Homochirality In Nature Research Articles

Regioselective and Homochiral Supramolecular Polymerization of Nanotadpole Aggremers of Poly(phenylacetylene) Derivatives.

Biological homochirality is a signature of life. Supramolecular polymerization is effective to achieve high hierarchical homochirality in nature, but has not been well-explored. Herein, we report regioselective and homochiral supramolecular polymerization of chiral nanotadpole aggregates made of either synthetic helical poly(phenylacetylene)s or chirality-amplified co-assembly of chiral and achiral poly(phenylacetylene)s. The twisted nanotadpole aggregates with high screw-sense preference polymerized as monomers (aggremers) into supramolecular chains in a head-to-tail regioselective and stepwise manner. Supramolecular copolymerization of enantiomeric aggremers favored formation of homochiral hierarchical supramolecular structures as visualized by TEM. Chiral hexagonal columnar mesophase of aggremers was responsive for the stereoselectivity. The work opens a gate to controllably and effectively construct functional chiral supramolecular materials and deepens the understanding of hierarchical biological homochirality.

Regioselective and Homochiral Supramolecular Polymerization of Nanotadpole Aggremers of Poly(phenylacetylene) Derivatives

Biological homochirality is a signature of life. Supramolecular polymerization is effective to achieve high hierarchical homochirality in nature, but has not been well‐explored. Herein, we report regioselective and homochiral supramolecular polymerization of chiral nanotadpole aggregates made of either synthetic helical poly(phenylacetylene)s or chirality‐amplified co‐assembly of chiral and achiral poly(phenylacetylene)s. The twisted nanotadpole aggregates with high screw‐sense preference polymerized as monomers (aggremers) into supramolecular chains in a head‐to‐tail regioselective and stepwise manner. Supramolecular copolymerization of enantiomeric aggremers favored formation of homochiral hierarchical supramolecular structures as visualized by TEM. Chiral hexagonal columnar mesophase of aggremers was responsive for the stereoselectivity. The work opens a gate to controllably and effectively construct functional chiral supramolecular materials and deepens the understanding of hierarchical biological homochirality.

Selective Chiral Interactions between Hydrophilic/Hydrophobic Amino Acids and Growing Gypsum Crystals.

In nature, selective interactions between chiral amino acids and crystals are important for the formation of chiral biominerals and provide insight into the mysterious origin of homochirality. Here, we show that chiral amino acids with different hydrophilicities/hydrophobicities exhibit different chiral selectivity preferences in the dynamically growing gypsum [001] steps. Hydrophilic amino acids show a chiral selectivity preference for their d-isomers, whereas hydrophobic amino acids prefer their l-isomers. These differences in chiral recognition can be attributed to the different stereochemical matching between the hydrophilic and hydrophobic amino acids on the [001] steps of growing gypsum. These different chiral selectivities resulting from the amino acid hydrophilicity/hydrophobicity are confirmed by the experimental crystallization investigations from nano regulation on dynamic steps, to microscopic modification of gypsum morphology, and to macroscopic precipitation. Furthermore, as the hydrophilicity of amino acids increases, the disparity in chiral selection rises; conversely, the increase in the hydrophobicity of amino acids results in a decline in chiral selection. These insights improve our understanding of the interaction mechanism between amino acids and crystals and provide insights into the formation process of chiral biominerals and the origin of homochirality in nature.

Regulating the Chiroptical Expression of Aggregated Solvophobic Core by Solvophilic Segments.

