Crystallisation Based Deracemisation and Chiral Resolution of Small Molecules

Enantiomeric separation of chiral molecules is pivotal for exploring fundamental questions about life's origin and various other scientific domains. Crystallisation is an important platform for the separation of chiral molecules, elegantly applied to many systems, for instance, the formation of conglomerates, where the enantiomers crystallise as separate phases. Many approaches have been proposed to explore crystallisation-driven enantiomeric separation with fewer insights into the complex pathways associated with the separation processes. Controlling derecemisation or chiral resolution through crystallisation by applying mechanical forces, magnetic substrates, and seeds have attracted significant attention due to better outcomes. Despite these advancements, the mechanisms behind the separation of enantiomers using crystallisation as a tool, especially the delicate balance between kinetically and thermodynamically controlled pathways, are unclear, particularly for small organic molecules critical to pharmaceutical applications. This review aims to address many of these challenges, providing synthetic chemists with a deeper understanding of the role of crystallisation for innovative enantiomeric separation strategies.

On Solute Recovery and Productivity in Chiral Resolution through Solid-State Deracemization by Temperature Cycling

Temperature cycling represents an effective means for the deracemization of chiral compounds that crystallize as conglomerates and racemize in solution. In such a process, a suspension enriched in the desired enantiomer is converted into an enantiopure one through periodic cycles of crystal dissolution and crystal growth. We show that performing temperature cycling at higher temperatures leads to faster deracemization and, consequently, higher productivity. However, this comes at the cost of lower recovery, as the solution contains potentially relevant amounts of solute due to the higher solubility at an elevated temperature. In this work, we introduce and compare two process variants that mitigate this issue. The first involves temperature cycling, followed by linear cooling, whereas the second is based on merging the temperature cycles and cooling crystallization. Experiments carried out with the chiral compound N-(2-methylbenzylidene)-phenylglycine amide show that the former variant is faster than the latter, and it is easier to design and implement. In this process, the choice of an appropriate cooling rate is essential to avoid nucleation of the undesired enantiomer.

Exact and Ubiquitous Condition for Solid-State Deracemization in Vitro and in Nature

Solid-state deracemization is the amplification of an enantiomeric excess in suspensions of conglomerate-forming chiral compounds. Although numerous chemical and biochemical compounds deracemize, its governing mechanism has remained elusive. We introduce a novel formulation of the classical population-based model of deracemization through temperature cycles to prove that suspensions deracemize whenever a simple and ubiquitous condition is met: crystal dissolution must be faster than crystal growth. Such asymmetry is a known principle of crystallization, hence explaining the generality of deracemization. Through both experiments and a theoretical analysis, we demonstrate that this condition applies even for very small temperature cycles and for random temperature fluctuations. These findings establish solid-state deracemization as an attractive route to the manufacture of enantiopure products and as a plausible pathway toward the emergence of homochirality in nature.

Strategies for chiral separation: from racemate to enantiomer

Chiral separation has become a crucial topic for effectively utilizing superfluous racemates synthesized by chemical means and satisfying the growing requirements for producing enantiopure chiral compounds. However, the remarkably close physical and chemical properties of enantiomers present significant obstacles, making it necessary to develop novel enantioseparation methods. This review comprehensively summaries the latest developments in the main enantioseparation methods, including preparative-scale chromatography, enantioselective liquid-liquid extraction, crystallization-based methods for chiral separation, deracemization process coupling racemization and crystallization, porous material method and membrane resolution method, focusing on significant cases involving crystallization, deracemization and membranes. Notably, potential trends and future directions are suggested based on the state-of-art "coupling" strategy, which may greatly reinvigorate the existing individual methods and facilitate the emergence of cross-cutting ideas among researchers from different enantioseparation domains.

A Complex Reaction Network Model for Spontaneous Mirror Symmetry Breaking in Viedma Deracemizations

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.

