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Secondary nucleation that leads to chiral symmetry breaking in stirred crystallization
Abstract
We have recently reported the breaking of chiral symmetry in stirred crystallization. One of the most important aspect of the mechanism that leads to the symmetry breaking is the phenomenon of secondary nucleation. The empirical expressions currently used for the rates of secondary nucleation are not based on a clear physical model. We present here some experimental observations on NaClO3 crystallization that gives us a physical model to develop an expression for the rates. Such expressions are important for a quantitative and statistical understanding of the phenomenon of chiral symmetry breaking in stirred crystallization.
... Crystallization of NaClO 3 from a static solution results in equal population of two enantiomorphs [2]: chirally symmetric state. However, it has been reported that perturbations on solution during crystallization causes deviation from the chirally symmetric state [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Various experiments of crystallization with the perturbations have indicated that the chiral symmetry breaking is accomplished during the early stages of NaClO 3 crystallization. ...
... Various experiments of crystallization with the perturbations have indicated that the chiral symmetry breaking is accomplished during the early stages of NaClO 3 crystallization. As the cause of amplification of one-single chirality, for instance, secondary nucleation [3][4][5][6][7][8][9][10][11][12][13], chiral recognition of subcritical chiral cluster [14][15][16] have been proposed. However, the processes that lead to the chiral symmetry breaking still remain ambiguous because the most of crystallization experiments of chiral symmetry breaking has exclusively focused on the crystals that are not growing. ...
... Many crystallization experiments of chiral symmetry breaking are explained by secondary nucleation model [3][4][5][6][7][8][9][10][11][12][13]. The scenario of the model is as follows: (1) a one-single "mother" crystal of Phase I appears at primary nucleation, (2) many "daughter" crystals, which possess the same chirality as the "mother" crystal, are generated from the "mother" crystal though secondary nucleation, and (3) the generation of the "daughter" crystals reduces a concentration of solution, suppressing the generation of the opposite enentiomorph. ...
The solubility of the metastable achiral monoclinic phase in NaClO3 crystallization from an aqueous solution, which appears prior to the nucleation of chiral crystals, was successfully measured in the range from 10 °C to 23 °C. Antisolvent crystallization method was used to obtain metastable crystals for the measurement. The solubility was determined to be about 1.6 times higher than that of the chiral stable cubic phase by observing growth or dissolution of the crystal in aqueous solution at the temperature and concentration of which is predetermined.
... [13] NaClO 3 is a particularly interesting compound, for which spontaneous deracemization has been already observed during stirred crystallization in the pioneering work by Kondepudi. [17][18][19][20] It crystallizes in the chiral space group P2 1 3, which can easily be analysed by optical rotation. [21,22] It turns into an achiral solute upon dissolution, [23] i. e. it shows intrinsically the effect of "chiral amnesia". ...
... During grinding in the presence of glass beads, crystals are fragmented by collisions and shear-forces into smaller crystals of the same handedness. [2,20,54,77,81,82] The system is driven out of equilibrium by an influx of mechanical energy; there is a continuous circulating flux in the reaction loop. The smaller crystals quickly dissolve, [80] while collisions generate secondary nuclei playing the role of seed crystals. ...
Attrition‐enhanced chiral symmetry breaking in crystals, known as Viedma deracemization, is a promising method for converting racemic solid phases into enantiomerically pure ones under non‐equilibrium conditions. However, many aspects of this process remain unclear. In this study, we present a new investigation into Viedma deracemization using a comprehensive kinetic rate equation continuous model based on classical primary nucleation theory, crystal growth, and Ostwald ripening. Our approach employs a fully microreversible kinetic scheme with a size‐dependent solubility following the Gibbs–Thomson rule. To validate our model, we use data from a real NaClO3 deracemization experiment. After parametrization, the model shows spontaneous mirror symmetry breaking (SMSB) under grinding. Additionally, we identify a bifurcation scenario with a lower and upper limit of the grinding intensity that leads to deracemization, including a minimum deracemization time within this window. Furthermore, this model uncovers that SMSB is caused by multiple instances of concealed high‐order autocatalysis. Our findings provide new insights into attrition‐enhanced deracemization and its potential applications in chiral molecule synthesis and understanding biological homochirality.
... Kondepudi et al. [46,47] who observed the exact same phenomenon for NaClO 3 , using vigorous magnetic stirring to induce secondary nucleation. In a follow-up study, Chen et al. [41] used sequential seeding of single NaClO 3 crystals of both chiralities in a supersaturated solution (100 mL) under the presence of ultrasound (bath 40 kHz and 50 W nominal electrical power). ...
... In primary nucleation, crystals of both chiralities will be formed stochastically for these compounds, whereas in secondary nucleation, the parent nucleus or crystal will transfer its chirality to new nuclei. That way, any initial chiral asymmetry (formed stochastically) will be amplified autocatalytically by secondary nucleation, as shown in the work of Kondepudi at al. [46,47]. The fact that this effect is also observed in the presence of ultrasound, as shown in the studies discussed here, could support a secondary nucleation-dominated mechanism for sonocrystallization processes in general. ...
Industrial synthesis of enantiopure compounds is nowadays heavily based on the separation of racemates through crystallization processes. Although the application of ultrasound in solution crystallization processes (sonocrystallization) has become a promising emerging technology, offering several benefits (e.g. reduction of the induction time and narrowing of the metastable zone width, control over the product size, shape and polymorphic modification), little attention has been paid so far to the effects of ultrasound on chiral crystallization processes. Several recent studies have reported on the application of acoustic energy to crystallization processes that separate enantiomers, ranging from classical (diastereomeric) resolution and preferential crystallization to new and emerging processes such as attrition-enhanced deracemization (Viedma ripening). A variety of interesting effects have been observed, which include among others, enhanced crystallization yield with higher enantiomeric purity crystals, spontaneous mirror symmetry breaking crystallization, formation of metastable conglomerate crystals and enhanced deracemization rates. The objective of this review is to provide an overview of the effects of ultrasound on chiral crystallization and outline several aspects of interest in this emerging field.
... According to most above studies, it was suggested that the secondary nucleation was the main mechanism in the CSB and supersaturation was the most controlling factor of the CSB mechanism. 5,13 Meanwhile, a few studies suggest a different mechanism for the CSB. 18 In Viedma's study, he obtained complete CSB even in high supersaturation (above ∼1.5) with massive instantaneous primary nucleation. ...
... Besides spontaneous nucleation, seed particles may induce nucleation in two manners: secondary nucleation, which in the sense of chiral crystals would be chirality-preserving nucleation on top of a seed particle, as also suggested for the chiral NaClO 3 system, 24,25 or more generally, heterogeneous nucleation, which would be a non-chirality-preserving, surface-catalyzed effect. Evidence for the existence of chirality-preserving secondary nucleation is found by comparing seeding L-NC seeds into D-TA containing synthesis at the beginning and end of the induction period (Figures 2 and 3). ...