The investigation of chiral supramolecular stacking is of essential significance for the understanding of the origin of homochirality in nature. Unlike structurally well-defined amphiphilic liposomes, it remains unclear whether the solvophilic segments of the amphiphilic block copolymer play a decisive role in the construction of asymmetric superstructures. Herein, insights are presented into the stacking patterns and morphological regulation in azobenzene-containing block copolymer assemblies solely by modulating the solvophilic chain length. The solvophilic poly(methacrylic acid) (PMAA) segments of different molecular weights could cause multi-mode chirality inversions involving stacking transitions between intra-chain π-π stacking, inter-chain H- and J-aggregation. Furthermore, the length of the solvophilic PMAA also affects the morphology of the chiral supramolecular assemblies; rice grain-like micelles, worms, nanofibers, floccules, and lamellae can be prepared at different solvophilic-solvophobic balance. The comprehensive mechanism is collectively revealed by utilizing various measurement methods, such as including circular dichroism (CD), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD). This study highlights the critical importance of fully dissolved solvophilic segments for the chiroptical regulation of the aggregated core, providing new insights into the arrangement of chiral supramolecular structures in polymer systems.

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.

Selective Chiral Recognition between Amino Acids and Growing Gypsum Crystals.

In nature, selective chiral interactions between biomolecules and minerals provide insight into the mysterious origin of homochirality. Here, we show growing gypsum crystals in a nonequilibrium state can recognize chiral enantiomers of amino acids. The chiral selection for amino acids with different functional groups by growing minerals are distinct. For 11 amino acids, the d-isomer slows dynamic gypsum growth more than the l-isomer, whereas for another 7 amino acids, the opposite was observed. These differences in chiral recognition are attributed to the different stereochemical matching between the chiral amino acids and the dynamic steps of growing gypsum. These stereoselective interactions between amino acid enantiomers and dynamic growing crystals can be applied toward the fabrication of gypsum cements to regulate their structure and mechanical properties. These findings provide insight into understanding the mechanism of the origin of homochirality in nature and suggest a pathway for constructing advanced functional materials.

Condensation and asymmetric amplification of chirality in achiral molecules adsorbed on an achiral surface

The origin of homochirality in nature is an important but open question. Here, we demonstrate a simple organizational chiral system constructed by achiral carbon monoxide (CO) molecules adsorbed on an achiral Au(111) substrate. Combining scanning tunneling microscope (STM) measurements with density-functional-theory (DFT) calculations, two dissymmetric cluster phases consisting of chiral CO heptamers are revealed. By applied high bias voltage, the stable racemic cluster phase can be transformed into a metastable uniform phase consisting of CO monomers. Further, during the recondensation of a cluster phase after lowering down bias voltage, an enantiomeric excess and its chiral amplification occur, resulting in a homochirality. Such asymmetry amplification is found to be both kinetically feasible and thermodynamically favorable. Our observations provide insight into the physicochemical origin of homochirality through surface adsorption and suggest a general phenomenon that can influence enantioselective chemical processes such as chiral separations and heterogeneous asymmetric catalysis.

Formation of Multiple Supramolecular Helices for Spontaneous Resolution

AbstractRelating supramolecular helices to the spontaneous resolution that generates conglomerate crystals is meaningful to the production of enantiopure compounds and to the understanding of natural homochirality as well. While homochiral elongation is favored along the axis of 1D supramolecular helical chains, conglomerate crystallization requires a 3D homochiral framework. This inspires us to explore new avenues to the spontaneous resolution through the formation of multiple supramolecular helices during crystallization. This Concept outlines and reviews multiple supramolecular helices found in the conglomerate crystals, i. e. three supramolecular helices along one crystallographic axis and two or three supramolecular helices along different crystallographic axes, followed by our observations of homochiral elongation of the helicity of the helical building blocks, the azapeptides, during the formation of supramolecular helices, as well as the spontaneous resolution of amino acid alanine that is made into a helical azapeptide derivative via the formation of supramolecular helices along two different crystallographic axes.

Chiral supramolecular polymers.

