitation and to the lowest triplet T1 state induced by four-photon excitation were well temporally and spectrally resolved. The S1 decay time and T1 formation time were determined to be 9.5 and 20 ps, respectively. We show the availability of multiphoton excitation spectroscopy for clarifying the ultrafast relaxation kinetics of a complex system.
... [6] To gain more information about photosynthetic systems and artificially designed molecular systems involving carotenoids, it is important to understand excited states, photophysics and electrochemical properties of carotenoids. Thus, several techniques such as fluorescence, [7,8] resonance Raman, [9,10] two-photon, [11][12][13] and transient absorption spectroscopy have been employed to obtain information about excited states and photophysics of carotenoids. Cyclic and square-wave voltammetry are extensively used for characterization of oxidoreductive reactions of carotenoids. ...
... [19][20][21][22][23] The ICT state is manifested in transient absorption spectra as a band that is red-shifted from the S 1 -S n band as well as by a weak stimulated emission in the near-IR region. [18] The ICT state couples to the S 1 state, forming a state usually denoted as S 1 /ICT [12,17,24,25] resulting in shortening of the S 1 /ICT state lifetime in polar environment. [16,20,26,27] Carotenoid 8'-apo-β-carotenal and its analogues are widely used as synthetic models of carbonyl carotenoids because of their availability for spectroscopic experiments. ...
This work examines the influence of applied external voltage in bulk electrolysis on the excited‐state properties of 8′‐apo‐β‐carotenal in acetonitrile by steady‐state and ultrafast time‐resolved absorption spectroscopy. The data collected under bulk electrolysis were compared with those taken without applied voltage. The steady‐state measurements showed that although intensity of the S0‐S2 absorption band varies with the applied voltage, the spectral position remain nearly constant. Comparison of transient absorption spectra shows that the magnitude of the ICT‐like band decreases during the experiment under applied voltage condition, and is associated with a prolongation of the S1/ICT‐like lifetime from 8 ps to 13 ps. Furthermore, switching off the applied voltage resulted in returning to no‐voltage data within about 30 min. Our results show that the amplitude of the signal associated with the ICT state can be tuned by applying an external voltage.
... The enhancement of S ICT is obtained when Per is placed in protic solvents, like methanol or ethylene glycol, due to hydrogen bonding via the carbonyl group in the lactone ring that is part of the delocalized system. The S ICT state was found to be stabilized in PCP and suggested to be a key property for highly efficient transfer of energy from Per to Chl [35][36][37][38][39]. ...
... In other words, S 1 has an 1 A g character and S 2 an 1 B u + one (as previously remarked they are "symmetry-like" states). Thus for the nature of the S ICT state different models were proposed: (i) the S ICT state is distinct from S 1 [14,24,37]; (ii) the S ICT and S 1 are coherently mixed (or the same state); [36,[40][41][42]; (iii) the S ICT state is the S 1 state with a large intrinsic dipole moment [25]; (iv) S ICT is formed by mixing S 1 and S 2 state moving away from Franck-Condon portion of the S 1 excited state surface [32]. ...
Peridinin-Chlorophyll-a-Proteins (PCPs) are water-soluble light harvesting complexes from dinoflagellates.
They have unique light-harvesting and energy transfer properties which have been studied in details in the last 15 years.
This review aims to give an overview on all the main aspects of PCPs photophysics, with an emphasis on some aspects
which have not been reviewed in details so far, such as vibrational spectroscopy studies, theoretical calculations, and
magnetic resonance studies. A paragraph on the present development of PCPs towards technological applications is also
included.
... The mechanism of energy transfer is facilitated by formation of the ICT state, which strongly enhances the weak transition dipole moment that is characteristic of the S 1 state and leads to stronger electronic coupling with Chl-a (14). Several studies of carotenoids further suggest that~25% of the excitation energy is transferred directly from S 2 to the Q x state of Chl-a (4,18,21,22). ...
... Several models have been proposed to explain the structural origin of the peridinin ICT-state dynamics in solution and the role that the peridinin ICT state plays in energy transfer between peridinin and Chl-a in the PCP complex. Previous experimental and theoretical observations of this interaction suggest that either 1), the ICT state is an electronic state that is distinct from S 1 (S 1 þ ICT) (9,13,23,24), 2), the ICT state and the S 1 state are coherently mixed (S 1 /ICT) (15,22), or 3), the ICT state is simply the S 1 state with a large intrinsic dipole moment (S 1 (ICT)) (12). ...
Experimental and theoretical evidence is presented that supports the theory that the intramolecular charge transfer (ICT) state of peridinin is an evolved state formed via excited-state bond-order reversal and solvent reorganization in polar media. The ICT state evolves in <100 fs and is characterized by a large dipole moment (∼35 D). The charge transfer character involves a shift of electron density within the polyene chain, and it does not involve participation of molecular orbitals localized in either of the β-rings. Charge is moved from the allenic side of the polyene into the furanic ring region and is accompanied by bond-order reversal in the central portion of the polyene chain. The electronic properties of the ICT state are generated via mixing of the "1(1)Bu(+)" ionic state and the lowest-lying "2(1)Ag(-)" covalent state. The resulting ICT state is primarily (1)Bu(+)-like in character and exhibits not only a large oscillator strength but an unusually large doubly excited character. In most solvents, two populations exist in equilibrium, one with a lowest-lying ICT ionic state and a second with a lowest-lying "2(1)Ag(-)" covalent state. The two populations are separated by a small barrier associated with solvent relaxation and cavity formation.
... A few reports applying transient absorption spectroscopy, however, showed that the carotenoid S 1 state is indeed excited by 2PE in both isolated carotenoids 25−28 and light-harvesting systems. 23,29,30 2PE-induced transient absorption spectroscopy has not been applied to LHCII yet, and the assumption of selective 2PE of S 1 of the carotenoids in LHCII has been recently challenged by measurements of 2PE profiles of Chl a and Chl b, which clearly can be excited by 2PE with excitation wavelengths in the 1100−1300 nm spectral region. 31 2PE spectra of Chl a and Chl b, including 2PE spectra of carotenoid−tetrapyrrole dyads, have also been reported by Gacek et al., 32 but a direct comparison of carotenoid and Chl a/Chl b 2PE cross sections is missing. ...
Femtosecond transient absorption spectroscopy following two-photon excitation (2PE) is used to determine the contributions of carotenoids and chlorophylls to the 2PE signals in the main plant light-harvesting complex (LHCII). For 2PE, excitation at 1210 and 1300 nm was used, being within the known 2PE profile of LHCII. At both excitation wavelengths, the transient absorption spectra exhibit a shape characteristic of excited chlorophylls with only a minor contribution from carotenoids. We compare the 2PE data measured for LHCII with those obtained from 2PE of a lutein/chlorophyll a mixture in acetone. We estimate that although the 2PE cross section of a single carotenoid in acetone is ∼1.7 times larger than that of a Chl a, due to the 1:3.5 carotenoid/Chl ratio in LHCII, only one-third of the absorbed 2PE photons excite carotenoids in LHCII in the 1200-1300 nm range.
... The blue emission lifetimes of BPEI-CQDs and CQD-AuNC nanosatellite were 9.5 ns and 5.8 ns, respectively. While the red emission lifetimes of DHLA-AuNCs and CQD-AuNC nanosatellite were 1.9 s and 2.1 s, revealing the changes of chemical environment around the AuNCs [43]. The FRET efficiency was calculated to be ∼40% based on the fluorescence lifetime measurements (Table S1) [44]. ...
... The ultrafast spectroscopy of carotenoids, like the examples shown in Fig. 8 has been recently reviewed [172]. In particular, detailed fs dynamics has been measured for energy transfer between intramolecular CT states in carotenoids, as well as the rates of excitation transfer from these species to Chl-a Q x and Q y bands [173][174][175][176][177][178][179][180][181][182]. ...