Investigation of Temperature Cycling with Coupled Vessels for Efficient Deracemization of NMPA

Chiral compounds can exist as pairs of nonsuperimposable stereoisomers (enantiomers) possessing the same physical properties but interacting differently with biological systems. This makes them interesting materials to be explored by the pharmaceutical and food industries. In this study, to obtain pure enantiomers from their conglomerates, a method that involves using a two-vessel system for deracemization of N-(2-methylbenzylidene) phenylglycine amide (NMPA) was developed. In this method, a suspension was transferred with a pulsating pumping profile between two inter-connected stirred vessels that were set at constant temperatures. As the suspension was exposed to more rapid changes in temperature, it resulted in the speeding up of the process and thus enhancing productivity in comparison to a single vessel system. The results confirmed successful deracemization of NMPA. A modified pumping profile and tubing design eliminated the issue of clogging of the transfer tubes and ensured effective suspension transfer for longer durations. Operating parameters, such as initial enantiomeric excess, vessel residence time, and suspension density were also investigated. In this method, optimization of residence time was necessary to enhance the efficiency of the process further. Results confirmed that this methodology has the potential to be more adaptable and scalable as it involved no mechanical attrition.

Conversion of stable crystals to metastable crystals in a solution by periodic change of temperature

Using a Becker-Döring-type model including cluster incorporation, we study the possibility of conversion of stable crystals to metastable crystals in a solution by a periodic change of temperature. At low temperature, both stable and metastable crystals are assumed to grow by coalescence with monomers and corresponding small clusters. At high temperature, a large amount of small clusters produced by the dissolution of crystals inhibits the dissolution of crystals, and the imbalance in the amount of crystals increases. By repeating this process, the periodic temperature change can convert stable crystals into metastable crystals.

Rapid deracemization through solvent cycling: proof-of-concept using a racemizable conglomerate clopidogrel precursor

We demonstrate that a conglomerate-forming clopidogrel precursor undergoing solution phase racemization can be deracemized through cyclic solvent removal and re-addition. We establish that the combination of slow growth and fast dissolution of crystals is ideal for rapid deracemization, which we achieve by repurposing a Soxhlet apparatus to realize the slow removal and fast re-addition of solvent autonomously.

Ultrasound-Directed Symmetry Breaking and Spin Filtering of Supramolecular Assemblies from only Achiral Building Blocks

Herein we describe the self-assembly of an achiral molecule into macroscopic helicity as well as the emergent chiral-selective spin-filtering effect. It was found that a benzene-1,3,5-tricarboxamide (BTA) motif with an aminopyridine group in each arm could coordinate with Ag^I and self-assemble into nanospheres. Upon sonication, symmetry breaking occurred and the nanospheres transferred into helical nanofibers with strong CD signals. Although the sign of the CD signals appeared randomly, it could be controlled by using the as-made chiral assemblies as a seed. Furthermore, it was found that the charge transport of the helical nanofibers was highly selective with a spin-polarization transport of up to 45 %, although the chiral nanofibers are composed exclusively from achiral building blocks. This work demonstrates symmetry breaking under sonication and the chiral-selective spin-filtering effect.

Chiral Amplification through the Interplay of Racemizing Conditions and Asymmetric Crystal Growth

Amplification of enantiomeric excesses (ee) is routinely observed during chiral crystallization of conglomerate crystals for which the enantiomers undergo racemization in solution. Although routes comprising a combination of crystal growth and dissolution are frequently used to obtain enantiopure molecules, crystal growth by itself has rather been considered as a source of enantiomeric erosion and discounted as a potential source of enantiomeric amplification. Counterintuitively, we here demonstrate striking enantiomeric amplification during crystal growth for clopidogrel and tert-leucine precursors. Based on a mechanistic framework, we identify that the interplay between racemization and crystal growth rates elicits this surprising effect. The asymmetric amplification of the solid-phase ee can be enhanced by increasing the mass of grown material relative to the product such that small amounts of seeds of only 60% ee already result in virtually exclusive growth of the majority phase. These results impact our understanding of asymmetric amplification mechanisms during crystallization and offer a tangible basis for practical production of enantiopure molecules.

Mathematical Models of Chiral Symmetry-breaking – A Review of General Theories, and Adiabatic Approximations of the APED System

We review the literature surrounding chiral symmetry-breaking in chemical systems, with a focus on understanding the mathematical models underlying these chemical processes. We comment in particular on the toy model of Sandars, Viedma’s crystal grinding systems and the APED model. We include a few new results based on asymptotic analysis of the APED system.

