Showing papers on "Homochirality published in 2016"

Discovery of the interstellar chiral molecule propylene oxide (CH3CHCH2O)

Abstract: Life on Earth relies on chiral molecules—that is, species not superimposable on their mirror images. This manifests itself in the selection of a single molecular handedness, or homochirality, across the biosphere. We present the astronomical detection of a chiral molecule, propylene oxide (CH_3CHCH_2O), in absorption toward the Galactic center. Propylene oxide is detected in the gas phase in a cold, extended molecular shell around the embedded, massive protostellar clusters in the Sagittarius B2 star-forming region. This material is representative of the earliest stage of solar system evolution in which a chiral molecule has been found.

Discovery of the Interstellar Chiral Molecule Propylene Oxide (CH$_3$CHCH$_2$O)

Abstract: Life on Earth relies on chiral molecules, that is, species not superimposable on their mirror images. This manifests itself in the selection of a single molecular handedness, or homochirality, across the biosphere. We present the astronomical detection of a chiral molecule, propylene oxide (CH$_3$CHCH$_2$O), in absorption toward the Galactic Center. Propylene oxide is detected in the gas phase in a cold, extended molecular shell around the embedded, massive protostellar clusters in the Sagittarius B2 star-forming region. This material is representative of the earliest stage of solar system evolution in which a chiral molecule has been found.

Host–Guest Chirality Interplay: A Mutually Induced Formation of a Chiral ZMOF and Its Double-Helix Polymer Guests

Abstract: A novel homochiral zeolite-like metal-organic framework (ZMOF), [(Cu4I4) (dabco)2]·[Cu2(bbimb)]·3DMF (JLU-Liu23, dabco =1,4-diazabicyclo[2.2.2]-octane, H2bbimb =1,3-bis(2-benzimidazol)benzene, DMF = N,N-dimethylformamide), has been successfully constructed to host unprecedented DNA-like [Cu2(bbimb)]n polymers with double-helicity. The host-guest chirality interplay permitted the induced formation of an unusual gyroid MOF with homochirality and helical channels in the framework for the first time, JLU-Liu23. Importantly, the enantiomeric pairs (23P, 23M) can be promoted and isolated in the presence of appropriate chiral inducing agents, affording enantioselective separation of chiral molecules as well as small gas molecules.

Supramolecular Double-Helix Formation by Diastereoisomeric Conformations of Configurationally Enantiomeric Macrocycles

Abstract: Solid-state superstructures, resulting from assemblies programmed by homochirality, are attracting considerable attention. In addition, artificial double-helical architectures are being investigated, especially in relation to the ways in which homochiral small molecules can be induced to yield helical forms as a result of chiral induction. Herein, we report the highly specific self-assembly upon crystallization of a double-helical superstructure from an enantiopure macrocyclic dimer which adopts two diastereoisomeric conformations in a molar ratio of 1.5:1 in dimethyl sulfoxide. These two conformational diastereoisomers self-organize—and self-sort—in the crystalline phase in equimolar proportions to form two single-handed helices which are complementary to each other, giving rise to the assembly of a double helix that is stabilized by intermolecular [C–H···O] and π–π stacking interactions. The observed self-sorting phenomenon occurs on going from a mixed-solvent system containing two equilibrating conform...

Necessary conditions for the emergence of homochirality via autocatalytic self-replication.

Abstract: We analyze a recent proposal for spontaneous mirror symmetry breaking based on the coupling of first-order enantioselective autocatalysis and direct production of the enantiomers that invokes a critical role for intrinsic reaction noise. For isolated systems, the racemic state is the unique stable outcome for both stochastic and deterministic dynamics when the system is in compliance with the constraints dictated by the thermodynamics of chemical reaction processes. In open systems, the racemic outcome also results for both stochastic and deterministic dynamics when driving the autocatalysis unidirectionally by external reagents. Nonracemic states can result in the latter only if the reverse reactions are strictly zero: these are kinetically controlled outcomes for small populations and volumes, and can be simulated by stochastic dynamics. However, the stability of the thermodynamic limit proves that the racemic outcome is the unique stable state for strictly irreversible externally driven autocatalysis. These findings contradict the suggestion that the inhibition requirement of the Frank autocatalytic model for the emergence of homochirality may be relaxed in a noise-induced mechanism.