As an active branch within the field of supramolecular polymers, chiral supramolecular polymers (SPs) are an excellent benchmark to generate helical structures that can clarify the origin of homochirality in Nature or help determine new exciting functionalities of organic materials. Herein, we highlight the most utilized strategies to build up chiral SPs by using chiral monomeric units or external stimuli. Selected examples of transfer of asymmetry, in which the point or axial chirality contained by the monomeric units is efficiently transferred to the supramolecular scaffold yielding enantioenriched helical structures, will be presented. The importance of the thermodynamics and kinetics associated with those processes is stressed, especially the influence that parameters such as the helix reversal and mismatch penalties exert on the achievement of amplification of asymmetry in co-assembled systems will also be considered. Remarkable examples of breaking symmetry, in which chiral supramolecular polymers can be attained from achiral self-assembling units by applying external stimuli like stirring, solvent or light, are highlighted. Finally, the specific and promising applications of chiral supramolecular polymers are presented with recent relevant examples.

Amplification of Molecular Asymmetry during the Hierarchical Self-Assembly of Foldable Azobenzene Dyads into Nanotoroids and Nanotubes.

The amplification of molecular asymmetry through self-assembly is a phenomenon that not only comprehends the origin of homochirality in nature but also produces chiroptically active functional materials from molecules with minimal enantiomeric purity. Understanding how molecular asymmetry can be transferred and amplified into higher-order structures in a hierarchical self-assembly system is important but still unexplored. Herein, we present an intriguing example of the amplification of molecular asymmetry in hierarchically self-assembled nanotubes that feature discrete and isolatable toroidal intermediates. The hierarchical self-assembly is initiated via asymmetric intramolecular folding of scissor-shaped azobenzene dyads furnished with chiral side chains. When scalemic mixtures of the enantiomers are dissolved in a non-polar solvent and cooled to 20 °C, single-handed nanotoroids are formed, as confirmed using atomic force microscopy and circular dichroism analyses. A strong majority-rules effect at the nanotoroid level is observed and can be explained by a low mismatch penalty and a high helix-reversal penalty. The single-handed nanotoroids stack upon cooling to 0 °C to exclusively afford their respective single-handed nanotubes. Thus, the same degree of amplification of molecular asymmetry is realized at the nanotube level. The internal packing efficiency of molecules within nanotubes prepared from the pure enantiomers or their scalemic mixtures is likely different, as suggested by the absence of higher-order structure (supercoil) formation in the latter. X-ray diffraction analysis of the anisotropically oriented nanotube films revealed looser molecular packing within the scalemic nanotubes, which clearly reflects the lower enantiomeric purity of their internal chiral side chains.

Modulating the Macroscopic Helicity of Poly(m‐phenylenediamine) by Achiral Monomer Copolymerization

AbstractDesigning artificial systems to mimic chiral superstructures and exploring how achiral molecules precisely modulate their chirality is crucial to understand the origin of homochirality in nature. Herein, the tunability nature of supramolecular chirality is subtly applied to fabricate robust chiral covalent polymers with controlled macroscopic helicity. When using conventional supramolecular transcription methods to prepare chiral poly(m‐phenylenediamine), simply introducing various small amounts of achiral diaminopyridines (DAPs) to copolymerize with m‐phenylenediamine (MPD) enables the resulting chiral copolymers with adjustable helical configurations and chiroptical activities in an enantiopure template system. The strong hydrogen bonds between DAPs and the template molecules ((S)/(R)‐PhgC16) proved to be the driving force for guiding the chiral polymerization mode of MPD. Impressively, the exquisite self‐sorting phenomenon is observed by tuning the stoichiometric ratio of DAP and (R)‐PhgC16, where the as‐prepared copolymers with opposite helicity can exist simultaneously to form a double network structure and their chiroptical activity can be well regulated. In addition, based on such a copolymerization strategy, a conceptual library of helical functional copolymers is built with editable polymer types including conducting polymers and designable amine‐aldehyde condensation‐based polymers. This copolymerization‐mediated synthetic strategy may facilitate the development of chiral polymers with controlled morphologies and dynamic compositions.