While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.
... Chl a [18]. MFPCP from Amphidinium had a lifetime ranging between 2 and 4.5 ns383940, which is substantially shorter than 7−7.4 ns measured in the Symbiodinium PCP [18]. The shortening of the lifetime in MFPCP may be associated with a slow phase of annihilation of Chl a excited states occurring between PCP trimers even at low excitation intensities [38]. ...
The peridinin-chlorophyll a-protein (PCP) is one of the major light harvesting complexes (LHCs) in photosynthetic dinoflagellates. We analyzed the oligomeric state of PCP isolated from the dinoflagellate Symbiodinium, which has received increasing attention in recent years because of its role in coral bleaching. Size-exclusion chromatography (SEC) and small angle neutron scattering (SANS) analysis indicated PCP exists as monomers. Native mass spectrometry (native MS) demonstrated two oligomeric states of PCP, with the monomeric PCP being dominant. The trimerization may not be necessary for PCP to function as a light-harvesting complex.
Copyright © 2015. Published by Elsevier B.V.
... The resulting weak coupling between the two Chls and the high Crt-Chl ratio of 4:1 indicate a particular strategy for LH. Upon absorption of light by the peridinin, the energy is then transferred to Chl a with a quantum efficiency close to 100% [19,20]. The energy transfer between the two Chl a can be described in terms of Förster energy transfer, with a transfer rate of 0.082 ps-1 [21]. ...
Light harvesting complexes developed by living organisms render themselves as an excellent system for understanding basic physical and chemical processes behind the conversion of sunlight energy. Although light harvesting complexes are pretty robust, biochemical reconstitution and genetic modifications have proven the flexibility to tailor their absorption spectra and energy transfer. Importantly, the refolding of the protein and the exchanging of the pigment in micellar media results in very similar pigment arrangement within the native complexes. Here, we show reconstitution approaches with different pigments that have been carried out in PCP, LHCII, and LHI complexes. Monitoring on the spectral changes and energy transfer has also been described.
The coding regions for the N-domain, and full length peridinin–chlorophyll a apoprotein (full length PCP), were expressed in Escherichia coli. The apoproteins formed inclusion bodies from which the peptides could be released by hot buffer. Both the above constructs were reconstituted by addition of a total pigment extract from native PCP. After purification by ion exchange chromatography, the absorbance, fluorescence excitation and CD spectra resembled those of the native PCP. Energy transfer from peridinin to Chl a was restored and a specific fluorescence activity calculated which was ~86% of that of native PCP. Size exclusion analysis and CD spectra showed that the N-domain PCP dimerized on reconstitution. Chl a could be replaced by Chl b, 3-acetyl Chl a, Chl d and Bchl using the N-domain apo protein. The specific fluorescence activity was the same for constructs with Chl a, 3-acetyl Chl a, and Chl d but significantly reduced for those made with Chl b. Reconstitutions with mixtures of chlorophylls were also made with eg Chl b and Chl d and energy transfer from the higher energy Qy band to the lower was demonstrated.
The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown. In order to contribute to this quest, here, we present a solvent-dependent 2D Electronic Spectroscopy (2DES) characterization of fucoxanthin, a carbonyl carotenoid involved in the light-harvesting process of brown algae. The 2DES technique allows probing its ultrafast relaxation dynamics in the first 1000 fs after photoexcitation with a 10 fs time resolution. The obtained results help shed light on the dynamics of the first electronic state manifold and, in particular, on an intramolecular charge-transfer state (ICT), whose photophysical properties are particularly elusive given its (almost) dark nature.
The structure and excited state properties of the H- and J-aggregates of the marine carbonyl carotenoid, fucoxanthin(Fx), were studied by various spectroscopic methods, and compared with those of Fx monomers in polar organic solvents. The fluorescent analysis indicated that the higher vibronic states of S2 contribute more to populating the S1 state, from which fluorescent emission mainly originates. Resonance Raman and density functional theory calculations confirmed the ‘card-packed’ and ‘head-to-tail’ structures of the H- and J-aggregates of Fx, respectively. An fs time-resolved absorption study proved the coexistence of S1 and intramolecular charge transfer relaxation pathways upon excitation to the S2 state for both the monomers and aggregates.
The attribution of quantum beats observed in the time-resolved spectroscopy of photosynthetic light-harvesting antennae to nontrivial quantum coherences has sparked a flurry of research activity beginning a decade ago. Even though investigations into the functional aspects of photosynthetic light-harvesting were supported by X-ray crystal structures, the non-covalent interactions between pigments and their local protein environment that drive such function has yet to be comprehensively explored. Using symmetry-adapted perturbation theory (SAPT), we have comprehensively determined the magnitude and compositions of these non-covalent interactions involving light-harvesting chromophores in two quintessential photosynthetic pigment-protein complexes — peridinin chlorophyll-a protein (PCP) from dinoflagellate Amphidinium carterae and phycocyanin 645 (PC645) from cryptophyte Chroomonas mesostigmatica. In PCP, the chlorophylls are dispersion-bound to the peridinins, which in turn are electrostatically anchored to the protein scaffold via their polar terminal rings. This might be an evolutionary design principle in which the relative orientation of the carotenoids towards the aqueous environment determines the arrangement of the other chromophores in carotenoid-based antennas. On the other hand, electrostatics dominate the non-covalent interactions in PC645. Our ab initio simulations also suggest full protonation of the PC645 chromophores in physiological conditions, and that changes to their protonation states result in their participation as switches between folded and unfolded conformations.
To demonstrate the value of the multi-pulse method in revealing the nature of coupling between excited states and explore the environmental dependencies of S1 and ICT state equilibration, we performed ultrafast transient absorption pump-dump-probe and pump-repump-probe spectroscopies on fucoxanthin in various solvent conditions. The effects of polarity, proticity, and temperature were tested in solvents methanol at 293 and 190 K, acetonitrile, and isopropanol. We show that manipulation of the kinetic traces can produce one trace reflecting the equilibration kinetics of the states which reveals that lower polarity, proticity, and temperature delays S1/ICT equilibration. Based upon a two-state model representing the S1 and ICT states on the same S1/ICT potential energy surface, we were able to show that the kinetics are strictly dependent on the initial relative populations of the states as well as the decay of the ICT state to the ground state. Informed by global analysis, a systematic method for target analysis based upon this model allowed us to quantify the population transfer rates throughout the life of the S1/ICT state as well as separate the S1 and ICT spectral signatures. The results are consistent with the concept that the S1 and ICT states are part of one potential energy surface.
The nature of intramolecular charge transfer (ICT) and the mechanism of intramolecular singlet fission (SF) in peridinin remain open research questions. Obtaining an understanding of the population evolution from the bright state to dark states following a photoinduced electronic transition is critical. Unambiguously describing this evolution in peridinin, and light-harvesting carotenoids in general, has proven elusive experimentally and computationally. To offer a balanced description of bright- and dark-state electronic structure, we here apply ab initio multireference perturbation theory quantum chemistry—the density matrix renormalization group self-consistent-field (DMRG-SCF) and complete-active space self-consistent field (CASSCF) with second-order N-electron valence perturbation theory (NEVPT2). At traditional bright- (S2) and dark-state (S1) optimized geometries, we find that an additional correlated triplet pair (CTP) state and ICT state are derived from the canonical polyene Bu (S3) and 3Ag (S4) dark singlet excited states, respectively. While the S3 state’s physical properties are insensitive to peridinin’s allene-tail donor and lactone-ring acceptor functionalization, the S4 state exhibits a markedly enhanced oscillator strength and HOMO-LUMO transition density. These changes suggest that ICT character stems from mixing between the bright S2 and putatively dark S4.