Temperature-Triggered Chiral Self-Assembly of Achiral Molecules at the Liquid–Solid Interface

Abstract: Temperature triggered chiral nanostructures have been investigated on two-dimensional (2D) surfaces by means of scanning tunneling microscopy. Achiral molecules 1 and 2 tend to self-assemble into strip structures on graphite before heating. However, R and S flower-like structures are observed when heated to certain temperature. The transition temperatures of 1 and 2 systems are 55 and 60 °C, respectively. The density functional theory calculations demonstrate that R and S flower-like structures are more stable than strip structures. The coexistence of flower-like structures and strip structures demonstrates the thermodynamic equilibrium. Further, when chiral solvent is added to the sample with other conditions remaining the same, the racemic phenomenon disappears and homochirality emerges. This is an efficient method to control the chirality of 2D molecular assemblies.

A Dual Modulated Homochiral Helical Nanofilament Phase with Local Columnar Ordering Formed by Bent Core Liquid Crystals: Effects of Molecular Chirality

Abstract: Helical nanofilament (HNF) phases form as a result of an intralayer mismatch between top and bottom molecular halves in bent-core liquid crystals (BC-LCs) that is relieved by local saddle-splay geometry. HNFs are immensely attractive for photovoltaic and chiral separation applications and as templates for the chiral spatial assembly of guest molecules. Here, the synthesis and characterization of two unichiral BC-LCs and one racemic mixture with tris-biphenyl-diester cores featuring chiral (R,R) and (S,S) or racemic 2-octyloxy aliphatic side chains are presented. In comparison to the achiral compound with linear side chains forming an intralayer modulated HNF phase (HNFmod ), synchrotron small angle X-ray diffraction indicates that the unichiral derivatives form a dual modulated HNF phase with intra- as well as interlayer modulations (HNFmod2 ) suggesting a columnar local structure of the nanofilaments. Transmission electron microscopy and circular dichroism spectropolarimetry confirm that the unichiral materials exclusively form homochiral HNFs with a twist sense-matching secondary twist. A contact preparation provides the first example of two identical chiral liquid crystal phases only differing in their handedness that do not mix and form an achiral liquid crystal phase with an entirely different structure in the contact zone.

Molecular Asymmetry in Prebiotic Chemistry: An Account from Meteorites

Abstract: Carbonaceous Chondrite (CC) meteorites are fragments of asteroids, solar planetesimals that never became large enough to separate matter by their density, like terrestrial planets. CC contains various amounts of organic carbon and carry a record of chemical evolution as it came to be in the Solar System, at the time the Earth was formed and before the origins of life. We review this record as it pertains to the chiral asymmetry determined for several organic compounds in CC, which reaches a broad molecular distribution and enantiomeric excesses of up to 50%–60%. Because homochirality is an indispensable attribute of extant polymers and these meteoritic enantiomeric excesses are still, to date, the only case of chiral asymmetry in organic molecules measured outside the biosphere, the possibility of an exogenous delivery of primed prebiotic compounds to early Earth from meteorites is often proposed. Whether this exogenous delivery held a chiral advantage in molecular evolution remains an open question, as many others regarding the origins of life are.

Synthesis of Metal–Organic Zeolites with Homochirality and High Porosity for Enantioselective Separation

Abstract: Using lactic acid derivatives as chiral ligands, a pair of unprecedented homochiral metal–organic zeolites have been synthesized that feature zeotype CAN topology and have high porosity for enantioselective separation of racemates.

Chiral selection of single helix formed by diblock copolymers confined in nanopores

Abstract: Chiral selection has attracted tremendous attention from the scientific communities, especially from biologists, due to the mysterious origin of homochirality in life. The self-assembly of achiral block copolymers confined in nanopores offers a simple but useful model of forming helical structures, where the helical structures possess random chirality selection, i.e. equal probability of left-handedness and right-handedness. Based on this model, we study the stimulus-response of chiral selection to external conditions by introducing a designed chiral pattern onto the inner surface of a nanopore, aiming to obtain a defect-free helix with controllable homochirality. A cell dynamics simulation based on the time-dependent Ginzburg–Landau theory is carried out to demonstrate the tuning effect of the patterned surface on the chiral selection. Our results illustrate that the chirality of the helix can be successfully controlled to be consistent with that of the tailored surface patterns. This work provides a successful example for the stimulus response of the chiral selection of self-assembled morphologies from achiral macromolecules to external conditions, and hence sheds light on the understanding of the mechanism of the stimulus response.