Nucleation of crystals as well as their growth is difficult to study experimentally. We have recently demonstrated that chiral Eu3+‐doped terbium phosphate nanocrystals are an interesting system for studying nanocrystal formation mechanisms and chiral symmetry breaking, occurring during their formation, directed by chiral ligands, such as tartaric acid. In this paper, we show how simultaneous, in situ monitoring of both total emission intensity and circularly polarized luminescence magnitude and sign versus time during nanocrystal formation provides considerable information on the mechanisms of nanocrystal nucleation and growth. Specifically, we show that the presence of tartaric acid leads to the formation of chiral prenucleation clusters, which deterministically transform into nanocrystals of a specific handedness. Additionally, we demonstrate that both unseeded and seeded nanocrystal syntheses behave differently mechanistically and that the addition of seed nanocrystals catalyses both enantio‐specific (also called secondary nucleation) as well as nonspecific nucleation. In situ circularly polarized and total luminescence were measured during the synthesis of chiral terbium phosphate nanocrystals and revealed many details about their nucleation and growth mechanisms.
... A considerable number of theoretical and experimental attempts were performed to annotate the creation of ee and the proliferation of such imbalance to yield an enantiopure system. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] The first experimental elucidations of Frank's concept were reported by Kondepudi and Sabanayagam 18 and Soai et al. 19 in the context of asymmetric crystals and molecules, respectively. Soai et al. ...
We present a theoretical model to study the origin of chiral symmetry breaking of a racemic mixture of optically active biomolecules. We consider a collection of Brownian particles, which can stay in any of the three possible isomeric states: one achiral and two enantiomers. Isomers are undergoing self-regulatory reaction along with chiral inhibition and achiral decay processes. The reaction rates of the isomeric states are guided by their neighbors as well as the thermal fluctuations of the system. We find that an alteration in the relative dominance of self-regulation, chiral inhibition, and achiral decay processes breaks the chiral symmetry of the system, which is either partial or complete. This results in four different asymmetric population states, viz., three-isomer coexistence, enantiomeric coexistence, chiral–achiral coexistence, and homochiral state. A change in the reaction condition induces nonequilibrium transition among these states. We also report that a fast stochastic self-regulation and a slow chiral inhibition and achiral decay process along with a threshold population of interacting neighbors suffice for the requisite for transition toward a completely symmetry broken state, i.e., homochirality.
... Namely, it is considered that nucleation of chiral mother crystal influence the production of generations of secundary nucleated crystals of the same handedness. [7][8][9][10][11][12][13][14][15][16][17] In the paper [18] it was shown that chiral symmetry breaking occurs in primary nucleation, and that secondary nucleation only increase it. Due to that, process of chiral symmetry breaking cannot be explained by the nucleation of mother crystal, i.e. mechanisms which induce domination of one enantiomer group are still unclear. ...
... Fragmentation was further confirmed by performing experiments at different stirring/shear rates, with the fastest growth observed at the highest rate ( Supplementary Fig. 7). Others have shown that stirring can lead to fragmentation 15 or increased secondary nucleation 16 . ...
Supramolecular polymers, such as microtubules, operate under non-equilibrium conditions to drive crucial functions in cells, such as motility, division and organelle transport¹. In vivo and in vitro size oscillations of individual microtubules2,3 (dynamic instabilities) and collective oscillations⁴ have been observed. In addition, dynamic spatial structures, like waves and polygons, can form in non-stirred systems⁵. Here we describe an artificial supramolecular polymer made of a perylene diimide derivative that displays oscillations, travelling fronts and centimetre-scale self-organized patterns when pushed far from equilibrium by chemical fuels. Oscillations arise from a positive feedback due to nucleation–elongation–fragmentation, and a negative feedback due to size-dependent depolymerization. Travelling fronts and patterns form due to self-assembly induced density differences that cause system-wide convection. In our system, the species responsible for the nonlinear dynamics and those that self-assemble are one and the same. In contrast, other reported oscillating assemblies formed by vesicles⁶, micelles⁷ or particles⁸ rely on the combination of a known chemical oscillator and a stimuli-responsive system, either by communication through the solvent (for example, by changing pH7–9), or by anchoring one of the species covalently (for example, a Belousov–Zhabotinsky catalyst6,10). The design of self-oscillating supramolecular polymers and large-scale dissipative structures brings us closer to the creation of more life-like materials¹¹ that respond to external stimuli similarly to living cells, or to creating artificial autonomous chemical robots¹². © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
... The initial imbalance may be caused by statistical fluctuations, the parity violation energy difference, the influence of circularly polarized light or selective adsorption on chiral mineral surfaces. 1 In conglomerate crystal systems, this initial imbalance can be amplified through, for example, the eutectic model, 2 Kondepudi's model 3,4 and more recently, Viedma ripening. 5 Generating chirality in the absence of chiral reagents (i.e. ...
1,2-bis(N-benzoyl-N-methylamino)benzene (2) forms centrosymmetric hydrate crystals (2•xH2O) but non-centrosymmetric anhydrous crystals. Dehydration of this hydrate (30 min at 140 °C) resulted in the formation of chiral crystals (i.e. a physical racemate of the conglomerate crystals) as verified using solid-state circular dichroism and powder X-ray diffraction. Subsequent attrition-enhanced deracemization, also known as Viedma ripening, was used to obtain homochiral crystals of 2 within 5 h.
... If a substance crystallizes as an equal mixture of dextro and levo crystals when unstirred, its chiral symmetry can be disrupted by stirring (Kondepudi et al., 1990;McBride & Carter, 1991). This phenomenon has been attributed to the effect of stirring on secondary nucleation (Kondepuddi & Sabanayagam, 1994). ...
... Breakage of chiral symmetry has been discovered by Kondepudi and co-workers in their studies of sodium chlorate crystallization from stirred solution (Kondepudi et al., 1990). The phenomenon has been investigated both experimentally and theoretically (McBride & Carter 1991; Kondepudi et al., 1995 a,b; Kondepudi et al., 1993; Kondepudi et al., a,1995 Kondepudi & Sabanayagam, 1994; Qian & Botsaris, 1998, 2004). At present, efforts are made to establish the experimental conditions under which the phenomenon occurs, to determine all important parameters controlling its kinetics and mechanisms, and to develop a reliable theory (Veintemillas-Verdaguer et al, 2007; Viedma 2004; Cartwright et al., 2004; Martin et al. , 1996; Metcalfe & Ottino, 1994; Bushe et al., 2000). ...