The central-cis isomer of the carotenoid peridinin, presumably 13-cis, was separated and studied with spectroscopic methods including static absorption, fluorescence and femtosecond time-resolved absorption. The investigations exposed differences in the photophysical properties of this isomer in respect to all-trans peridinin. Steady-state absorption spectroscopy revealed the presence of an additional weak absorption band at the long wavelength tail of the main S0 → S2 transition. Modeling of the hypothetical vibronic progression of the S0 → S1 electronic transition demonstrated that this weak band can be associated with a higher (0-2) vibronic band of the transition and that lower vibronic bands have negligible intensities due to a large displacement between the S0 and S1 states energy curves as also suggested by the spectral shape of steady-state fluorescence emission. Transient absorption studies demonstrated that the lifetime of the S1 state of the central-cis isomer is shorter compared to the all-trans counterpart by 6-16%, depending on the polarity of the solvent. On the other hand, molecular isomerization negligibly affects the lifetime of intramolecular charge transfer (ICT), which for both isomers is ∼10 ps in the polar solvent methanol.
The prevalence of reactive oxygen species (ROS) production and the enzyme-containing intracellular milieu could lead to the fluorescence quenching of the bovine serum albumin (BSA)-capped gold nanoclusters (AuNCs). Here we report an efficient strategy to address this issue, where a polymer-like shielding layer is designed to wrapping around the Au core to significantly improve the stability of AuNCs against ROS and protease degradation. The key of our design is to covalently incorporate a thiolated AuNC into the BSA-AuNC via carbodiimide-activated coupling, leading to the formation of a AuNC pair inside the cross-linked BSA molecule. The as-designed paired AuNCs in BSA (or BSA-p-AuNCs for short) show improved performances in living cells.
Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative decay pathway from the S2 (11Bu+) state to the S1 (21Ag−) state of peridinin in methanol solution. Just as previously observed by this laboratory for β-carotene in benzonitrile, the real (absorption) and imaginary (dispersion) components of the transient grating signal obtained with Fourier transform spectral interferometry from peridinin exhibit ultrafast responses indicating that S2 state decays in 12 fs to produce an intermediate state, Sx. The excited state absorption spectrum from the Sx state of peridinin, however, is found to be markedly blue shifted from that of β-carotene because it makes a substantial contribution to the signal observed with 40 fs, 520 nm pulses. The results of a global target analysis and numerical simulations using nonlinear response functions and the multimode Brownian oscillator model support the assignment of Sx to a displaced conformation of the S2 state rather than to a vibrationally excited (or hot) S1 state. The Sx state in peridinin is assigned to a structure with a distorted conjugated polyene backbone moving past an activation-energy barrier between planar and twisted structures on the S2 potential surface. The lengthened lifetime of the Sx state of peridinin in methanol, 900 ± 100 fs, much longer than that typically observed for carotenoids lacking carbonyl substituents, ~150 fs, can be attributed to the slowing of torsional motions by solvent friction. In peridinin, the system–bath coupling is significantly enhanced over that in β-carotene solution most likely due to the intrinsic intramolecular charge transfer character it derives from the electron withdrawing nature of the carbonyl substituent. An important additional implication is that the Sx state and subsequent distorted S2 structures may serve as the principal excitation energy transfer donors to chlorophyll a in the peridinin–chlorophyll a protein from dinoflagellates.
Photosynthesis begins when photons are absorbed by the light-harvesting system. The light-harvesting system then transfers the absorbed energy to the reaction center (RC), where it is trapped and initiates the primary redox reactions of photosynthesis. The light-harvesting system therefore acts to increase the effective cross-sectional area of each RC for light absorption. This means that RCs can be kept supplied with sufficient numbers of photons even when the incident light intensity is rather low. The size of the antenna system and the types of pigments that are used depend on the ecological niche in which the particular photosynthetic species lives.
Excitation energy transfer (EET) in peridinin-chlorophyll-protein complexes is dominated by the S1 → Qy pathway, but the high efficiencies cannot be solely explained by this one pathway. We postulate that EET from peridinin S2 excitons may also be important. We use complete active space configuration interaction calculations and the transition density cube method to calculate Coulombic couplings between peridinin and chlorophyll a in the PCP complex of the dinoflagellate Amphidinium carterae, and compare monomeric and dimeric delocalized peridinin S2 excited states. Our calculations show that the S2 → Qy EET pathway from peridinin to chlorophyll a is the dominant energy transfer pathway from the S2 excited state in PCP, with several values in the sub-ps range. This result suggests that the S2 → Qy EET pathway may be responsible for the reported chlorophyll a bleaching signature seen in experiment at around 200 fs, and not the S2 → Qx pathway as previously suggested.
Photosynthetic eukaryotes exhibit very different light-harvesting proteins, but all contain membrane-intrinsic light-harvesting complexes (Lhcs), either as additional or sole antennae. Lhcs non-covalently bind chlorophyll a and in most cases another chlorophyll, as well as very different carotenoids, depending on the taxon. The proteins fall into two major groups: The well-defined Lhca/b group of proteins binds typically chlorophyll b and lutein, and the group is present in the ‘green lineage’. The other group consists of Lhcr/Lhcf, Lhcz and Lhcx/LhcSR proteins. The former are found in the so-called Chromalveolates, where they mostly bind chlorophyll c and carotenoids very efficient in excitation energy transfer, and in their red algae ancestors. Lhcx/LhcSR are present in most Chromalveolates and in some members of the green lineage as well. Lhcs function in light harvesting, but also in photoprotection, and they influence the organisation of the thylakoid membrane. The different functions of the Lhc subfamilies are discussed in the light of their evolution.
Fucoxanthin–chlorophyll-a/c protein (FCP) complexes from brown algae Cladosiphon okamuranus TOKIDA (Okinawa Mozuku in Japanese) contain the only species of carbonyl carotenoid, fucoxanthin, which exhibits spectral characteristics attributed to an intramolecular charge-transfer (ICT) property that arises in polar environments due to the presence of the carbonyl group in its polyene backbone. Here, we investigated the role of the ICT property of fucoxanthin in ultrafast energy transfer to chlorophyll-a/c in brown algal photosynthesis using femtosecond pump–probe spectroscopic measurements. The observed excited-state dynamics show that the ICT character of fucoxanthin in FCP extends its absorption band to longer wavelengths and enhances its electronic interaction with chlorophyll-a molecules, leading to efficient energy transfer from fucoxanthin to chlorophyll-a.
Results of infrared transient absorption measurements following two-photon excitation to the forbidden S1 state of the carotenoid β-apo-8' -carotenal are presented. This technique enables the study of subsequent structural dynamics without interference from higher states.
We modeled excitation energy transfer (EET) in the peridinin-chlorophyll a-protein (PCP) complex of dinoflagellate Amphidinium carterae, to determine which pathways contribute dominantly to the high efficiency of this process. We used complete active space configuration interaction (CAS-CI) to calculate electronic structure properties of the peridinin (PID) and chlorophyll a (CLA) pigments in PCP, and the transition density cube (TDC) method to calculate Coulombic couplings between energy transfer donors and acceptors. Our calculations show that the S1 → Qy EET pathway from peridinin to chlorophyll a is the dominant energy transfer pathway in PCP, with two sets of interactions -- between PID612 and CLA601, and betwen PID622 and CLA602 -- contributing most strongly. EET lifetimes for these two interactions were calculated to be 2.66 and 2.90 ps, with quantum efficiencies of 85.75% and 84.65%, respectively. The calculated Coulombic couplings for EET between two peridinin molecules in the strongly allowed S2 excited states are extremely large, and suggest excitonic coupling between pairs of peridinin S2 states. This methodology is also broadly applicable to the study of EET in other photosynthetic complexes and/or organic photovoltaics, where both single and double excitations are present and donor and acceptor molecules are tightly packed.