Elucidation of the Structures of Asymmetric Autocatalyst Based on X-ray Crystallography

Abstract: Much interest has been paid to the origin of homochirality, such as that of l-amino acids. The proposed origins of chirality have usually induced only low enantiomeric excess. Thus, asymmetric amplification by asymmetric autocatalysis has been invoked. Asymmetric autocatalysis of 5-pyrimidyl alkanol in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde is a unique example of amplification of chirality. Crystallization of possible intermediates, specifically zinc alkoxides of this 5-pyrimidyl alkanol, was investigated to obtain mechanistic insights into this reaction. Single-crystal X-ray crystallography of the formed zinc alkoxide crystals shows the existence of tetramer or higher oligomer structures in this asymmetric autocatalytic system.

Anisotropy spectroscopy of chiral alcohols, amines, and monocarboxylic acids: Implications for the analyses of extraterrestrial samples

Abstract: Stereoisomers of distinct chiral amino acids were observed to occur in l -enantioenriched form in carbonaceous chondrite meteorites. Meteoritic amines and monocarboxylic acids were recently shown to occur in racemic ratio. In this study we investigated the electronic circular dichroism and anisotropy spectra of chiral alcohols, chiral amines, and chiral monocarboxylic acids. We recorded circular dichroism and anisotropy spectra from 280 to 170 nm in aqueous solution using a synchrotron-radiation ultraviolet circular dichroism spectrophotometer. The obtained anisotropy spectra are employed to discuss the likely role of ultraviolet circularly polarized light leading to enantioenriched amino acids, as well as racemic amines and monocarboxylic acids during their primordial interstellar synthesis. These data will moreover accompany the European Space Agency’s Rosetta mission, which successfully landed Philae on the nucleus of comet 67P/Churyumov–Gerasimenko to search for chiral organic molecules.

Enantioselective Gas Chromatography in Search of the Origin of Biomolecular Asymmetry in Outer Space

Abstract: Since the pioneering work of Emanuel Gil-Av and his associates at the Weizmann Institute of Science the direct gas chromatographic resolution of enantiomers has found its application in different fields of natural sciences, even expanding to space research and astrobiology. In these domains the resolution of chiral molecules of prebiotic relevance is of particular interest, since the determination of ratios between l- and d-enantiomers within non-terrestrial samples is assumed to shed light on the origin of biomolecular homochirality and the beginnings of life itself. According to an astrophysical scenario, the first molecular symmetry breaking event occurred upon irradiation of chiral organic molecules with circularly polarized light present in the interstellar medium. To examine this hypothesis, among others, chiral gas chromatographic stationary phases have been widely used to provide the differentiation of enantiomers in extracts of meteorites, samples of simulated interstellar ices and Mars soil analogues. Moreover several chiral capillary columns have been selected for in situ analyses of extraterrestrial organic matter by Mars missions and the cometary Rosetta mission. This review will highlight current advances of enantioselective gas chromatography applied to space science.

Spontaneous symmetry breaking on ordered, racemic monolayers of achiral theophylline: formation of unichiral stripes on Au(111)

Abstract: We report the observation of spontaneous chiral symmetry breaking within ordered, racemic monolayers of theophylline, manifesting itself as extended, nanoscale unichiral stripes at the interface between molecular domains. Theophylline is a xanthine derivative playing an important role in several biochemical processes. Molecular chirality is induced by adsorption on the Au(111) surface, resulting in extended domains with two different racemic, ordered structures, coexisting with a disordered phase. By combining low-temperature scanning tunneling microscopy (LT-STM) and ab initio density functional theory calculations, we first provide a detailed picture of the interactions within the ordered assemblies, and we uncover the origin of the distinct contrast features in STM images. Secondly, experiments reveal the existence of nanoscale stripes of unichiral molecules separating racemic domains of one of the two ordered phases, giving rise to a local enantiomeric imbalance. Systematic theoretical investigation of their structure and chiral composition confirm their unichirality, with the specific handedness related to the registry between the two ordered domains facing the stripes. These findings can open the way to new insights into the elusive mechanisms leading to local chiral imbalances in racemic systems, possibly at the origin of biomolecular homochirality, as well as suggest novel approaches for stereoselective heterogeneous catalysis.