... Despite serious effort, however, only the first of these mechanisms has ever been reported to produce any net chirality in the laboratory, and even then the enantiomeric excess never exceeds a few percent under even the most extreme experimental conditions (e.g., Bailey, 2000;Keszthelyi, 2001;Podlech, 2001;Wang and Liang, 2001). And while beaker-scale syntheses have been reported that employ vigorous stirring (e.g., Kondepudi and Sabanayagam, 1994) to generate net chirality (apparently by crushing and mixing the first nucleating crystal and thus ensuring that all subsequent crystals nucleate with the same handedness), either enantiomer is equally likely to be produced in any given repeat of the experiment, and thus this effect is extremely unlikely to generate net chirality on a planetary scale (reviewed by Podlech, 2001). That said, a minor (2-9%), presumably abiologically produced enantiomeric excess has been observed in an astronomical context (e.g., Pizzarello and Cronin, 2000), though the mechanisms underlying these minor excesses have not been identified. ...
We propose a terahertz (far-infrared) circular dichroism-based life-detection technology that may provide a universal and unequivocal spectroscopic signature of living systems regardless of their genesis. We argue that, irrespective of the specifics of their chemistry, all life forms will employ well-structured, chiral, stereochemically pure macromolecules (>500 atoms) as the catalysts with which they perform their metabolic and replicative functions. We also argue that nearly all such macromolecules will absorb strongly at terahertz frequencies and exhibit significant circular dichroism, and that this circular dichroism unambiguously distinguishes biological from abiological materials. Lastly, we describe several approaches to the fabrication of a terahertz circular dichroism spectrometer and provide preliminary experimental indications of their feasibility. Because terahertz circular dichroism signals arise from the molecular machinery necessary to carry out life's metabolic and genetic processes, this life-detection method differs fundamentally from more well-established approaches based on the detection of isotopic fractionation, "signature" carbon compounds, disequilibria, or other by-products of metabolism. Moreover, terahertz circular dichroism spectroscopy detects this machinery in a manner that makes few, if any, assumptions as to its chemical nature or the processes that it performs.
Obtaining enantiopure chemicals is a major challenge in the chemical industry, in particular for the agrochemical and pharmaceutical industries. In this thesis, crystallization of chiral systems have been studied and two approaches are presented to cover various types of chirality and their features. In chapter two, we propose a reassessed phase diagram of an atropisomer that, despite having been reported as a conglomerate forming system, presents a stable racemic compound. The emergence of the heterochiral phase is rationalized by structural considerations. Moreover, the effect of the appearance of a more stable racemic compound when using a conglomerate for resolution purposes is discussed. In chapter three, we show that a chiral sulfoxide, forming a conglomerate system, and the corresponding nonchiral sulfone exhibit an isostructural crystal packing. Consistently, the existence of complete solid solutions between the prochiral sulfone and the enantiomers of sulfoxide have been demonstrated. The sulfone was deracemized by Viedma ripening and the sulfoxide was resolved by “classical” AS3PC mode of preferential crystallization. Moreover, a variation of the AS3PC mode, in which control over the chirality is induced by seeding with enantiopure crystals of a related compound, is proposed. Experiments demonstrate its feasibility and illustrate the possibility of a continuum between supramolecular and intrinsic chirality.
We have found that large chiral symmetry breaking in chiral crystallization can be achieved by irradiating several mW focused laser to a plasmonic nanolattice immersed in a stagnant NaClO3 saturated aqueous solution. Several hundreds of chiral crystals with the same handedness showed up in the solution after the laser irradiation in contrast to spontaneous crystallization. In-situ microscopic observation for the early stage of the crystallization in the vicinity of the focal spot revealed that microbubble generation followed by large supersaturation increment, in which supersaturation reaches to 360%, promotes several numbers of crystal nucleation in the vicinity of the bubble as “mother” crystal. The generation of the microbubble induced Marangoni convection, the velocity of which reaches to several hundreds of micrometer per second, crushing the first appeared chiral crystal into pieces by microfluidic shear. Namely, secondary nucleation caused by microfluidic shear amplified the number of “daughter” crystals with the same handedness. This spatiotemporally controllable micro-mixing experiment realized by laser irradiation gives us not only novel route bridging a light and chiral symmetry breaking but also the novel method to observe the early stage dynamics of the secondary nucleation, which was hard to observe by conventional observation technique, in real time.
Mueller matrix polarimetry distinguishes the different origins of the reversible and irreversible chiroptical effects emerging in stirred solutions of J-aggregate nanoparticles: the reversible effect is due to an anisotropic ordering in the solution and the irreversible one is due to a bias from the racemic composition of intrinsically chiral structures.
Several asymmetries in nature that might have led to excess production of a pair of enantiomers have been suggested, but they could produce only a very small enantiomeric excess. There are two conditions that must exist for a small initial enantiomeric excess to survive and increase. There must be a source of free energy that keeps the system far from equilibrium. There must also be some mechanism that amplifies the initial enantiomeric excess. Also, there are several mechanisms that can cause enantiomeric excess to increase in a system that is initially racemic or achiral. None of these can increase the enantiomeric excess to unity, and some of them produce only a transient increase that decays to a steady state having zero or very small enantiomeric excess. Mutual destruction is capable of producing enantiomeric excess that is close to unity, but mutual destruction causes the steady state concentrations of both S and R to be small. One of the most effective mechanisms for the production of enantiomeric excess close to unity is the random formation of a single homochiral oligomer molecule that catalyzes the production of its own monomers. Two or more mechanisms acting together are more effective than any one of them alone. Even a small or transient enantiomeric excess can lead to a large, permanent one because the chiral species can undergo favorable mutations, and the more abundant enantiomers will have greater probability of mutating. The relative importance of the several amplification mechanisms depends on the values of the rate constants. These values can be determined by experiments for specific reactions.
Glycine is the simplest achiral amino acid that undergoes spontaneous mirror symmetry breaking when it crystallizes in its chiral γ-polymorphic modification. As a result of Ostwald ripening, its racemic mixture stochastically becomes optically active. The sense of the resulting handedness can be controlled by addition of one enantiomer of a simple chiral amino acid, the alanine.
The Soai reaction and the Viedma deracemization of racemic conglomerate crystal mixtures are experimental pieces of evidence of the ability of enantioselective autocatalytic coupled networks to yield absolute asymmetric synthesis. Thermodynamically open systems or systems with non-uniform energy distributions may lead to chiral final states and, in systems able to come into thermodynamic equilibrium with their surroundings, to kinetically controlled absolute asymmetric synthesis. The understanding of network parameters and of the thermodynamic scenarios that may lead to spontaneous mirror symmetry breaking (SMSB) could assist in the development of new methods for asymmetric synthesis and enantioselective polymerizations (e.g., replicators), and to frame reasonable speculations on the origin of biological homochirality.