Fucoxanthin, containing a carbonyl group in conjugation with its polyene backbone, is a naturally occurring pigment in marine organisms and is essential to the photosynthetic light-harvesting function in brown alga and diatom. Fucoxanthin exhibits optical characteristics attributed to an intramolecular charge transfer (ICT) state that arises in polar environments due to the presence of the carbonyl group. In this study, we report the spectroscopic properties of fucoxanthin in methanol (polar and protic solvent) observed by femtosecond pump–probe measurements in the near-infrared region, where transient absorption associated with the optically allowed S2 (11B
u+) state and stimulated emission from the strongly coupled S1/ICT state were observed following one-photon excitation to the S2 state. The results showed that the amplitude of the stimulated emission of the S1/ICT state increased with decreasing excitation energy, demonstrating that the fucoxanthin form associated with the lower energy of the steady-state absorption exhibits stronger ICT character.
In this letter, we report the singlet ground state structure of the full carotenoid peridinin by means of variational Monte Carlo (VMC) calculations. The VMC relaxed geometry has an average bond length alternation of 0.1165(10) Å, larger than the values obtained by DFT (PBE, B3LYP, and CAM-B3LYP) and shorter than that calculated at the Hartree−Fock (HF) level. TDDFT and EOM- CCSD calculations on a reduced peridinin model confirm the HOMO−LUMO major contribution of the Bu+-like (S2) bright excited state. Many Body Green’s Function Theory (MBGFT) calculations of the vertical excitation energy of the Bu+-like state for the VMC structure (VMC/MBGFT) provide an excitation energy of 2.62 eV, in agreement with experimental results in n-hexane (2.72 eV). The dependence of the excitation energy on the bond length alternation in the MBGFT and TDDFT calculations with different functionals is discussed.
A theoretical analysis of linear and non-linear (two-photon absorption) electronic spectroscopy of all known porphyrinic pigments has been performed using linear and quadratic density functional response theory, with the long-range corrected CAM-B3LYP functional. We found that higher Soret transitions often contain non-Gouterman contributions and that each chlorophyll has the possibility for resonance enhanced TPA in the Soret region, although there is also significant TPA in the Q region. This journal is
The concentration dependence of the fluorescence spectra and lifetimes of chlorophyll a and sodium magnesium chlorophyllin are compared to explore the influences of intramolecular relaxation and intermolecular interaction of chlorophylls on the fluorescence dynamics. With different concentrations of chlorophyll a and sodium magnesium chlorophyllin in the suspensions, three different fluorescence emission bands are observed. Intermolecular interactions increase as the concentrations increase, which lead to redshifts in both fluorescence and lifetime spectra. Additionally, the lifetime is increased by re-absorption in the sodium magnesium chlorophyllin suspensions. These observations increase the understanding of the light harvesting and energy relaxation mechanisms of chlorophyll molecules.
We present a quantum-mechanical investigation of the photophysics of a specific carotenoid, peridinin, which is present in light-harvesting complexes. The fundamental role played by the geometry in determining position and character of its low-lying singlet electronic states is investigated using a multireference density functional theory approach in combination with a continuum solvation model. The main photophysical properties of peridinin appear to be governed by the lowest two singlet excited states as no evidence points to an intermediate S(*) state and the energies of the upper excited states are too high to allow their population with excitation in the visible range. These two excited states (S1, 2(1)A(-)g and S2, 1(1)B(+)u) are highly connected through the conjugation path here characterized by the value of the bond length alternation (BLA). The S1 and S2 states present distinct natures for small BLA values whereas for larger ones they become more similar in terms of both brightness and dipolar character as well as their energies become closer. The geometrical issue is thus of fundamental importance for a correct interpretation of the spectroscopic signatures of peridinin.
The fundamental interactions between naturally occurring pigments in light-har- vesting systems are responsible for the high efficiency of the photosynthetic apparatus. We describe the role of carotenoids (Cars) in light-harvesting systems, including our work eluci- dating the mechanism of energy transfer from the optically dark Car singlet excited state (S1) to chlorophyll (Chl) and calculations on the electronic structure of Cars by means of time- dependent density functional theory (TDDFT). We highlight new studies on the charge-trans- fer state of the Car, peridinin (Per), which enhances the light-harvesting efficiency of the Car by increasing the electronic coupling to Chl. The role of another Car, zeaxanthin (Zea), is dis- cussed with respect to its role in the mechanism of the feedback deexcitation quenching in green plants, a vital regulation process under light conditions which exceed photosynthetic capacity. Lastly, we provide insight on how the 96 Chls in Photosystem I are optimized to generate a pigment-protein complex which utilizes solar energy with near unit efficiency.
Ultrafast relaxation kinetics of fucoxanthin in polar and non-polar solvents have been studied by femtosecond pump-probe spectroscopy. Transient absorption associated with S1 or intramolecular charge transfer (ICT) excited state has been observed following either one-photon excitation to the optically allowed S2 state or two-photon excitation to the symmetry-forbidden S1 state. The results suggest that the ICT state formed after excitation of fucoxanthin in a polar solvent is a distinct excited state from S1.
Femtosecond one- and two-photon pump–probe dispersive spectroscopic measurements have been applied to the investigation of the vibrational relaxation kinetics of the dark S1 (21Ag-) state in β-carotene, combining a higher sensitive detection system with tunable visible and infrared excitation pulses. The two-photon excitation measurements enable the preferential detection of the dark S1 state. The tunable infrared excitation pulses allowed selective excitation to a different vibrational level of S1. The S1 dynamics at early delay times depend strongly on excitation energy. A dependence of the initial S1 dynamics on excitation energy is discussed in term of the vibrational relaxation of S1.
The ultrafast relaxation kinetics of fucoxanthin in polar and non-polar solvents have been studied by femtosecond dispersed one- and two-photon pump–probe spectroscopies. Transient absorption kinetics of the lowest singlet S1 (21Ag-) state and/or intramolecular charge transfer (ICT) state after excitation to the optically allowed S2(11Bu+) state depend strongly on solvent polarity. Transient absorption spectra and the kinetics of absorbance changes after direct two-photon excitation to S1/ICT depend strongly on excitation energy in non-polar solvent. The results suggest that the ICT state is a distinct state from S1 in polar solvent.
Inter-pigment interactions define the functioning of light-harvesting protein complexes. To describe the particularly complex molecular dynamics and interactions of peridinin and chlorophyll in the peridinin chlorophyll protein of Amphidinium carterae, we applied global and target analysis to a series of ultrafast transient absorption experiments. We have created and validated a model that consistently describes and characterizes the interactions and evolution of excited and ground-state populations after excitation in all different experiments. The series of energy transfer steps that follow excitation are described by our model of cascading populations and numerous rate constants that correspond to intra-molecular thermal relaxation, fast and slow peridinin-to-chlorophyll energy transfer steps, and chlorophyll excited-state annihilation. By analyzing the spectral response of ground-state peridinins to excited chlorophylls we have identified which specific peridinin molecule is most closely coupled to the chlorophylls. No evidence was found that the intra-molecular charge transfer (ICT) state of peridinin, identified in studies of peridinin in solution, is a separate entity in the protein. The peridinin that exhibited slow peridinin-to-chlorophyll energy transfer was characterized by a difference spectrum free from ICT features, consistent with the importance of coupled ICT and S1 states for energy transfer.