Possible chemical and physical scenarios towards biological homochirality

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

Combining Viedma Ripening and Temperature Cycling Deracemization

While much data are available for the Viedma ripening and temperature cycling deracemization processes, not much is known about the advantages (or disadvantages) of a combination of the two. We here try to elucidate what happens when Viedma ripening is used in combination with temperature cycling by comparing not only the deracemization times but also the change in the sizes of the crystals. We found that, in the case of NMPA (rac-(2-methylbenzylidene)-phenylglycine amide) as a model compound, combined experiments significantly increase the deracemization time. By tuning the process parameters, it is possible to approach experimental conditions where both Viedma ripening and temperature cycling control the deracemization. Under those conditions, however, the deracemization time is not significantly improved. Following our results, it seems unlikely that a combination of Viedma ripening and temperature cycling would shorten the deracemization time. Nevertheless, these experiments might provide clues for unraveling the mechanism of temperature cycling.

Functional Chirality: From Small Molecules to Supramolecular Assemblies

Many structures in nature look symmetric, but this is not completely accurate, because absolute symmetry is close to death. Chirality (handedness) is one form of living asymmetry. Chirality has been extensively investigated at different levels. Many rules were coined in attempts made for many decades to have control over the selection of handedness that seems to easily occur in nature. It is certain that if good control is realized on chirality, the roads will be ultimately open towards numerous developments in pharmaceutical, technological, and industrial applications. This tutorial review presents a report on chirality from single molecules to supramolecular assemblies. The realized functions are still in their infancy and have been scarcely converted into actual applications. This review provides an overview for starters in the chirality field of research on concepts, common methodologies, and outstanding accomplishments. It starts with an introductory section on the definitions and classifications of chirality at the different levels of molecular complexity, followed by highlighting the importance of chirality in biological systems and the different means of realizing chirality and its inversion in solid and solution-based systems at molecular and supramolecular levels. Chirality-relevant important findings and (bio-)technological applications are also reported accordingly.

Chirality and the Origin of Life

The origin of life, based on the homochirality of biomolecules, is a persistent mystery. Did life begin by using both forms of chirality, and then one of the forms disappeared? Or did the choice of homochirality precede the formation of biomolecules that could ensure replication and information transfer? Is the natural choice of L-amino acids and D-sugars on which life is based deterministic or random? Is the handedness present in/of the Universe from its beginning? The whole biosystem on the Earth, all living creatures are chiral. Many theories try to explain the origin of life and chirality on the Earth: e.g., the panspermia hypothesis, the primordial soup hypothesis, theory of parity violation in weak interactions. Additionally, heavy neutrinos and the impact of the fact that only left-handed particles decay, and even dark matter, all have to be considered.

Mutual Information Disentangles Interactions from Changing Environments

Real-world systems are characterized by complex interactions of their internal degrees of freedom, while living in ever-changing environments whose net effect is to act as additional couplings. Here, we introduce a paradigmatic interacting model in a switching, but unobserved, environment. We show that the limiting properties of the mutual information of the system allow for a disentangling of these two sources of couplings. Further, our approach might stand as a general method to discriminate complex internal interactions from equally complex changing environments.

Spontaneous and Controlled Macroscopic Chiral Symmetry Breaking by Means of Crystallization

In this paper, macroscopic chiral symmetry breaking refers to as the process in which a mixture of enantiomers departs from 50–50 symmetry to favor one chirality, resulting in either a scalemic mixture or a pure enantiomer. In this domain, crystallization offers various possibilities, from the classical Viedma ripening or Temperature Cycle-Induced Deracemization to the famous Kondepudi experiment and then to so-called Preferential Enrichment. These processes, together with some variants, will be depicted in terms of thermodynamic pathways, departure from equilibrium and operating conditions. Influential parameters on the final state will be reviewed as well as the impact of kinetics of the R ⇔ S equilibrium in solution on chiral symmetry breaking. How one can control the outcome of symmetry breaking is examined. Several open questions are detailed and different interpretations are discussed.