Chiral symmetry breaking during the chiral crystallization from a sodium chlorate (NaClO3) aqueous solution is an intriguing phenomenon because it provides insights into the prebiotic process of biohomochirality. However, a mechanism of the emergence and amplification of chirality remains controversial, especially for crystallization from highly supersaturated solution, and one of the hypotheses proposed before is a transition toward the homochiral state during the early stages of crystallization. In this contribution, we directly examined the early stage of crystallization by in situ polarized-light microscopy. The observation revealed that achiral crystals, which appear prior to the formation of chiral crystals, transform to the chiral crystal through two kinds of polymorphic transformations: (1) martensitic transformation (MT) and (2) solution-mediated phase transition (SMPT). The SMPT is remarkably facilitated by contact with a chiral crystal. Notably, the resulting enantiomorph through contact-facilitated SMPT is strongly directed by the contacting enantiomorph. In contrast, the MT yields two enantiomorphs in equal probability. The emergence and amplification of chirality has generally been considered to be a result of direct nucleation of a chiral crystal and its fragmentation. In contrast, our observations provide a possibility that the MT and contact-facilitated SMPT play a role for the emergence and amplification of chirality, respectively.
Chiral symmetry breaking in NaClO3 crystallization from an aqueous solution with perturbations has been of great interest. To understand the mechanism, several models focusing on the early stage of the crystallization have been proposed. However, they are ambiguous because the early stage has been barely explored directly. Here, we investigate the early stages of the crystallization process driven by droplet evaporation using a combination of direct in situ microscopic observations and cryogenic single-crystal XRD experiments. We demonstrate that an achiral crystal having P21/a symmetry, which is newly discovered for a solution growth, first appears in the droplet and then transforms into the chiral crystals. Additionally, determination of the lattice constants by XRD experiments (a = 8.42 Å, b = 5.26 Å, c = 6.70 Å, β = 109.71°) revealed that the achiral phase should be identical to Phase III (a = 8.78 Å, b = 5.17 Å, c = 6.83 Å, β = 110°), which is a high-temperature phase from a melt growth of NaClO3. We advocate further assessment of the achiral crystal and a new pathway for the formation of chiral crystals via crystalline phase transition from achiral Phase III.
A stochastic version of Mason’s modification of the Frank chiral amplification model is examined. The initial state of the system is chosen to be strictly racemic. Nevertheless, the system always evolves to a monochiral terminal state. Two characteristic times are considered: the separation time t0 – the moment after which the sign of the enantiomeric excess remains unchanged, and the parting time tp – the moment when the enantiomeric excess reaches 1% of its final value. Whereas tp depends (in average) linearly on the logarithm of the magnitude ξ of the fluctuations of the respective rate constants, the separation times are (in average) independent of ξ.
Parity violation represents an essential property of particle and atomic handedness used to cope with the complex phenomenon of asymmetry in the universe. At the molecular level, however, numerous experiments suggest that parity-violating energy differences have not determined the amplification and propagation of homochirality. Asymmetric transformations conducted under far-from-equilibrium conditions reveal the existence of non-linear autocatalysis which is stochastic in nature. In any event and, globally considered, chirality appears as a unifying characteristic of our visible environment with evolutionary implications, thereby suggesting areas for productive research.
Statistics of nucleation of chiral forms of sodium bromate from unstirred aqueous solutions was studied. It was established that bimodal, trimodal and unimodal distributions of enantiomers are obtained in unstirred crystallization. It was also found out that probabilities of the creation of L or D crystals and racemates R, as well as the presence of D, L, and R peaks in distributions depend on crystallizer size, supersaturation and temperature. Nucleation at low supersaturations in small, closed crystallizers leads to the formation of pure enantiomers, and to bimodal distributions with D and L peaks at any temperature. At high supersaturations in large, open crystallizers the formation of racemates and unimodal distributions with racemate R peaks results. In open crystallizers at the lowest temperatures and at the highest temperatures used in crystallization from aqueous solution racemates of sodium bromate are preferentially formed, but in a wide range of intermediate-temperatures apart from racemates, pure enantiomers are efficiently formed which leads to trimodal distributions. The spontaneous formation of pure enantiomers in crystallization from unstirred, unseeded solutions is caused by the chiral symmetry breaking phenomenon, the same as that discovered in stirred crystallization. The conservation of chiral symmetry is, in unstirred crystallization, one of the two possibilities, and the other one is the breakage of symmetry. Both of them occur in nature.
Statistics of nucleation of chiral forms was studied to establish the effect of the number of first crystals and their handedness on distributions of enantiomers. Various bimodal, trimodal and unimodal distributions are obtained in unstirred crystallization, depending on the number of initial crystals and growth conditions. The binomial distribution satisfactorily describes experimental distributions of enantiomeric excess and may be used to predict distributions and probabilities of nucleation of enantiomers. The first nucleated crystals determine the handedness of secondary crystals, and number of initial crystals governs statistics of chiral nucleation. According to the binomial distribution if single crystals nucleate as the first, the bimodal distributions result with D and L peaks. If LD, LL, and DD pairs are nucleated as first, trimodal distributions with D, R, and L peaks are created, and if groups of crystals of various handedness nucleate as the first the unimodal distributions of enantiomeric excess with racemate R peaks are formed. Chiral nucleation experiments on sodium bromate were the basis for the theoretical considerations and verifications of predictions resulting from binomial distributions on probabilities of the creation of L and D crystals, and racemates, and the presence of D, L, and R peaks in the distributions. Growth conditions affect the number of the first crystals and effectiveness of cloning, and as a result, the distributions of enantiomers. Formation of pure enantiomers and/or racemates proves that the conservation of chiral symmetry, and the breakage of chiral symmetry can occur in unstirred crystallization.
Chiral crystals of sodium chlorate and sodium bromate induced highly enantioselective organic synthesis in combination with asymmetric autocatalysis. (S)-5-Pyrimidyl alkanols with up to 98% ee were obtained in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehydes in the presence of d-sodium chlorate. On the other hand, (R)-5-pyrimidyl alkanols with up to 98% ee were obtained for the corresponding reaction in the presence of l-sodium chlorate. Chiral crystals of d- and l-sodium bromate were also found to work as chiral initiators.
Parity violation represents an essential property of particle and atomic handedness used to cope with the complex phenomenon of asymmetry in the universe. At the molecular level, however, numerous experiments suggest that parity-violating energy differences have not determined the amplification and propagation of homochirality. Asymmetric transformations conducted under far-from-equilibrium conditions reveal the existence of non-linear autocatalysis which is stochastic in nature. In any event and, globally considered, chirality appears as a unifying characteristic of our visible environment with evolutionary implications, thereby suggesting areas for productive research.
Annelated silicon ring systems have received extensive investigation as interesting targets that provide access to physical and chemical properties that are difficult to achieve with the corresponding carbon systems. The aim of this article is to describe the results of our investigation on the chemical and physical properties of the ladder oligosilanes and the ladder oligogermanes, such as bicyclo[2.2.0]hexasilane, tricyclo[4.2.0.02,5]octasilane, tetracyclo[4.4.0.02,5.07,10]decasilane, pentacyclo[6.4.0.02,7.03,6.09,12]dodecasilane, bicyclo[2.2.0]hexagermane, and tricyclo[4.2.0.02,5]octagermane systems. Such compounds can be prepared by the reductive coupling of the appropriate polychlorosilanes or polychlorogermanes with lithium or magnesium. The X-ray structural analyses of these compounds provide intriguing information about the stereochemistry of the multifused ring systems; each cyclotetrasilane ring or cyclotetragermane ring is not planar but folded, leading to a helical structure of the ladder frameworks of the anti type. The ladder oligosilanes and oligogermanes show absorption in the UV–vis region and have considerably low oxidation potentials. The following examples of the reactions of the ladder compounds are described: the ring-opening chlorination, mono- and polyoxidation, and reduction with alkali metals leading to stable radical ions. In addition, chiral symmetry breaking in the anti-tricyclo[4.2.0.02,5]octasilane is described. For the reactions of ladder oligosilanes, the comparison with octasilacubanes is also discussed.