Supramolecular chiral polymeric aggregates: Construction and applications

AbstractInspired by the mystery of natural homochirality, the design and construction of artificial chiral systems with special functions including mechanics, chiroptical devices, and chiral separation have aroused the great interest of scientists. Due to its diverse conformations and components, polymers become an excellent candidate to construct artificial chiral aggregates, which provides an opportunity to fabricate advanced chiral materials. Herein, in this review, we summarized not only the different strategies for construction and regulation of supramolecular chirality in chiral or achiral polymer systems, mainly including post‐assembly and polymerization‐induced chiral self‐assembly in chiral polymer systems and several chirality transfer and induction strategies in achiral polymer systems, but also the formation mechanisms of chiral aggregates by the chirality amplification from the molecular level to supramolecular level. Furthermore, we present the typical applications of chiral polymeric aggregates in circularly polarized light emission, chiral recognition and separation, and chiral elastomer. Finally, the current challenges and future directions of chiral polymeric aggregates are also elaborated.

Jiannan Zhao.

"I am waiting for the day when someone will discover the origin of homochirality in nature … A key experience in my education was a trip to Germany and attending the 65th Lindau Nobel Laureates Meeting with ambitious young scientists from all natural science disciplines…" Find out more about Jiannan Zhao in his Introducing … Profile.

Exploring the enantiomeric 13C position-specific isotope fractionation: challenges and anisotropic NMR-based analytical strategy.

Trying to answer the intriguing and fundamental question related to chiral induction/amplification at the origin of homochirality in Nature: "Is there a relationship between enantiomeric and isotopic fractionation of carbon 13 in chiral molecules?" is a difficult but stimulating challenge. Although isotropic 13C-PSIA NMR is a promising tool for the determination of (13C/12C) ratios capable of providing key 13C isotopic data for understanding the reaction mechanisms of biological processes or artificial transformations, this method does not provide access to any enantiomeric 13C isotopic data unless mirror-image isomers are first physically separated. Interestingly, 13C spectral enantiodiscriminations can be potentially performed in situ in the presence of enantiopure entities as chiral-europium complexes or chiral liquid crystals (CLCs). In this work, we explored for the first time the capabilities of the anisotropic 13C-{1H} NMR using PBLG-based lyotropic CLCs as enantiodiscriminating media in the context of the enantiomeric position-specific 13C isotope fractionation (EPSIF), within the requested precision of the order of the permil. As enantiomeric NMR signals are discriminated on the basis of a difference of 13C residual chemical shift anisotropy (13C-RCSA) prior to being deconvoluted, analysis of enantiomeric mixtures becomes possible. The analytical potential of this approach when using poly-γ-benzyl-L-glutamate (PBLG) is presented, and the preliminary quantitative results on small model chiral molecules obtained at 17.5T with a cryogenic NMR probe are reported and discussed. A promising analytical approach based on anisotropic irm-13C-NMR spectrometry to potentially reveal the natural 13C/12C isotopic enantiofractionation effects in organic chiral molecules is proposed and discussed.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Racemic Tartrate/Malate Anions Combine with Racemic Complex Cations to Form Optically Active Ionic Crystals.

Spontaneous resolution has attracted continuing attention in various research fields since Pasteur's work on the crystallization behavior of racemic tartrate. Here, a unique example of this phenomenon is reported, involving ionic crystals generated from racemic RR/SS- tartrate or R/S-malate and racemic ΔΔ/ΛΛ-[Ag3 Rh2 (2-aminoethanethiolato)6 ]3+ (ΔΔ/ΛΛ-[1]3+ ) in water. RR- and SS-tartrate selectively recognize the ΛΛ and ΔΔ isomers of [1]3+ to produce ionic crystals of (ΛΛ-[1])2 (RR-tartrate)3 and (ΔΔ-[1])2 (SS-tartrate)3 , respectively, which can undergo spontaneous resolution. While spontaneous resolution also occurs when using R/S-malate, R- and S-malate select the opposite isomers of [1]3+ to give ionic crystals of (ΔΔ-[1])2 (R-malate)3 and (ΛΛ-[1])2 (S-malate)3 , respectively. In the presence of S-aspartate, (ΛΛ-[1])2 (R-tartrate)3 and (ΔΔ-[1])2 (S-tartrate)3 are preferentially crystallized from ΔΔ/ΛΛ-[1]3+ and RR/SS-tartrate at solution pH values of 6 and 10, respectively. This finding provides significant insight into the optical resolution of chemical species by spontaneous resolution and the origin of homochirality in nature.