The lifetime of the lowest excited singlet state of carbonyl-containing carotenoids typically depends on the polarity of the solvent, an effect that has been attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this ICT state has yet to be clarified. In the present work, steady-state and ultrafast time-resolved optical spectroscopic experiments have been performed on peridinin and three synthetic analogues, C33-peridinin, C35-peridinin, and C39-peridinin, which have different extents of π-electron conjugation. Steady-state absorption at cryogenic temperatures revealed new absorption bands on the long-wavelength side of the strongly allowed S0 (1(1)Ag(-)) → S2 (1(1)Bu(+)) transition that can be assigned to S0 (1(1)Ag(-)) → S1 (2(1)Ag(-)) absorption. Analysis of the time-resolved absorption and fluorescence data sets revealed that the influence of polarity of the solvent on the excited state lifetime is unique for each molecule, leading to subtle differences in the values in highly polar solvents. Measurements in the most polar solvent, acetonitrile, demonstrated that the ICT state lifetime is shortest at 6.4 ps for C39-peridinin and gradually increases as the extent of π-electron conjugation decreases, becoming 10.6 ps for C33-peridinin. This suggests that the energy of the ICT state is dependent on the number of conjugated carbon-carbon double bonds.
The spectroscopic properties of the peridinin-Chl a-protein (PCP) from the coral symbiotic dinoflagellate Symbiodinium have been characterized by application of various ultrafast optical spectroscopies including femto and nanosecond time-resolved absorption and picosecond time-resolved fluorescence (TRF) at 77 K. Excited states properties of peridinin and Chl a and their intramolecular interaction characteristics have been obtained from global fitting analysis and directed kinetic modeling of the datasets and compared to their counterparts known for the PCP from Amphidinium carterae. The lifetimes of the excited state of peridinin show close agreement with those known for the counterpart PCP demonstrating that molecular interactions have the same characteristics in both complexes. More variances have been recorded for the exited state properties of Chl a including elongation of both the intramolecular energy transfer time between Chls within the pair in the protein monomer and the exited state lifetime of the long wavelength form of Chl a (terminal acceptor). Kinetic modeling of formation of the peridinin triplet state has shown that the PCP is protected from potential photodamage due to an extremely fast peridinin triplet state formation of k_TT = (14.4 ± 2.3)×109 s-1 ((70 ± 12)-1(ps)-1) that guarantees instantaneous depletion of Chl a triplets and prevents formation of harmful singlet oxygen (1ΔgO2).
Femtosecond time-resolved transient absorption spectroscopy was performed on the chlorophyll a–chlorophyll c
2–peridinin-protein-complex (acpPC), a major light-harvesting complex of the coral symbiotic dinoflagellate Symbiodinium. The measurements were carried out on the protein as well on the isolated pigments in the visible and the near-infrared region at 77 K. The data were globally fit to establish inter-pigment energy transfer paths within the scaffold of the complex. In addition, microsecond flash photolysis analysis was applied to reveal photoprotective capabilities of carotenoids (peridinin and diadinoxanthin) in the complex, especially the ability to quench chlorophyll a triplet states. The results demonstrate that the majority of carotenoids and other accessory light absorbers such as chlorophyll c
2 are very well suited to support chlorophyll a in light harvesting. However, their performance in photoprotection in the acpPC is questionable. This is unusual among carotenoid-containing light-harvesting proteins and may explain the low resistance of the acpPC complex against photoinduced damage under even moderate light conditions.
Quantum chemical calculations have been employed for the investigation of the lowest excited electronic states of lutein, with particular reference to its function within light harvesting antenna complexes of higher plants. Through comparative analysis obtained by using different methods based on gas-phase calculations of the spectra, it was determined that variations in the lengths of the long C-C valence bonds and the dihedral angles of the polyene chain are the dominant factors in determining the spectral properties of Lut 1 and Lut 2 corresponding to the deformed lutein molecules taken from crystallographic data of the major pigment-protein complex of photosystem II. By MNDO-CAS-CI method, it was determined that the two singlet B(u) states of lutein (nominally 1B(u)(-)* and 1B(u)(+)) arise as a result of mixing of the canonical 1B(u)(-) and 1B(u)(+) states of the all-trans polyene due to the presence of the ending rings in lutein. The 1B(u)(-)* state of lutein is optically allowed, while the 1B(u)(-) of a pure all-trans polyene chain is optically forbidden. As demonstrated, the B(u) states are much more sensitive to minor distortions of the conjugated chain due to mixing of the canonical states, resulting in states of poorly defined particle-hole symmetry. Conversely, the A(g) states are relatively robust with respect to geometric distortion, and their respective inversion and particle-hole symmetries remain relatively well-defined.
The spectroscopic properties and dynamics of the excited states of two different synthetic analogues of peridinin were investigated as a function of solvent polarity using steady-state absorption, fluorescence, and ultrafast time-resolved optical spectroscopy. The analogues are denoted S-1- and S-2-peridinin and differ from naturally occurring peridinin in the location of the lactone ring and its associated carbonyl group, known to be obligatory for the observation of a solvent dependence of the lifetime of the S(1) state of carotenoids. Relative to peridinin, S-1- and S-2-peridinin have their lactone rings two and four carbons more toward the center of the π-electron system of conjugated carbon-carbon double bonds, respectively. The present experimental results show that as the polarity of the solvent increases, the steady-state spectra of the molecules broaden, and the lowest excited state lifetime of S-1-peridinin changes from ∼155 to ∼17 ps which is similar to the magnitude of the effect reported for peridinin. The solvent-induced change in the lowest excited state lifetime of S-2-peridinin is much smaller and changes only from ∼90 to ∼67 ps as the solvent polarity is increased. These results are interpreted in terms of an intramolecular charge transfer (ICT) state that is formed readily in peridinin and S-1-peridinin, but not in S-2-peridinin. Quantum mechanical computations reveal the critical factors required for the formation of the ICT state and the associated solvent-modulated effects on the spectra and dynamics of these molecules and other carbonyl-containing carotenoids and polyenes. The factors are the magnitude and orientation of the ground- and excited-state dipole moments which must be suitable to generate sufficient mixing of the lowest two excited singlet states.
A model for the third order optical response of carotenoids is used to analyse transient grating and pump–probe data of peridinin in solution and bound in the peridinin–chlorophyll protein (PCP). For peridinin in solution, the transient grating signal detected at 505 nm exhibits a bi-exponential recovery whose fast phase is assigned to relaxation from the S2 state that has a lifetime of 75 ± 25 fs. The slower, solvent-dependent rise component is assigned to equilibration of the (S1/ICT) state, taking place on a time scale of 0.6 and ∼2.5 ps in acetontrile and benzene, respectively. These dynamics match those obtained from pump–probe measured in the spectral region of the ICT state, implying that the ICT state contributes to the signal at 505 nm. In PCP, the transient grating signal shows distinctly different kinetics, and the signal shows no recovery. This difference is explained by energy transfer from peridinin to chlorophyll-a.
Photosynthetic organisms utilize (bacterio) chlorophylls and carotenoids as main light-harvesting pigments. In this chapter,
we review bacteriochlorophyll light-harvesting in photosynthetic purple bacteria; we discuss intra- and intercomplex energy
transfer processes as well as energy trapping by reaction centers. From the viewpoint of light-harvesting, in most organisms
carotenoids are accessory pigments absorbing in the blue–green region of the solar spectrum, where chlorophylls and bacteriochlorophylls
have weak absorption. Here, we discuss carotenoid light-harvesting in a pigment–protein complex having carotenoids as main
lightharvesting pigment, the peridinin chlorophyll protein (PCP).