Rapid Discrimination of Crystal Handedness by X-ray Natural Circular Dichroism (XNCD) Mapping

An original method for determining the handedness of individual non-centrosymmetric crystals in a mixture using a tightly-focused, circularly polarized X-ray beam is presented. The X-ray natural circular dichroism (XNCD) spectra recorded at the metal K-edge on selected crystals of [Δ-M(en)₃ ](NO₃ )₂ and [Λ-M(en)₃ ](NO₃ )₂ (M=Co^II , Ni^II ) show extrema at the metal pre-edge (7712 eV for Co, 8335 eV for Ni). A mapping of a collection of some 220 crystals was performed at the respective energies by using left and right circular polarizations. The difference in absorption for the two polarizations, being either negative or positive, directly yielded the handedness of the crystal volume probed by the beam. By using this technique, it was found that the addition of l-ascorbic acid during the synthesis of [Co(en)₃ ](NO₃ )₂ resulted in an enantiomeric enrichment of the Λ-isomer of 67±13 %, whereas the Ni analogue was similarly, but conversely, enriched in the Δ-isomer (65±22 %).

On the Effect of Secondary Nucleation on Deracemization through Temperature Cycles

Herein, the pivotal role of secondary nucleation in a crystallization-enhanced deracemization process is reported. During this process, complete and rapid deracemization of chiral conglomerate crystals of an isoindolinone is attained through fast microwave-assisted temperature cycling. A parametric study of the main factors that affect the occurrence of secondary nucleation in this process, namely agitation rate, suspension density, and solute supersaturation, confirms that an enhanced stereoselective secondary nucleation rate maximizes the deracemization rate. Analysis of the system during a single temperature cycle showed that, although stereoselective particle production during the crystallization stage leads to enantiomeric enrichment, undesired kinetic dissolution of smaller particles of the preferred enantiomer occurs during the dissolution step. Therefore, secondary nucleation is crucial for the enhancement of deracemization through temperature cycles and as such should be considered in further design and optimization of this process, as well as in other temperature cycling processes commonly applied in particle engineering.

High-throughput on demand access of single enantiomers by a continuous flow crystallization process

A novel continuous flow reactive crystallization process for the in situ on-demand access of single enantiomer crystals is reported and exemplified for a chiral pharmaceutical intermediate that crystallizes as a racemic conglomerate.

Transition-metal(ii) complexes with a tripodal hexadentate ligand, 1,1,1-tris[2-aza-3-(imidazol-4-yl)prop-2-enyl]ethane, exhibiting incomplete total or absolute spontaneous resolution

Crystal structures and solid-state CD spectra of the compounds, [M(H₃L)]Cl(ClO₄) (M = Mn, Fe, Co, Ni and Zn) were examined.

Supramolecular chiral surface of nickel sulfate hexahydrate crystals and its ability to chirally recognize enantiomers by adsorption data

The chiral recognition ability of the surface of α-NiSO₄·6H₂O was found using gas and liquid chromatography.

Highly Sensitive, Tunable Chirality Amplification through Space Visualized for Gold Nanorods Capped with Axially Chiral Binaphthyl Derivatives

The creation and transmission of chirality in molecular systems is a well-known, widely applied notion. Our understanding of how the chirality of nanomaterials can be controlled, measured, transmitted through space, and applied is less well understood. Dynamic assemblies for chiral sensing or metamaterials engineered from chiral nanomaterials require exact methods to determine transmission and amplification of nanomaterial chirality through space. We report the synthesis of a series of gold nanorods (GNRs) with a constant aspect ratio of ∼4.3 capped with C₂-symmetric, axially chiral binaphthyl thiols, preparation of dispersions in the nematic liquid crystal 5CB, measurements of the helical pitch, and the determination of the helical twisting power as well as the average distance between the chiral nanomaterial additives. By comparison to the neat organic chiral derivatives, we demonstrate how the amplification of chirality facilitated by GNRs decorated with chiral molecules can be used to clearly distinguish the chiral induction strength of a homologous series of binaphthyl derivatives, differing only in the length of the nontethered aliphatic chain, in the induced chiral nematic liquid crystal phase. Considering systematic errors in sample preparation and optical measurements, these chiral molecules would otherwise be deemed identical with respect to chiral induction. Notably, we find some of the highest ever-reported values of the helical twisting power. We further support our experimentally derived arguments of a more comprehensive understanding of chirality transfer by calculations of a suitable pseudoscalar chirality indicator.