Investigations into Spin- and Unpolarized Secondary Electron-Induced Reactions in Self-Assembled Monolayers of Cysteine.

Cysteine is the simplest thiolated, chiral amino acid and is often used as the anchor for studies of self-assembled monolayers (SAMs) of complex biomolecules such as peptides. Understanding the interaction of SAMs of cysteine with low-energy secondary electrons (SEs) produced by X-rays can further our understanding of radiation damage in biomolecules. In particular, if the electrons are polarized, chiral-selective chemistry could have bearing on the origin of homochirality in nature. In the present paper, we use synchrotron radiation-based X-ray photoelectron spectroscopy to determine the changes that occur in the bonding of self-assembled layers of cysteine on gold as a result of soft X-ray irradiation. To investigate the possibility of chiral selectivity resulting from the interaction of low-energy, spin-polarized SEs (SPSEs), measurements were conducted on cysteine adsorbed on a 3 nm-thick gold layer deposited on a CoPt thin-film multilayer with perpendicular magnetic anisotropy. Time-dependent measurements of the C 1s, N 1s, O 1s, S 2p, and Au 4f core levels are used to follow the changes in surface chemistry and determine reaction cross-sections as a function of SE exposure. Analysis of the data results in cross-sections in the range of 5-7 Mb and suggests possible reaction pathways. Changing the magnetization direction of the CoPt multilayer produces SPSEs with opposite polarity. Some evidence of spin-dependent reactions is indicated but is inconclusive. Possible reasons for the discrepancy are posited.

Supramolecular chirality of multicomponent self-assembled systems and their applications

<p indent=0mm>Chirality is closely associated with life, such as amino acids at the molecular level, secondary structures of proteins (DNA, RNA), nanoscale helices and macroscopic conches. Due to the dynamic property of non-covalent bond, self-assembly of molecule shows environmental adaptability. The stacking form of molecule can be regulated via tunning non-covalent interaction. The asymmetric aggregation of molecular building blocks is called supramolecular chirality which is not only related to the structure of molecule and molecular chirality, but also influenced by non-covalent interaction. Regulation of the emergence, amplification, transfer and inversion of supramolecular chirality is an important field. The assemblies with supramolecular chirality are usually driven by various non-covalent interactions with unique chiral micro-environment. Design and fabrication of supramolecular assemblies with two or more kinds of components are much more complex than the single component systems due to the complexity of possible pathways. Compared with single-component systems which have already matured, both basic and application research of multi-component supramolecular chiral assemblies are still limited. However, the study of supramolecular chirality of multicomponent self-assembled systems not only benefit to simulate complex biological systems, but also contribute to the deep understanding of the origin of homochirality in nature to further apply in bionics, which is of great significance. Therefore, in this review article, we mainly discuss the construction of multicomponent self-assembled systems including solvent effect, hydrogen bond, structural analogues, organic/inorganic co-assembly. Solvent polarity has a great influence on self-assembled process of molecules especially for amphiphiles whose molecular stacking layer that exposes to solvents should be solvophilic. Besides the effect of polarity, solvent molecules also could act as an active component to participate self-assembled process generating significant influence on self-assembly behavior and even supramolecular chirality. Due to the directionality and relatively strong intensity compared with other non-covalent interactions, hydrogen bonding has been considered as the important interaction to construct supramolecular assemblies. Hydrogen bonding could be widely observed among amide/imide, ureido and carboxylic acid groups which has been introduced in this review in detail. And the influence of hydrogen bond to supramolecular chirality also has been fully discussed. Introducing structural analogues into building block with chiral center is another effective method to construct multicomponent supramolecular assemblies. Two different paths upon the aggregation of structural analogues including self-sorting and coassembly might occur which are controlled via equilibrium of kinetically or thermodynamically. The realization of coassembly between structural analogues requires strong interactions between components or similarly staking mode. Because of the excellent properties of inorganic material especially in luminescence, inorganic/organic coassembled materials which could be applied in chiroptical with tunable chiral and optical properties have emerged. Moreover, a serial of three component supramolecular assemblies with tunable chiral and optical properties based on the research of double components have been developed. Strong non-covalent interactions including hydrogen bond and π-π stacking interaction are benefit for forming multicomponent supramolecular chiral assemblies. Besides design and fabrication multicomponent supramolecular chiral assemblies, we also introduced their applications in chiral recognition, circularly polarized luminescence (CPL), chiral catalysis and chiral template. Chiral recognition not only displays the interaction between achiral molecules with various chiral molecules, but also between supramolecular assemblies with chiral molecules. We mainly discussed the recognition of supramolecular assemblies to enantiomers. CPL is a nonnegligible application field of multicomponent supramolecular chiral assemblies which has attracted increasing attention due to its wide potential applications in optical displays, encryption and storage of information, photoelectric devices, chiroptical materials, and so on. Catalysts for asymmetric reactions under mild conditions have been widely studied based on different supramolecular assemblies including helical nanorods, nanotubes and vesicles. We expect this review will serve as an introduction and reference for researchers who are interested in exploring this field.