Over the past ten years, the techniques and sophistication of analysis of ultrafast spectroscopy have advanced to a remarkable
extent. Following the demonstration and validation of new methods using simple dilute dye solutions, many applications of
these new techniques have been to photosynthetic systems. The reasons for this are not hard to find: photosynthetic pigment
protein complexes function through a delicate interplay of interpigment and pigment-environment interactions, and standard
methods provide an unsatisfactory level of microscopic insight because signals are dominated by inhomogeneous effects, or
the methods themselves are insensitive to the interactions between states of interest. The developments described in this
chapter are designed to address all these issues. Multiphoton transient absorption spectroscopy helps to unravel complex energy
transfer and relaxation pathways. Two-photon excitation spectroscopy prepares states that are not ac cessible directly from
the ground state. Photon echo methods defeat inhomogeneous broadening, measure it, and exploit it to observe energy transfer
between chemically identical donors and acceptors. Two-dimensional techniques explicitly reveal electronic couplings and the
pathways of energy flow. New methods based on Raman spectroscopy reveal vibrational frequencies in excited states, and indeed
a wide range of transient species.
The theoretical description of the initial steps in photosynthesis has gained increasing importance over the past few years. This is caused by more and more structural data becoming available for light-harvesting complexes and reaction centers which form the basis for atomistic calculations and by the progress made in the development of first-principles methods for excited electronic states of large molecules. In this Review, we discuss the advantages and pitfalls of theoretical methods applicable to photosynthetic pigments. Besides methodological aspects of excited-state electronic-structure methods, studies on chlorophyll-type and carotenoid-like molecules are discussed. We also address the concepts of exciton coupling and excitation-energy transfer (EET) and compare the different theoretical methods for the calculation of EET coupling constants. Applications to photosynthetic light-harvesting complexes and reaction centers based on such models are also analyzed.
Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.
The peridinin-chlorophyll a-protein (PCP) is a light-harvesting pigment-protein complex found in many species of marine algae. It contains the highly substituted carotenoid peridinin and chlorophyll a, which together facilitate the transfer of absorbed solar energy to the photosynthetic reaction center. Photoexcited peridinin exhibits unorthodox spectroscopic and kinetic behavior for a carotenoid, including a strong dependence of the S(1) excited singlet state lifetime on solvent environment. This effect has been attributed to the presence of an intramolecular charge transfer (ICT) state in the molecule. The present work explores the effect of changing the extent of π-electron conjugation and attached functional groups on the nature of the ICT state of peridinin and how these factors affect the excited singlet and triplet state spectra and kinetics of the carotenoid. In this investigation three peridinin analogues denoted C-1-R-peridinin, C-1-peridinin, and D-1-peridinin were synthesized and studied using steady-state absorption and fluorescence techniques and ultrafast time-resolved transient absorption spectroscopy. The study explores the effect on the singlet and triplet state spectra and dynamics of removing the allene group from the peridinin structure and either replacing it with a rigid furanoid ring, replacing it with an epoxide group, or extending the polyene chain into the β-ionylidine ring.
The photophysical properties of a carbonyl-containing carotenoid analogue in an s-cis configuration, relative to the conjugated π system, 2-(all-trans-retinylidene)-indan-1,3-dione (C20Ind), were investigated by femtosecond time-resolved spectroscopy in various solvents. The lifetime of the optically forbidden S(1) state of C20Ind becomes long as solvent polarity increases. This trend is completely opposite to the situation of S(1-ICT) dynamics of carbonyl-containing carotenoids, such as peridinin and fucoxanthin. Excitation energy dependence of the transient absorption measurements shows that the transient absorption spectra in nonpolar solvents were originated from two distinct transient species, while those in polar and protic solvents are due to a single transient species. By referring to the results of MNDO-PSDCI (modified neglect of differential overlap with partial single- and double-configuration interaction) calculations, we conclude: (1) in polar and protic solvents, the S(1) state is generated following excitation up to the S(2) state; (2) in nonpolar solvents, however, both the S(1) and the (1)nπ* states are generated; and (3) C20Ind does not generate the S(1-ICT) state, despite the fact that it has two conjugated carbonyl groups.
Stark absorption spectra of peridinin (Per) and five allene-modified analogues and their angular dependence as a function of an externally applied electric field were measured in methyl methacrylate polymer at 77K. In all cases, the energetically lowest absorption band has a significant change of static dipole moment upon photoexcitation (Δμ). In particular, Per has the largest value of |Δμ|. The angles between Δμ and the transition dipole moment of all the analogues were determined. It is suggested that the allene group in Per plays a key role as the electron donor in the charge transfer process following photoexcitation. The results of MNDO-PSDCI calculations support this idea.
Numerous femtosecond time-resolved optical spectroscopic experiments have reported that the lifetime of the low-lying S(1) state of carbonyl-containing polyenes and carotenoids decreases with increasing solvent polarity. The effect becomes even more pronounced as the number of double bonds in the conjugated π-electron system decreases. The effect has been attributed to an intramolecular charge transfer (ICT) state coupled to S(1), but it is still not clear what the precise molecular nature of this state is, and how it is able to modulate the spectral and dynamic properties of polyenes and carotenoids. In this work, we examine the nature of the ICT state in three substituted polyenes: crocetindial, which contains two terminal, symmetrically substituted carbonyl groups in conjugation with the π-electron system, 8,8'-diapocarotene-8'-ol-8-al, which has one terminal conjugated carbonyl group and one hydroxyl group, and 8,8'-diapocarotene-8,8'-diol, which has two terminal, symmetrically positioned, hydroxyl groups but no carbonyls. Femtosecond time-resolved optical spectroscopic experiments on these molecules reveal that only the asymmetrically substituted 8,8'-diapocarotene-8'-ol-8-al exhibits any substantial effect of solvent on the excited state spectra and dynamics. The data are interpreted using molecular orbital theory which shows that the ICT state develops via mixing of the low-lying S(1) (2(1)A(g)-like) and S(2) (1(1)B(u)-like) excited singlet states to form a resultant state that preferentially evolves in polar solvent and exhibits a very large (∼25 D) dipole moment. Molecular dynamics calculations demonstrate that the features of the ICT state are present in ∼20 fs.
Spectroscopic properties of spheroidene and a series of spheroidene analogs were studied using steady-state absorption, fluorescence, fluorescence excitation, and time-resolved absorption spectroscopy. A systematic series of molecules that control energy transfer to bacteriochlorophyll is provided. The molecules were purified by high-pressure liquid chromatographic techniques. Absorption spectra were observed to red-shift with increasing extent of Ï-electron conjugation. The transient data of the energy gap law for radiationless transitions allow a prediction of the Sâ energies of the molecules. The process of light harvesting by carotenoids in photosynthesis is discussed. 44 refs., 8 figs., 3 tabs.
Carotenoids are, along with chlorophylls, crucial pigments involved in light-harvesting processes in photosynthetic organisms. Details of carotenoid to chlorophyll energy transfer mechanisms and their dependence on structural variability of carotenoids are as yet poorly understood. Here, we employ femtosecond transient absorption spectroscopy to reveal energy transfer pathways in the peridinin-chlorophyll-a-protein (PCP) complex containing the highly substituted carotenoid peridinin, which includes an intramolecular charge transfer (ICT) state in its excited state manifold. Extending the transient absorption spectra toward near-infrared region (600-1800 nm) allowed us to separate contributions from different low-lying excited states of peridinin. The results demonstrate a special light-harvesting strategy in the PCP complex that uses the ICT state of peridinin to enhance energy transfer efficiency.
The two-photon fluorescence excitation spectrum of LH2, measured by directly exciting the carotenoid S1 transition and monitoring fluorescence from the bacteriochlorophyll, provides the first direct experimental verification of carotenoid S1 to bacteriochlorophyll energy transfer. It also provides an estimate of the in situ spheroidene S1 transition energy of 13 900±150 cm−1, slightly red-shifted from solution estimates. The relative abilities of the carotenoids spheroidene (10 conjugated double bonds) and rhodopin glucoside (11 conjugated double bonds) to transfer energy via their S1 states are discussed in terms of spectral overlap factors and competing processes such as S1→S0 internal conversion.