Symmetry Breaking in Self-Assembled Nanoassemblies

The origin of biological homochirality, e.g., life selects the L-amino acids and D-sugar as molecular component, still remains a big mystery. It is suggested that mirror symmetry breaking plays an important role. Recent researches show that symmetry breaking can also occur at a supramolecular level, where the non-covalent bond was crucial. In these systems, equal or unequal amount of the enantiomeric nanoassemblies could be formed from achiral molecules. In this paper, we presented a brief overview regarding the symmetry breaking from dispersed system to gels, solids, and at interfaces. Then we discuss the rational manipulation of supramolecular chirality on how to induce and control the homochirality in the self-assembly system. Those physical control methods, such as Viedma ripening, hydrodynamic macro- and micro-vortex, superchiral light, and the combination of these technologies, are specifically discussed. It is hoped that the symmetry breaking at a supramolecular level could provide useful insights into the understanding of natural homochirality and further designing as well as controlling of functional chiral materials.

Chemical Basis of Biological Homochirality during the Abiotic Evolution Stages on Earth

Spontaneous mirror symmetry breaking (SMSB), a phenomenon leading to non-equilibrium stationary states (NESS) that exhibits biases away from the racemic composition is discussed here in the framework of dissipative reaction networks. Such networks may lead to a metastable racemic non-equilibrium stationary state that transforms into one of two degenerate but stable enantiomeric NESSs. In such a bifurcation scenario, the type of the reaction network, as well the boundary conditions, are similar to those characterizing the currently accepted stages of emergence of replicators and autocatalytic systems. Simple asymmetric inductions by physical chiral forces during previous stages of chemical evolution, for example in astrophysical scenarios, must involve unavoidable racemization processes during the time scales associated with the different stages of chemical evolution. However, residual enantiomeric excesses of such asymmetric inductions suffice to drive the SMSB stochastic distribution of chiral signs into a deterministic distribution. According to these features, we propose that a basic model of the chiral machinery of proto-life would emerge during the formation of proto-cell systems by the convergence of the former enantioselective scenarios.

Сhiral and Racemic Fields Concept for Understanding of the Homochirality Origin, Asymmetric Catalysis, Chiral Superstructure Formation from Achiral Molecules, and B-Z DNA Conformational Transition

The four most important and well-studied phenomena of mirror symmetry breaking of molecules were analyzed for the first time in terms of available common features and regularities. Mirror symmetry breaking of the primary origin of biological homochirality requires the involvement of an external chiral inductor (environmental chirality). All reviewed mirror symmetry breaking phenomena were considered from that standpoint. A concept of chiral and racemic fields was highly helpful in this analysis. A chiral gravitational field in combination with a static magnetic field (Earth’s environmental conditions) may be regarded as a hypothetical long-term chiral inductor. Experimental evidences suggest a possible effect of the environmental chiral inductor as a chiral trigger on the mirror symmetry breaking effect. Also, this effect explains a conformational transition of the right-handed double DNA helix to the left-handed double DNA helix (B-Z DNA transition) as possible DNA damage.

The Strecker reaction coupled to Viedma ripening: a simple route to highly hindered enantiomerically pure amino acids

The Strecker reaction is broadly used for the preparation of α-amino acids. However, control of enantioselectivity remains challenging. We here couple the Strecker reaction to Viedma ripening for the absolute asymmetric synthesis of highly sterically hindered α-amino acids. As proof-of-principle, the enantiomerically pure α-amino acids tert-leucine and α-(1-adamantyl)glycine were obtained.

Applications of ultrasound to chiral crystallization, resolution and deracemization

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.

Chiral symmetry breaking due to impeller size in cooling crystallization of sodium chlorate

We report, for the first time, that the size of impellers is the exception to Denk and Botsaris's work where a stirrer of the smallest surface area led to close to 100% deracemization, while the data from the largest impeller agreed with the results of Denk and Botsaris, displaying a racemic mixture.

Intensified deracemization via rapid microwave-assisted temperature cycling

Rapid cooling and microwave heating substantially speed up temperature cycling-enhanced deracemization, while limiting the concomitant side reactions. During fast cooling, secondary nucleation is shown to enable deracemization.

Effect of surface scraping on chiral symmetry in seeded cooling crystallization of sodium chlorate

Product crystal handedness similar to seed decreases with the number of scratches.