Homochirality of organic matter – Objective law or curious incident

Abstract: Of the many hypotheses on the origin of biological homochirality on the Earth, the combination of the notion of subatomic particles composing a homochiral pool of primary matter, on the one hand, with the suggestion of the synthesis of chiral organic molecules in the plasma torch generated by super-high-velocity impacts (SHVI) of meteorites, on the other hand, was chosen as the working assumption.

Photochemical Chiral Symmetry Breaking in Alanine.

Abstract: We introduce a general theoretical approach for the simulation of photochemical dynamics under the influence of circularly polarized light to explore the possibility of generating enantiomeric enrichment through polarized-light-selective photochemistry. The method is applied to the simulation of the photolysis of alanine, a prototype chiral amino acid. We show that a systematic enantiomeric enrichment can be obtained depending on the helicity of the circularly polarized light that induces the excited-state photochemistry of alanine. By analyzing the patterns of the photoinduced fragmentation of alanine we find an inducible enantiomeric enrichment up to 1.7%, which is also in good correspondence to the experimental findings. Our method is generally applicable to complex systems and might serve to systematically explore the photochemical origin of homochirality.

The Role of Conglomerate Crystallization in Spontaneous Resolution and Chiral Amplification

Abstract: The mystery of the origin of homochirality in Nature has been thoroughly investigated due to the essential role it plays in Chemistry, Biochemistry and Biology. For example, enantiopure compounds are often essential in the pharmaceutical and agrochemical industries. New aspects of chemistry and chirality can be explored by effectively utilizing the unique nature of conglomerate crystals. In some cases, conglomerate crystals can be easily resolved by carefully observing the features of the crystals. Resolution is possible through observation of hemihedrism, macromorphology, circular polarization or various surface features. Cytosine and 1,2-bis(N-benzoyl-N-methylamino)benzene are two examples of compounds that form centrosymmetric monohydrate racemic crystals but non-centrosymmetric anhydrous conglomerate crystals. Herein, a novel approach to asymmetric amplification by coupling conglomerate crystal formation via dehydration, with subsequent Viedma ripening (i.e. attrition-enchanced deracemization) is being explored. A systematic search of the Cambridge Crystallographic Database for other crystal systems which can undergo desolvation and subsequent chiral amplification is ongoing. Enantiomer-specific oriented attachment, an essential process in Viedma ripening, was also investigated in guanidine carbonate crystals. Simply boiling or shaking powdered racemic guanidine carbonate in saturated solution leads to the formation of large crystalline clusters. These clusters, characterized by solid-state circular dichroism and X-ray powdered diffraction, were found to be nearly homochiral. Enantiomer-specific oriented attachment can be thought of as a mesoscale analogue of conglomerate crystallization.

Theoretical and Computational Studies of Condensed-Phase Phenomena: The Origin of Biological Homochirality, and the Liquid-Liquid Phase Transition in Network-Forming Fluids

Abstract: ............................................................................................................................ iii Acknowledgements ........................................................................................................... v

Enantiomeric Separation of Chiral Amines and Amino Alcohols Using Acetylated β-Cyclodextrin Stationary Phase by High-Performance Liquid Chromatography

Abstract: Enantiomeric separation ability on high-performance liquid chromatography (HPLC) was compared between native and acetylated β-cyclodextrin stationary phases. In the chiral stationary phases (CSP) used in this work, β-cyclodextrin is chemically bonded to aminopropyl silica gel via sugar chain spacer. The acetylated β-cyclodextrin phase (SUMICHIRALTM OA-7700) exhibits excellent performance for the separation of chiral amines and amino alcohols in reversed phase mode. Conventional phosphate buffer can be used in mobile phase without special modifier and the analytical method is easy to develop. OA-7700 is very useful practically for the determination of optical purity of many important chiral amines. It can separate enantiomers of 1-phenyl-2-(p-tolyl)ethylamine (PTE) which is used for industrial scale resolution of chrysanthemic acids and norephedrin (phenylpropanolamine) which is regulated as stimulant raw material.