Efficient Amplification in Soai's Asymmetric Autocatalysis by a Transient Stereodynamic Catalyst.

Mechanisms leading to a molecular evolution and the formation of homochirality in nature are interconnected and a key to the underlying principles that led to the emergence of life. So far proposed mechanisms leading to a non-linear reaction behavior are based mainly on the formation of homochiral and heterochiral dimers. Since homochiral and heterochiral dimers are diastereomers of each other, the minor enantiomer is shifted out of equilibrium with the major enantiomer by dimer formation and thus a reaction or catalysis can be dominated by the remaining molecules of the major enantiomer. In this article a mechanism is shown that leads to homochirality by the formation of a highly catalytically active transient intermediate in a stereodynamically controlled reaction. This is demonstrated by Soai's asymmetric autocatalysis, in which aldehydes are transformed into the corresponding alcohols by addition of dialkylzinc reagents. The mechanism of chirogenesis proposed here shows that an apparently inefficient reaction is the best prerequisite for a selection mechanism. In addition, stereodynamic control offers the advantage that the minor diastereomeric intermediate can be interconverted into the major diastereomer and thus be stereoeconomically efficient. This is supported by computer simulation of reaction kinetics.

Asymmetric catalysis mediated by a mirror symmetry-broken helical nanoribbon

Although chirality has been recognized as an essential entity for life, it still remains a big mystery how the homochirality in nature emerged in essential biomolecules. Certain achiral motifs are known to assemble into chiral nanostructures. In rare cases, their absolute geometries are enantiomerically biased by mirror symmetry breaking. Here we report the first example of asymmetric catalysis by using a mirror symmetry-broken helical nanoribbon as the ligand. We obtain this helical nanoribbon from a benzoic acid appended achiral benzene-1,3,5-tricarboxamide by its helical supramolecular assembly and employ it for the Cu2+-catalyzed Diels–Alder reaction. By thorough optimization of the reaction (conversion: > 99%, turnover number: ~90), the enantiomeric excess eventually reaches 46% (major/minor enantiomers = 73/27). We also confirm that the helical nanoribbon indeed carries helically twisted binding sites for Cu2+. Our achievement may provide the fundamental breakthrough for producing optically active molecules from a mixture of totally achiral motifs.