Spontaneous mirror symmetry breaking and origin of biological homochirality

Recent reports on both theoretical simulations and on the physical chemistry basis of spontaneous mirror symmetry breaking (SMSB), that is, asymmetric synthesis in the absence of any chiral polarizations other than those arising from the chiral recognition between enantiomers, strongly suggest that the same nonlinear dynamics acting during the crucial stages of abiotic chemical evolution leading to the formation and selection of instructed polymers and replicators, would have led to the homochirality of instructed polymers. We review, in the first instance, which reaction networks lead to the nonlinear kinetics necessary for SMSB, and the thermodynamic features of the systems where this potentiality may be realized. This could aid not only in the understanding of SMSB, but also the design of reliable scenarios in abiotic evolution where biological homochirality could have taken place. Furthermore, when the emergence of biological chirality is assumed to occur during the stages of chemical evolution leading to the selection of polymeric species, one may hypothesize on a tandem track of the decrease of symmetry order towards biological homochirality, and the transition from the simple chemistry of astrophysical scenarios to the complexity of systems chemistry yielding Darwinian evolution.

Solid Phase Deracemization of an Atropisomer

The scope of Viedma ripening and temperature cycling with respect to chiral molecules has remained mostly limited to molecules with a single stereogenic center, while racemization proceeds through inversion at that particular stereocenter. In this article we demonstrate for the first time that atropisomers, chiral rotamers that possess an axis of chirality, can be successfully deracemized in the solid phase by either applying temperature cycling or Viedma ripening.

Nonlinear effects at the soft interface of an emulsion in the List–Lerner–Barbas aldol reaction

“Soft” yet promising: Nonlinear asymmetric induction of the List–Lerner–Barbas aldol reaction observed at the soft interface of an emulsion opens up hitherto unexplored possibilities which were previously restricted to solid–liquid phase equilibria.

The RNA World as a Model System to Study the Origin of Life

Understanding how life arose is a fundamental problem of biology. Much progress has been made by adopting a synthetic and mechanistic perspective on originating life. We present a current view of the biochemistry of the origin of life, focusing on issues surrounding the emergence of an RNA World in which RNA dominated informational and functional roles. There is cause for optimism on this difficult problem: the prebiotic chemical inventory may not have been as nightmarishly complex as previously thought; the catalytic repertoire of ribozymes continues to expand, approaching the goal of self-replicating RNA; encapsulation in protocells provides evolutionary and biophysical advantages. Nevertheless, major issues remain unsolved, such as the origin of a genetic code. Attention to this field is particularly timely given the accelerating discovery and characterization of exoplanets.

Cation-mediated optical resolution and anticancer activity of chiral polyoxometalates built from entirely achiral building blocks

We report the crystallization of homochiral polyoxometalate (POM) macroanions {CoSb6O4(H2O)3[Co(hmta)SbW8O31]3}15- (1, hmta = hexamethylenetetramine) via the counter cation-mediated chiral symmetry breaking and asymmetric autocatalytic processes. In the presence of low Co2+ concentrations both Δ- and Λ-enantiomers of 1 formed in the reaction, crystallizing into the racemic crystal rac-1. At a high Co2+ concentration, the polyoxoanion enantiomers showed a high level of chiral recognition via H-bonding interactions to crystallize into enantiopure crystals of Δ- or Λ-[Co(H2O)6{CoSb6O4(H2O)3[Co(hmta)SbW8O31]3}]13-. During crystallization, a microscale symmetry-breaking event and a nonlinear asymmetric autocatalysis process make the enantiomers crystallize in different batches, which provides an opportunity to isolate the homochiral bulk materials. The defined structures of the racemic and homochiral crystals thus provide a molecular-level illustration that H-bonding interactions are responsible for such high-level chiral recognition, in a process similar to the supramolecular chirality frequently observed in biology. These POM macroanions showed a high cytotoxicity against various cancer cells, particularly ovarian cancer cells. The antitumor activity of these compounds resulted at least in part from the activation of the apoptotic pathways, as shown by the flow cytometry, Annexin V staining, DNA ladder, and TUNEL assay, likely by blocking the cell cycle and complexing with proteins in cells. The POM macroanions reported herein provide promising and novel antitumor agents for the potential treatment of various cancers.