Enantioselective Gas Chromatography in Search of the Origin of Biomolecular Asymmetry in Outer Space

Abstract: Since the pioneering work of Emanuel Gil-Av and his associates at the Weizmann Institute of Science the direct gas chromatographic resolution of enantiomers has found its application in different fields of natural sciences, even expanding to space research and astrobiology. In these domains the resolution of chiral molecules of prebiotic relevance is of particular interest, since the determination of ratios between l- and d-enantiomers within non-terrestrial samples is assumed to shed light on the origin of biomolecular homochirality and the beginnings of life itself. According to an astrophysical scenario, the first molecular symmetry breaking event occurred upon irradiation of chiral organic molecules with circularly polarized light present in the interstellar medium. To examine this hypothesis, among others, chiral gas chromatographic stationary phases have been widely used to provide the differentiation of enantiomers in extracts of meteorites, samples of simulated interstellar ices and Mars soil analogues. Moreover several chiral capillary columns have been selected for in situ analyses of extraterrestrial organic matter by Mars missions and the cometary Rosetta mission. This review will highlight current advances of enantioselective gas chromatography applied to space science.

Chiral autocatalysis: reaction noise, micro-reversibility and chiral inhibition in mirror symmetry breaking

Abstract: Applying the constraints dictated by the principle of detailed balance, we analyze a recent proposal for spontaneous mirror symmetry breaking (SMSB) based on enantioselective autocatalysis coupled to a linear decay of the enantiomers and in the presence of reaction noise. We find the racemic state is the final stable outcome for both deterministic as well as for stochastic dynamics, and for both well-mixed and small spatially-coupled systems. The racemic outcome results even when the autocatalytic cycles are driven irreversibly by external reagents, in manifestly non-equilibrium conditions. Our findings suggest that first-order autocatalysis coupled to reactions involving \textit{non-linear} heterochiral dynamics is a necessary pre-condition for any mechanism purporting to lead to molecular homochirality.

Synchronization of conformational chirality in mixtures of achiral bent-core molecules

Abstract: Molecular conformational chirality has been proposed as a possible driving force for chiral symmetry breaking in fluids composed of achiral molecules, with deracemization being initiated by segregation of enantiomeric conformers. Using binary mixtures of liquid crystals made of achiral bent-core molecules, we now show that enantiomeric excess depends on chirality synchronization between conformers. This is manifested by the formation of two types of lamellar dark mesophases viz. the dark enantiomeric phase characterized by only one sense of chirality and the dark conglomerate phase composed of homochiral domains with opposite handedness. The severity of layer deformation in the dark phases, resulting from the frustration caused by the packing of chiral conformers within the layers is also found to depend on concentration. Electro-optic and dielectric responses reflect the layer deformation and the compactness of the sponge like layer structure and hence the efficiency of conformer packing. The severity of layer deformation has been monitored by visualizing the layer profiles using SEM. This type of study can contribute to the understanding of selective homochirality at the molecular level in achiral systems.

Chiral symmetry breaking : asymmetry and origin of chiral molecules relevant in prebiotic evolution

Abstract: Since the Pasteur’s discovery of chirality more than 150 years ago, the origin of homochirality remains to be an unresolved mystery, which is probably linked to the origin of life-itself. This puzzle was named by the journal Nature as one of the five biggest unresolved puzzles in modern science.Today there exist two ways to address the question of the origin of molecular homochirality. It implies either deterministic or chance scenario. The latest is based on the spontaneous resolution of enantiomers by phase transition, and in consequence, it cannot be tested. On the other hand, deterministic theories can be subjected to experimental confirmation, since if there was, in fact, a chiral influence that imposed its chirality this should in principle be reproducible.The presented manuscript is a synthesis of bibliographic research and experimental studies that focus on the interaction of CPL with chiral molecules which are put in the context of the origin of homochirality of life. This work is aiming to advance our understanding of chiroptical properties of chiral molecules as well as to determine chiral species susceptible to be present in cometary ice

Mineral-Induced Enantioenrichment of Tartaric Acid

Abstract: Interaction of mixtures of d - and l -tartrates with dissolved gibbsite minerals produces homochiral and heterochiral dimers in a stochastic manner, providing a mechanism for enantioenrichment given a small initial imbalance. This work offers the first suggestion for a path towards the emergence of homochirality in prebiotic metabolism via the glyoxylate scenario.