Homochirality is an essential feature of biology, but how it developed in early life remains unclear. Our aim in this paper is to add to the discussion by taking a somewhat arbitrary but definite sequence of events and examining carefully the chirality at each stage. Our scenario for the development of life starts with the prebiotic, continues through a pre-RNA stage to the RNA world. This leads to the development of proteins and then to the incorporation of DNA in RNA–DNA–protein biochemistry.
We argue that homochirality probably did not develop prebiotically. We see the likely chiral bifurcation point to be during the development of RNA from an achiral pre-RNA. Surprisingly, we find the driving force for this to be enantiomeric cross-inhibition in which the addition of a ‘wrong-handed’ enantiomer to a growing polymer brings the polymerization to a halt. This, it is often argued, is a serious impediment to the development of chiral purity. We suggest that the sign of handedness at this stage was probably determined by chance. We then point out that homochiral RNA was unlikely by itself to lead to homochiral proteins. We identify the additional nonlinear feature required for bifurcation to be the rapid increase in enzymic power available when the amino acids in proteins approach single handedness and structural features such as α-helices and β-sheets become viable. If this is correct, then the handedness of the protein sector is indeed linked to that of RNA. However, the relative sign of the two handednesses will have depended on the precise stereo-sensitive interaction between the RNA and protein systems. We suggest that the most plausible scenario is via the recently discovered RNA binding sites for amino acids, which are both stereo-selective and have contact with the developing genetic code. The detailed steps that determine the handedness are not yet clear and may be specific to the precise development path.
It follows that if biochemistry similar to that on Earth developed extraterrestrially, we would surely have homochirality but at this stage we cannot be sure about the handedness of either the nucleic acid or the protein components.
Symmetry breaking processes in both equilibrium and nonequilibrium systems are associated with critical parameters. In the general theory of symmetry breaking transitions, critical parameters have an important role. In this article, we present our study of rpm as a critical parameter in chiral symmetry breaking crystallization. The random distribution of crystal enantiomeric excess is obtained for various rpm. The results show how the probability distribution initially spreads and then develops two peaks in a way similar to that observed in a second-order symmetry breaking phase transition. It is also shown that the stochastic kinetic equations that we had proposed for this process could reproduce the experimentally observed relation between crystal enantiomeric excess and rpm.
The crystallization of NaClO3 from supersaturated boiling solutions leads to a strong bias of enantiomorphic crystals of the same chiral sign, which in the range of the experimental errors cannot be distinguished from that of a homochiral crystal mixture. The crystallization reactor is a closed system but with a temperature gradient between the walls of the reactor and the air/liquid interface that entails an intense recycling of the subcritical nuclei formed during the induction period of the primary nucleation in the bulk. During this period, the evolution of the population of subcritical nuclei takes place without any other noticeable crystal growth process. In these experimental conditions, the fast evolution of a myriad of supercritical nuclei and the immediate separation of the crystals strongly suggest that the evolution toward homochirality should occur during the primary nucleation process. The formation of stationary chiral compositions in the closed system by recycling through the irreversible step of the primary nucleation is discussed in relation to the chiral recognition in the primary nucleation process and the role of the heterogenity created by the phase transition and the nonuniform temperature distribution.
Creation of enantiomorphous forms in sodium chlorate and enantiomorphous nature of surface micromorphology were studied by optical microscopy. It was shown that when the small number of crystals nucleates spontaneously in unstirred, unseeded solutions then exclusively left- (L) or right-handed (D) crystals can be formed. At the large number of nucleated crystals the probability of the creation of L and D form is nearly the same. L and D crystals can be distinguished by the shape of growth or dissolution centers.
Links-rechts-Asymmetrie ist in der Natur allgegenwärtig. Neueren Studien zufolge können sich Kristalloberflächen, an die D- oder L-Aminosäuren gebunden sind, in ihrer Energie und ihrer Wachstumsgeschwindigkeit unterscheiden, wobei für das Anknüpfen der Aminosäuren die stereochemische Übereinstimmung entscheidend ist. Auch Oligomerisierungen von Aminosäuren scheinen chiroselektiv zu sein, sodass das Wachstum von Sequenzen definierter Händigkeit möglich wird.[[Zur Definition des Begriffs chiroselektive Selbstorganisation siehe: M. Bolli, R. Micura, A. Eschenmoser, Chem. Biol.1997, 4, 309–320.]] Diese Befunde liefern zusammen mit Resultaten zur Symmetriebrechung sowie zur weiteren Verstärkung der Asymmetrie auf supramolekularem Niveau neue Hinweise auf den Ursprung der Homochiralität in Lebewesen.
Morphologically chiral crystals of sodium chlorate induce highly enantioselective synthesis. Thus the addition of diisopropylzinc to 2-(tert- butylethynyl)pyrimidine-5-carbaldehyde in the presence of d- and l-NaClO3 gives the (S)- and (R)-alkanol, respectively, in high yields and with high ee values.
Chiral asymmetry generation, the predominant production of one enantiomer in a non-chiral environment, could occur in the production of the chiral complex cis-[CoBr(NH3)(en)2]Br2 by the reaction of [Co(H2O)2{(OH)2Co(en)2}2](SO4)2 with ammonium bromide in an aqueous medium. The main kinetic steps in the reaction system have been determined. During the reaction, the product crystallizes at an early stage. When a very small amount of crystalline enantiomer was added to the reaction system at an early stage, the same enantiomer was produced preferentially; in addition, the enantiomeric excess of the product increased with increasing the stirring rate. Thus, it seems that each enantiomer generates chiral crystals that could self-replicate through secondary nucleation when the solution is stirred; these crystals in turn enhance the production of the same enantiomer. With a computer code that simulates such a kinetic mechanism, it is shown that enantiomeric excess observed in the experiments could be reproduced. Chirality 10:343–348, 1998. © 1998 Wiley-Liss, Inc.