The spectroscopic properties of peridinin in solution, and the efficiency and dynamics of energy transfer from peridinin to chlorophyll a in the peridinin−chlorophyll−protein (PCP) from Amphidinium carterae, were studied by steady-state absorption, fluorescence, fluorescence excitation, and fast transient optical spectroscopy. Steady-state measurements of singlet energy transfer from peridinin to chlorophyll revealed an 88 ± 2% efficiency. Fast-transient absorption experiments showed that the excited S1 state of peridinin decayed in 13.4 ± 0.6 ps in methanol and 3.1 ± 0.4 ps in the PCP complex after direct excitation of the carotenoid. The onset of the bleaching of the chlorophyll absorption band at 672 nm, signifying the arrival of the excitation from the carotenoid, occurred in 3.2 ± 0.3 ps. These data show that the primary route of energy transfer from peridinin to chlorophyll in the PCP complex is through the S1 state of peridinin. Nanosecond time-resolved transient optical spectroscopy revealed that chlorophyll triplet states are efficiently quenched by peridinin whose triplet state subsequently decays with a lifetime of 10 ± 1 μs in the PCP complex. Close association between the peridinins and chlorophylls, which is clearly evident in the 3-D structure of the PCP complex, along with proper alignment of pigments and energy state matching are responsible for the high efficiencies of the photochemical processes.
We present theoretical confirmation of an intramolecular charge-transfer (CT) state in peridinin in agreement with experimental results of Frank and co-workers (J. Phys. Chem. B 1999, 103, 8751 and J. Phys. Chem. B 2000, 104, 4569). Quantum chemical calculations using time-dependent density functional theory under the Tamm−Dancoff approximation were made on two structures: fully optimized peridinin and a molecule from the crystal structure of peridinin−chlorophyll−protein. The CT state appears as the third and second excited singlet state, respectively, for the two structures. A dipole-in-a-sphere model is used to estimate the solvation stabilization energies of each state in a variety of solvents. The energy of the CT state is shown to decrease dramatically in solvents of increasing polarity while the energy of the dark S1 (Ag--like) state remains comparatively constant.
The dynamics of the excited states of the carotenoid peridinin in polar solvents were studied using femtosecond transient absorption spectroscopy in the spectral range of 500−1900 nm. A broadening of the absorption spectrum in polar solvents is caused by a distribution of conformers having different ground-state properties. In addition, the dependence of the peridinin lifetime on the excitation wavelength reveals that two peridinin forms coexist in protic solvents, where a “red”-absorbing form is produced by hydrogen bonding via the carbonyl group. The observed dynamics show that the S1 and intramolecular charge transfer (ICT) states of peridinin are strongly coupled, forming a collective S1/ICT state whose lifetime is determined by the degree of ICT character. In nonpolar solvent, pure S1 character with a lifetime of 160 ps is observed, whereas in polar solvents an increase in the ICT character leads to a lifetime as short as 10 ps in methanol and 13 ps in ethylene glycol. In protic solvents, the ICT character of the S1/ICT state of the red peridinin form is further enhanced by hydrogen bonding, resulting in lifetimes shorter than 6 ps. A weak dependence of peridinin dynamics on viscosity shows that the ICT state is not formed via a twisted ICT mechanism. At 190 K in methanol, a significant increase in the S1/ICT lifetime is observed, suggesting that thermal coupling is involved in the S1/ICT state mixing. At 77 K in ethylene glycol glass, a multiexponential decay is revealed, indicating the presence of several conformers with different S1/ICT state properties.
Peridinin chlorophyll a-protein (PCP) is a unique light harvesting protein found in dinoflagellates, that contains a large amount of the carotenoid peridinin. Carotenoids have unusual spectroscopic properties due to their approximate C2h symmetry, which makes transitions from their ground states to their S1 (S2) states one-photon forbidden (allowed). To gain information about one-photon forbidden electronic states in peridinin, fluorescence excitation spectra were measured after two-photon excitation for peridinin in benzene and in the PCP. The samples were excited using 920−1320 nm light. Fluorescence of the isolated peridinin S1 state was then measured at 750 nm. In PCP, the excited peridinin transfers energy to chlorophyll whose fluorescence was monitored at 670 nm. Surprisingly, two-photon absorption was observed in both the peridinin S1 and S2 regions, with the spectrum slightly red-shifted in the protein sample. The peridinin S1 energy was found to be higher than that of typical light harvesting carotenoids, making its S1 state very close in energy to its S2 state. We suggest that peridinin's polar groups, symmetry breaking, and possible mixing of electronic states lead to two-photon character of the normally one-photon allowed S0−S2 transition.
Spectroscopic properties as well as excited state dynamics of the carotenoid peridinin in several solvents of different polarities were investigated by time-resolved fluorescence and transient absorption techniques. A strong dependence of the peridinin lowest excited states dynamics on solvent polarity was observed after excitation into the strongly allowed S2 state. Peridinin relaxes to the ground state within 10 ps in the strongly polar solvent methanol, while in the nonpolar solvent n-hexane a 160 ps lifetime was observed, thus confirming the previous observations revealed by transient absorption spectroscopy in the visible region (Bautista, J. A.; et al. J. Phys. Chem. B 1999, 103, 8751). In addition, the solvent dependence in the near-IR region is demonstrated by a strong negative feature in the transient absorption spectrum of peridinin in methanol, which is not present in n-hexane. This band, characterized by a 1 ps rise time, is ascribed to stimulated emission from an intramolecular charge-transfer (ICT) state. Time-resolved fluorescence data support assignment of this band to the emissive singlet state, whose dynamic characteristics depend strongly on the dielectric strength of the medium. On the basis of all our time-resolved measurements combined with simulations of the observed kinetics, we propose the following model: the initially populated S2 state decays to the S1 state within less than 100 fs for both solvents. Then, the population is transferred from the S1 state to the S0 and ICT states. The S1 → ICT transfer is controlled by a solvent polarity dependent barrier. In n-hexane the barrier is high enough to prevent the S1 → ICT transfer and only S1 → S0 relaxation characterized by a time constant of 160 ps is observed. An increase of solvent polarity leads to a significant decrease of the barrier, enabling a direct quenching of the S1 state by means of the S1 → ICT transfer, which is characterized by a time constant of 148 ps for tetrahydrofuran, 81 ps for 2-propanol, and 11 ps for the most polar solvent methanol. The ICT state is then rapidly depopulated to the ground state. This relaxation also exhibits solvent dependence, having a time constant of 1 ps in methanol, 2.5 ps in 2-propanol, and 3.5 ps in tetrahydrofuran.
The spectroscopic properties and dynamics of the lowest excited singlet states of peridinin, fucoxanthin, neoxanthin, uriolide acetate, spheroidene, and spheroidenone in several different solvents have been studied by steady-state absorption and fast-transient optical spectroscopic techniques. Peridinin, fucoxanthin, uriolide acetate, and spheroidenone, which contain carbonyl functional groups in conjugation with the carbon-carbon pi-electron system, display broader absorption spectral features and are affected more by the solvent environment than neoxanthin and spheroidene, which do not contain carbonyl functional groups. The possible sources of the spectral broadening are explored by examining the absorption spectra at 77 K in glassy solvents. Also, carotenoids which contain carbonyls have complex transient absorption spectra and show a pronounced dependence of the excited singlet state lifetime on the solvent environment. It is postulated that these effects are related to the presence of an intramolecular charge transfer state strongly coupled to the S-1 (2(1)A(g)) excited singlet state. Structural variations in the series of carotenoids studied here make it possible to focus on the general molecular features that control the spectroscopic and dynamic properties of carotenoids.