The Life Story of Hydrogen Peroxide III: Chirality and Physical Effects at the Dawn of Life

It is a remarkable observed fact that all life on Earth is homochiral, its biology using exclusively the D-enantiomer of ribose, the sugar moiety of the ribonucleic acids, and the L-enantiomers of the chiral amino acids. Motivated by concurrent work that elaborates further the role of hydrogen peroxide in providing an oscillatory drive for the RNA world (Ball & Brindley 2015a, J. R. Soc. Interface 12, 20150366, and Ball & Brindley 2015b, this journal, in press), we reappraise the structure and physical properties of this small molecule within this context. Hydrogen peroxide is the smallest, simplest molecule to exist as a pair of non-superimposable mirror images, or enantiomers, a fact which leads us to develop the hypothesis that its enantiospecific interactions with ribonucleic acids led to enantioselective outcomes. We propose a mechanism by which these chiral interactions may have led to amplification of D-ribonucleic acids and extinction of L-ribonucleic acids.

Mechanism of chirality conversion by periodic change of temperature: Role of chiral clusters

By grinding crystals in a solution, the chirality of crystal structure (and the molecular chirality for the case of chiral molecules as well) can be converted, and the cause of the phenomenon is attributed to crystal growth with chiral clusters. We show that the recently found chirality conversion with a periodic change of temperature can also be explained by crystal growth with chiral clusters. With the use of a generalized Becker-Döring model, which includes enantio-selective incorporation of small chiral clusters to large solid clusters, the change of cluster distribution and the mass flow between clusters are studied. The chiral clusters act as a reservoir to pump out the minority species to the majority, and the exponential amplification of the enantiomeric excess found in the experiment is reproduced in the numerical calculation.

Viedma ripening: a reliable crystallisation method to reach single chirality

Crystallisation processes have evolved to practical methods that allow isolation of an enantiopure product in high yield. Viedma ripening in particular enables access to enantiopure products in a reliable way, simply through grinding of crystals in a solution. This tutorial review covers the basic principles behind asymmetric crystallisation processes, with an emphasis on Viedma ripening, and shows that to date many novel organic molecules can be obtained in enantiopure solid form.

Calculating anisotropic piezoelectric properties from texture data using the MTEX open source package

This paper presents the background for the calculation of anisotropic piezoelectric properties of single crystals and the graphical display of the results in two or three dimensions, and the calculation of the aggregate properties from constituent crystals and the texture of the aggregate in a coherent manner. The texture data can be obtained from a wide range of sources, including pole figure diffraction and single orientation measurements (electron backscattered diffraction, electron channelling pattern, Laue Pattern, optical microscope universal-stage). We consider the elastic wave propagation in piezoelectric crystals as an example of the interaction of electrical (2nd rank tensor), piezoelectric (3rd rank tensor) and elastic properties (4th rank tensor). In particular, we give explicit formulae for the calculation of the Voigt averaged tensor from individual orientations or from an orientation distribution function. For the latter we consider numerical integration and an approach based on the expansion into spherical harmonics. We illustrate the methods using single crystals, polycrystalline quartz measured using electron channelling patterns and ideal Curie limiting groups applied to quartz aggregates. This paper also serves as a reference paper for the mathematical tensor capabilities of the texture analysis software MTEX.

Chiral symmetry breaking dictated by electric-field-driven shape transitions of nucleating conglomerate domains in a bent-core liquid crystal

Generating and controlling chiral symmetry breaking and enantiomeric excess is not only interesting from a fundamental perspective but can also lead to novel functional materials. In this work, we show how the dark conglomerate (DC) liquid crystalline phase characterized by macroscopic chiral domains offers such a possibility if formed under an electric field. In addition the chiral domains are electro-optically switchable. The chiral segregation in the DC phase can be tuned by using dc or ac fields at different frequencies. Consequently, the enantioselectivity, dielectric parameters and switching polarization in the DC phase become tunable. Another interesting aspect is that the nucleating conglomerate domains formed under ac fields exhibit frequency dependent shape transitions which have a striking resemblance to domain shape changes observed in two-dimensional monolayers. This can therefore be used as a model experimental system to get a physical insight into the effects of chiral and electrostatic interactions, under external fields, on domain growth and interface structures. The domain shape transitions can also be used to investigate the role of growth morphology in coarsening and scaling hypotheses. From a technological point of view this opens up the possibility of obtaining chiral thin films with preferential sense of chirality which can be useful in chiroptical and nonlinear optical applications.