Optically active cis-[CoBr(NH3)(en)2]Br2 could be readily obtained by the reaction of optically inactive [Co(H2O)2{(OH)2Co(en)2}2](SO4)2 with ammonium bromide in a water suspension. The optically active cis-[CoBr(NH3)(en)2]Br2 was not readily racemized in its aqueous solution, indicating it not to be produced by total spontaneous resolution but by asymmetric synthesis. This synthesis was exclusively observed when the weight ratio of[Co(H2O)2{(OH)2Co(en)2}2](SO4)2 to water was above 0.31. Since the crystal of each enantiometer of cis-[CoBr(NH3)(en)2]Br2 introduced into the reaction system lead to the preferential production of the same enantiomer, chiral autocatalysis was shown to be performed during the reaction. In the concentrated suspension, crystallization of cis-[CoBr(NH3)(en)2]Br2 that was produced by the reaction would be started during the relatively early stage of the reaction. If several crystals to be initially produced are enantiomerically unequivalent by statistical fluctuation, it may cause the spontaneous generation of the enantiomeric predominance.
The effect of the stirring rate on the crystallization of sodium chlorate was studied in more than 200 experiments using a set-up that allows to perform sets of 20 simultaneous experiments. The crystallization conditions were those that according to previous results lead to only one of both L and D enantiomorphic forms under stirring. The probability of D crystallization was determined using a statistical analysis based on the central limit theorem. The stirring dependence of induction times and crystal size distribution is reported. The stirring effect was correlated with differences between surface and bulk crystallization. The probability of D crystallization was detected to be slightly higher than that of the L, in contradiction with some previous reports, and that might be attributed to the chiral contamination of the bio-organic world on the starting materials and on the system. Our analysis allows to detect that the D-enantiomorph induction by chiral contamination occurs mainly in the nucleation at the interface air/water, which can be related to the expected higher concentration of the contaminants (bioorganic chiral amphiphiles e.g. DNA or proteins) on the surface of the solution.
We describe results of computer simulation calculations of mixed and homochiral oligoribotides showing that it is possible to bind L(D)-monomers at the terminal positions of chains built from D(L)-ribotides, but insertion at internal positions produces a large instability of the chains. Coupled with a proposed new mechanism for the growth of these linear polymers in prebiotic conditions, our results imply that enantiomeric cross-inhibition could have been bypassed to make all-D(L) oligoribotides dominant. We also show that racemic amino acids can form stable peptide P-strands, which rules out a similar origin for modern L-amino acids. Alternative schemes are proposed to account for the homochiralities of the amino acids and of other biomolecules.
Chiral crystallization of optically inactive (achiral) compounds has not been a commonly known phenomenon in organic chemistry. However, large numbers of achiral compounds such as benzophenone, phenol, phenanthrene, etc. are known to crystallize into chiral crystals from their solutions. This review deals with the general aspects of this phenomenon: what is chiral crystallization, how to find organic compounds undergoing chiral crystallization, the preparation of chiral crystals from achiral organic compounds, and the generation of chirality.
The recent progress in the theoretical investigation of the symmetry breaking (the existence of a stable state of a system, in which the symmetry is lower than the symmetry of the system itself) for classical and quantum fluids is reviewed. The emphasis is on the conditions which cause symmetry breaking in the density distribution for one component fluids and binary mixtures confined in a closed nanoslit between identical solid walls. The existing studies have revealed that two kinds of symmetry breaking can occur in such systems. First, a one-dimensional symmetry breaking occurs only in the direction normal to the walls as a fluid density profile asymmetric with respect of the middle of the slit and uniform in any direction parallel to the walls. Second, a two-dimensional symmetry breaking occurs in the fluid density distribution which is nonuniform in one of the directions parallel to the walls and asymmetrical in the direction normal to the walls. It manifests through liquid bumps and bridges in the fluid density distribution. For one component fluids, conditions of existence of symmetry breaking are provided in terms of the average fluid density, strength of fluid-solid interactions, distance at which the solid wall generates a hard core repulsion, and temperature. In the case of binary mixtures, the occurrence of symmetry breaking also depends on the composition of the confined mixtures.
The solution of the Klemm model for spontaneous chiral stereoselection has been obtained. It is shown that a system whose time-evolution is described by the Klemm model, independently of the initial value of the enantiomeric excess, always reaches a racemic terminal state. In a system described by the Klemm model bistability never occurs.
When chemical reactions whose rate increases with the concentration of a product species are carried out in imperfectly mixed systems, a variety of complex behaviours can occur. These phenomena, which have relevance for biological processes as well, include chaotic and stochastic behaviour and selection of one final state over an equally probable alternative.
The effects of mechanical stirring on nucleation and chiral symmetry breaking have been investigated for a simple inorganic molecule, sodium chlorate (NaClO3). In contrast to earlier findings, our experiment suggests that the symmetry breaking may have little to do with hydrodynamic convection. Rather the effect can be reasonably accounted for by mechanical damage to incipient crystals. The catastrophic events, creating numerous small 'secondary' crystals, produce statistical domination of one chiral species over the other. Our conclusion is supported by a number of observations using different mixing mechanisms.
The chirality amplification mechanism proposed by Yamagata in 1966, relying on an Accumulation Principle which involved the parity violating energy difference (1 + epsilon) presumed to be operative at each step in the formation of a homochiral biopolymer, is briefly surveyed historically. The Accumulation Principle is then examined analytically and found to be incapable of producing a unique homochiral polymer in any realistic polymerization process. The extension of the Accumulation Principle to crystallizations which afford enantiomorphic crystals is also scrutinized and found to be misapplied and invalid.
Historically, parity violation at the contemporary biomolecular level (i.e., only L-amino acids in proteins and D-sugars in DNA and RNA) has been postulated to be the inevitable result of parity violations at the elementary particle level, involving either beta-decay electrons or parity violating energy differences (PVEDs) between enantiomers. These two chiral biases have in turn allegedly impressed a small but persistent chirality onto prebiotic chemistry which, after appropriate amplification, has culminated in our contemporary homochiral biopolymers. Experiments and controversies pertaining to the efficacy of these two chiral biases are reviewed briefly, with the conclusions that: a) there is no experimental evidence supporting the capability of beta-decay electrons or other spin-polarized chiral particles to generate chiral molecules, and b) only theoretical calculations, but no experimental evidence, support the allegation of a causal relation between PVEDs and biomolecular homochirality. We here attempt to examine the latter allegation experimentally. Spontaneous resolution under racemization conditions (SRURC) during the crystallization of the bromofluoro-1,4-benzodiazepinooxazole derivative I is capable of affording products of high enantiomeric purity. This process, which involves very efficient stereoselective autocatalysis, has now been examined statistically. If PVED effects are operative, the SRURC of racemic I should provide, either exclusively or with a strong and consistent bias, only one enantiomer of crystalline I. However, crystallization experiments of racemic I showed no bias in its SRURC, leading to the conclusion that PVED effects are ineffective in dictating a preferred chirality in this system. Several earlier experiments in the literature leading to a similar conclusion as to the inefficacy of PVED effects in promoting a preferred chirality are noted.