The spectroscopic properties and dynamic behavior of peridinin in several different solvents were studied by steady-state absorption, fluorescence, and transient optical spectroscopy. The lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in the nonpolar solvents cyclohexane and benzene. The lifetimes show no obvious correlation with solvent polarizability, and hydrogen bonding of the solvent molecules to peridinin is not an important factor in determining the dynamic behavior of the lowest excited singlet state. The wavelengths of emission maxima, the quantum yields of fluorescence, and the transient absorption spectra are also affected by the solvent environment. A model consistent with the data and supported by preliminary semiempirical calculations invokes the presence of a charge transfer state in the excited state manifold of peridinin to account for the observations. The charge transfer state most probably results from the presence of the lactone ring in the pi-electron conjugation of peridinin analogous to previous findings on aminocoumarins and related compounds. The behavior of peridinin reported here is highly unusual for carotenoids, which generally show little dependence of the spectral properties and lifetimes of the lowest excited singlet state on the solvent environment.
Common fluorescence properties of carotenoids functioning as an efficient antenna in algal pigment systems are elucidated in relation to their molecular structure. Those carotenoids contain eight conjugated double bonds and one keto group associated with the double bond. The origin of the fluorescence is the optically forbidden S1 state and its radiative lifetime is longer than that of carotenoids without a keto group. These characteristics are discussed in relation to the excited state of polyenes.
An energy transfer pathway in a carotenoid-chlorophyll a protein complex of dinoflagellates was studied by the femtosecond up-conversion method. The energy levels of the S2 state of peridinin and their lifetime were essentially identical in methanol and in the complex. The S1 lifetime of peridinin in solvents was more than 30-fold longer than in the complex. These results account for an observed transfer efficiency (higher than 85%) and indicate that an energy transfer occurs between the S1 states of peridinin and chlorophyll a after a rapid internal conversion. This pathway is unique in photosynthetic organisms.
Trimers of light harvesting complex II from Arabidopsis thaliana were studied by femtosecond fluorescence upconversion. The average lifetime of the carotenoid S2 state was ∼57 fs for wild type trimers and ∼70 fs for trimers from a mutant plant with a distinctly different carotenoid composition. We estimate that ∼56% of the energy transferred from carotenoids to chlorophylls proceeds via the carotenoid S2 state in the wild type and ∼46% in the mutant. By comparison with the fluorescence excitation spectra, we find that ∼20% of the energy transferred in both samples proceeds through the carotenoid S1 state.
Optical control of excited states of α-perylene crystal was realized by a femtosecond optimized pulse shaping method using Genetic Algorithm (GA). We succeeded in controlling the emission spectral feature of an α-perylene crystal; the intensity of E-emission was increased by a factor of 1.4 without the change of Y-emission intensity. Furthermore, we found a near-infrared pulse shape whose multi-photon excitation efficiency is larger than that of a single femtosecond pulse by a factor of two. On the auto-correlation traces of these shaped pulses, the several satellite peaks appeared beside the main peak. The origin and mechanism of the attained change were discussed.
1. The peridinin.chlorophyll a.protein complex from Amphidinium carterae (Plymouth 450) shows spectroscopic characteristic (absorption, CD, fluorescence polarization, lifetime and energy transfer) essentially identical with peridinin.chlorophyll a.protein complexes from Glenodinium sp., Gonyaulax polyedra and Amphidinium rhyncocephaleum. 2. The apoprotein of peridinin.chlorophyll a.protein complexes is globular, with an isotropic rotational relaxation time (e.g. 33 ns for the A. caterae peridinin.chlorophyll a.protein), as deduced from the dynamic depolarization data. 3. The chromophores (4 peridinins and 1 chlorophyll a for peridinin.chlorophyll a.protein complexes from Glenodinium sp., G. polyedra and A. rhyncocephaleum and 9 and 2, respectively, for peridinin.chlorophyll a.protein of A. carterae) are accommodated in a hydrophobic crevice and not exposed to the solvent. The surface of the protein is highly hydrophilic. 4. No evidence for chlorophyll-chlorophyll interactions in the A. carterae peridinin.chlorophyll a.protein was obtained. This implies that binding crevices for two chlorophylls and half of peridinins (four to five) are located at some distance from each other. 5. The peridinin.chlorophyll a.protein complexes function as the photosynthetic antenna pigment. In addition, peridinins effectively protect chlorophyll a from photodecomposition.
Time-resolved excited-state absorption intensities after direct two-photon excitation of the carotenoid S(1) state are reported for light-harvesting complexes of purple bacteria. Direct excitation of the carotenoid S(1) state enables the measurement of subsequent dynamics on a fs time scale without interference from higher excited states, such as the optically allowed S(2) state or the recently discovered dark state situated between S(1) and S(2). The lifetimes of the carotenoid S(1) states in the B800-B850 complex and B800-B820 complex of Rhodopseudomonas acidophila are 7+/-0.5 ps and 6+/-0.5 ps, respectively, and in the light-harvesting complex 2 of Rhodobacter sphaeroides approximately 1.9+/-0.5 ps. These results explain the differences in the carotenoid to bacteriochlorophyll energy transfer efficiency after S(2) excitation. In Rps. acidophila the carotenoid S(1) to bacteriochlorophyll energy transfer is found to be quite inefficient (phi(ET1) <28%) whereas in Rb. sphaeroides this energy transfer is very efficient (phi(ET1) approximately 80%). The results are rationalized by calculations of the ensemble averaged time constants. We find that the Car S(1) --> B800 electronic energy transfer (EET) pathway ( approximately 85%) dominates over Car S(1) --> B850 EET ( approximately 15%) in Rb. sphaeroides, whereas in Rps. acidophila the Car S(1) --> B850 EET ( approximately 60%) is more efficient than the Car S(1) --> B800 EET ( approximately 40%). The individual electronic couplings for the Car S(1) --> BChl energy transfer are estimated to be approximately 5-26 cm(-1). A major contribution to the difference between the energy transfer efficiencies can be explained by different Car S(1) energy gaps in the two species.
Peridinin-chlorophyll-protein (PCP) is a unique light-harvesting protein that uses carotenoids as its primary light-absorbers. This paper theoretically investigates excitation transfer between carotenoids and chlorophylls in PCP of the dinoflagellate Amphidinium carterae. Calculations based on a description of the electronic states of the participating chromophores and on the atomic level structure of PCP seek to identify the mechanism and pathways of singlet excitation flow. After light absorption the optically allowed states of peridinins share their electronic excitation in excitonic fashion, but are not coupled strongly to chlorophyll residues in PCP. Instead, a gateway to chlorophyll Q(y) excitations is furnished through a low-lying optically forbidden excited state, populated through internal conversion. Carbonyl group and non-hydrogen side groups of peridinin are instrumental in achieving the respective coupling to chlorophyll. Triplet excitation transfer to peridinins, mediated by electron exchange, is found to efficiently protect chlorophylls against photo-oxidation.
The peridinin chlorophyll-a protein (PCP) of dinoflagellates differs from the well-studied light-harvesting complexes of purple bacteria and green plants in its large (4:1) carotenoid to chlorophyll ratio and the unusual properties of its primary pigment, the carotenoid peridinin. We utilized ultrafast polarized transient absorption spectroscopy to examine the flow of energy in PCP after initial excitation into the strongly allowed peridinin S2 state. Global and target analysis of the isotropic and anisotropic decays reveals that significant excitation (25-50%) is transferred to chlorophyll-a directly from the peridinin S2 state. Because of overlapping positive and negative features, this pathway was unseen in earlier single-wavelength experiments. In addition, the anisotropy remains constant and high in the peridinin population, indicating that energy transfer from peridinin to peridinin represents a minor or negligible pathway. The carotenoids are also coupled directly to chlorophyll-a via a low-lying singlet state S1 or the recently identified SCT. We model this energy transfer time scale as 2.3 +/- 0.2 ps, driven by a coupling of approximately 47 cm(-1). This coupling strength allows us to estimate that the peridinin S1/SCT donor state transition moment is approximately 3 D.