Sodium chlorate is an achiral molecule that crystallizes from water in the chiral space group P2(1)3. In the absence of chiral perturbations, a random distribution of (+) and (-) crystals is obtained. Kondepudi(2) has shown that constantly stirring an evaporating NaClO(3) solution gives mostly either (+) or (-) crystals. Repeating this experiment many times gives equal numbers of (+) and (-) sets of crystals. Herein we report that when evaporating aqueous NaClO(3) is subjected to beta particles from an Sr-90 source, an asymmetric distribution of (+) and (-) crystals favoring the (+) crystals is obtained. The beta particles are energetic polarized electrons that are approximately 80% of left-handed helicity. By a poorly understood mechanism, the spin polarized electrons produce chiral nucleating sites that favor formation of the (+)-NaClO(3) crystals. Exposure of the evaporating solution instead to energetic positrons from an Na-22 source yields mainly (-)-NaClO(3) crystals. Polarized positrons are of predominantly right-handed helicity. One may conclude that the chirality of the radiation is correlated with the chirality of the crystals being generated.
Left-right asymmetry is ubiquitous in nature. Recent studies reveal changes in the energy and growth rate of crystal surfaces to which D or L amino acids bind, with the binding itself being dictated by stereochemical matching. Likewise, oligomerization of amino acids appears to be a chiroselective process that enables the propagation of sequences with defined handedness.[[For a definition of chiroselective self-assembly, see: M. Bolli, R. Micura, A. Eschenmoser, Chem. Biol. 1997, 4, 309-320.]] These results, along with related findings on symmetry breaking and further amplification of asymmetry at a supramolecular level, constitute new insights into the origin of homochirality in living species.
Discusses how handedness at one level may give rise to handedness at another. Presents examples from plants and animals, molecules, atoms, to elementary particles. Examines the chiral symmetry in life and when it starts. (YP)
The minimum seed crystal size or the minimum size, usually defined as the critical size greater than which seed crystals can produce secondary nuclei, was measured for potassium alum secondary nucleation using a seeded stirred-vessel crystallizer. The value was lowered with increasing stirrer speed both for stainless steel and acrylic resin impellers. The former impeller gave a smaller minimum size than the latter. The minimum size was newly interpreted as a measure of difficulty of nucleation, by defining it as the seed size at which the nucleation rate, being increased as the seed crystals grow, reached a certain detectable level. The behavior of the experimental results and literature data (effects of stirrer speed, supersaturation, impeller materials, impurities and system size) for laboratory crystallizers were reasonably but qualitatively explained in terms of secondary nucleation caused by crystal-impeller collisions. It was concluded that nucleation data obtained in a laboratory, using a small crystallizer, would be difficult to apply to industrial crystallizers because crystal-impeller collisions are unlikely to occur in large-scale industrial systems.
Solid samples (powders and crystals ) of optically active sodium chlorate, bromate, periodate trihydrate, uranylacetate, of benzil, and of nickel sulphate hexahydrate have been used in attempts to secure selective absorption or reaction with several racemic compounds, such as tris(pentane-2,4-dionato)cobalt(III) and α-lipoic acid. No genuine optical rotation has been observed. The origin of the spurious rotations reported in the literature in such systems( and readily reproduced here) is discussed.
The nucleation behavior of seeded sodium chlorate solutions was studied experimentally. Because sodium chlorate crystallizes in two easily distinguished enantiomorphic forms, the origin of the secondary nuclei (whether the seed crystal or the solution) could be easily identified.This work shows that secondary nucleation does not involve a singular mechanism but that different kinds of secondary nucleation can take place under different conditions of supersaturation, liquid velocity, and impurity concentration.It was found that under certain conditions the resulting nuclei were all of the same enantiomorphic form as the seed. It was then concluded that these nuclei resulted from the growth and detachment of surface irregularities such as dendrites from the seed crystal.In two other ranges of conditions, however, secondary nucleation produced both left and right handed crystals. In one case it was shown that the nucleation could have taken place in an impurity concentration gradient resulting from the rapid incorporation of dissolved impurities in the growing seed crystal. To explain the second case, a different mechanism (largely speculative) was proposed. This mechanism hypothesized that secondary nucleation could occur because of changes in the liquid layer surrounding the crystal. These changes in the liquid layer could be the result of an ordering of the solvent water molecules at the crystal-solution interface.
New insights into the mechanism of crystal nucleation emerge from a study by Kondepudi et al., on the crystallization of NaClO3 from a stirred solution. Rectangular prisms of the chiral space group P2(1)3 precipitate from aqueous solutions of the achiral ions. If the solution is not stirred, crystals of both enantiomorphic forms precipitate in equal amounts exactly as was reported almost one hundred years ago. However, when the NaClO3 solution is stirred, then almost exclusively only right-handed or only left-handed crystals are formed. There is a simple explanation for this extremely surprising and obviously reproducible phenomenon.
The parity-violating weak neutral current perturbation of the ground-state electronic energy has been calculated by ab initio methods for glycine over a range of chiral conformations, for L-alanine, L-alpha-aminopropionitrile, and for the peptide residue of polypeptides in the alpha-helix and the beta-sheet conformation. It is found that L-alanine in its preferred conformation in aqueous solution and the L-peptides in the alpha-helix and the beta-sheet conformation, have a lower ground-state energy than the corresponding D-enantiomers, because of the electroweak interaction. The enantiomer energy difference is small, of the order of 10-14 J mol-1, corresponding to an enantiomeric excess of 10^6 molecules of L-alanine or the L-peptide in one mole of the corresponding racemic mixture in thermodynamic equilibrium at ambient temperature. The significance of the energy difference between enantiomers arising from the electroweak interaction for the transition from racemic geochemistry to homochiral biochemistry in terrestrial evolution is discussed.
Contact nucleation experiments on the (100) face of KH2PO4 crystals have been performed using an advanced microscopic technique so that the contacting event and the results of the contact- secondary nucleation, disturbance of the step pattern and damage to the mother crystal-could be directly observed. We find that there is a critical impact energy Ec above which the mother crystal is damaged and pitting occurs on the crystal face. Below Ec the step pattern is disturbed by the impact, this is probably due to disturbance of the volume diffusion field. Secondary nuclei are formed above and below Ec but above Ec the nucleation rate is at least one order of magnitude larger than below Ec This indicates that there are different nucleation mechanisms for secondary nucleation. Crystal faces with many high macrosteps (bunches) are easier to damage than faces with shallow steps. This result could explain the dependence of the nucleation rate on supersaturation, equilibrium temperature and impurity concentration.
A new general and efficient method for kinetic resolution of racemic conglomerates by crystallization in the presence of "tailor-made" additives is described. The process is explained in terms of stereoselective adsorption of the resolved additive at the surface of the growing crystals of the enantiomer of the same absolute configuration, resulting in a drastic decrease in their rate of growth and thus allowing preferential crystallization of the opposite enantiomer ("rule of reversal"). Some empirical resolutions reported in the literature are rationalized through this mechanism, and appropriate additives for the resolution of new systems are designed and successfully applied. The crystallization of the conglomerates (R,S)-glutamic acid hydrochloride (Glu·HCl), (R,S)-threonine (Thr), (R,S)-(p-hydroxyphenyl)glycine p-toluenesulfonate (pHpgpTs), and (R,S)-asparagine hydrate (Asn·H2O) in the presence of other amino acids, used as additives, has been studied in particular. It is demonstrated that the additives are occluded in the bulk of the homochiral crystal in typical amounts of 0.5-1.5%, while they are not found in the bulk of the crystals of the antipode. The possible role of the additives in nucleation and dissolution of the affected crystals is considered. A new method for the assignment of absolute configuration of chiral molecules is proposed.
Results of our study of the kinetics of stirred crystallization that produces large (greater than 99%) asymmetry are presented. The change of concentration with time as the crystallization progresses is obtained for stirred and unstirred crystallizations. A sharp difference in the two concentration vs time curves, due to secondary nucleation in the stirred system, can be observed. Experimental results are compared with the predictions of a set of stochastic kinetic equations. It is found that the proposed kinetics can quite accurately reproduce the experimental data if it is assumed that secondary nuclei are produced in stirred systems only when the parent crystal reaches a minimum size.
It seems that statements and ideas to the effect that optically inactive starting materials in a symmetrical environment will always produce optically inactive products must at least be modified to incorporate a probability factor. Unless it is desired to establish that the crystallization of an enantiomeric crystal mixture is not influenced by trace impurities, a large number of individual crystallizations of a racemic mixture would not be carried out. In a few trials, rather than an exact 50-50 distribution of enantiomers in the product, it is very probable that an excess of one or the other prevails. In terms of common laboratory practice involving a few crystallizations of a racemic material, chance conditions alone will give rise to at least slight excess of a single dissymmetric form. Whether this may be increased by further crystallization or whether the slight excess may be noted by optical rotation measurements depends on the pecularities of the compound and on the care and persistence of the experimenter. It is clear that as a mechanism to account for the genesis of one sided optically active materials of nature, spontaneous resolutions seem very inefficient (1, 4, 6). Certainly as a means of practical laboratory resolutions the spontaneous crystallization method can apply only in rare cases. However, implications that dissymmetric material or forces must be present in order to generate other dissymmetric material are certainly invalid.
Aqueous solutions of the ionic compound NaClO3 are not optically active. These solutions can be made to supersaturate and precipitate chiral crystals of NaClO3. The undisturbed solution yields statistically equal numbers of l- and d-crystals. Stirring the solution causes breakage of chiral symmetry and very high enantiomeric excesses can be obtained. A computer program has been developed which closely simulates this phenomenon. A kinetic mechanism for the origin of chiral asymmetry in this system is discussed in relation to the current experimental and simulation data.
Optically active organic compounds origin on primordial earth, emphasizing role of asymmetric catalyst
Secondary nucleation of potassium dihydrogen phosphate (KDP) is studied from electrical conductometry and turbidimetry experiments. Different possible mechanisms are investigated: initial breeding is observed even in the absence of stirring; production of nuclei by collisions with the reactor walls (contact nucleation) is studied in a liquid medium in which KDP is not soluble so that effects other than hydrodynamical/mechanical phenomena are eliminated. Similarities between secondary and heterogenous nucleation could indicate the existence of a mechanism of surface nucleation in addition to the other two effects. Turbidimetry is particularly useful for the quantitative study of the dynamics of secondary nucleation: in this work, it is applied to the study of the inhibition of nuclei production by dissolved aluminium ions. An estimation of the size of the secondary nuclei is calculated from turbidity data.
Hydrodynamic models of secondary nucleus generation in mechanically agitated suspension crystallizers are analysed in detail and their predictions compared with experimental measurements using potassium sulphate crystals. The data indicate that, under operating conditions similar to those found in industrial crystallizers, secondary nuclei are produced by a particle attrition process consistent with a turbulent fluid-induced breeding mechanism having critical eddies in the viscous dissipation subrange of the turbulent energy spectrum.
This review approaches crystallizer performance from a reaction engineering viewpoint in which emphasis is placed on the interaction of crystallizer fluid mechanics and residence time distributions with the physical chemistry of nucleaton and growth kinetics to produce the crystal size distribution within a crystallizer. The present state of knowledge in these various areas is assessed, particular attention being paid to work carried out over the past decade. Great advances have been made during this period but significant gaps in our understanding still exist, particularly in the areas of crystallizer fluid mechanics (especially the role of micromixing), primary nucleation, habit modification, comparison of predicted crystallizer performance with actual operating characteristics, and the control of full scale crystallizers.
Classical mechanisms proposed for the transition from racemic geochemistry to homochiral biochemistry in terrestrial evolution generally ascribe to chance the particular handed choice of the L-amino acids and the D-sugars by self-replicating systems. The parity-violating weak neutral current interaction gives rise to an energy difference between a chiral molecule and its mirror-image isomer, resulting in a small stabilization of the L-amino acids and the L-peptides in the alpha-helix and the beta-sheet conformation relative to the corresponding enantiomer. The energy difference suffices to break the chiral symmetry of autocatalytic racemic reaction sequences in an open non-equilibrium system.
Some theoretical considerations and experimental results, which are rather difficulty accessible in the literature2, have been reported. These demonstrate the possibility of the spontaneous generation of optically active material starting from an inactive, closed system without interference of any directing, dissymmetric agency.RésuméL'auteur admet la possibilité d'une génération spontanée de matière optiquement active dans un système fermé, en l'absence de tout agent asymétrique. Quelques considerations théoriques ainsi que des expériences qui démontrent cette possibilité ont été rapportées2.ZusammenfassungEs wird über einige, weniger bekannt gewordene Überlegungen und Experimente berichtet, die die Möglichkeit geben zur Neuentstehung von optisch aktiver organischer Substanz aus inaktivem Ausgangsmaterial in einem geschlossen System ohne Zuhilfenahme irgendeines asymmetrischen Mittels2.
Sodium chlorate (NaClO3) crystals are optically active although the molecules of the compound are not chiral. When crystallized from an aqueous solution
while the solution is not stirred, statistically equal numbers of levo (L) and dextro (D) NaClO3 crystals were found. When the solution was stirred, however, almost all of the NaClO3 crystals (99.7 percent) in a particular sample had the same chirality, either levo or dextro. This result represents an experimental
demonstration of chiral symmetry breaking or total spontaneous resolution on a macroscopic level brought about by autocatalysis
and competition between L- and D-crystals.
The crystallization of 1, l'—binaphthyl from its racemic melt is an example of spontaneous generation of optical activity.
The distribution of specific rotations in 200 individual samples varied from [α]D = —218 degrees to [α]D = +206 degrees with a mean of +0.14 degree and standard deviation of 86.4 degrees. This resolution into enantiomers is determined
by chance development, with equal probability, of right- or left-handed crystallites; it can be controlled and made stereospecific
by addition of dissymmetric compounds at low concentrations.