Showing papers on "Homochirality published in 2012"

Selection of supramolecular chirality by application of rotational and magnetic forces

Abstract: Many essential biological molecules exist only in one of two possible mirror-image structures, either because they possess a chiral unit or through their structure (helices, for example, are intrinsically chiral), but so far the origin of this homochirality has not been unraveled. Here we demonstrate that the handedness of helical supramolecular aggregates formed by achiral molecules can be directed by applying rotational, gravitational and orienting forces during the self-assembly process. In this system, supramolecular chirality is determined by the relative directions of rotation and magnetically tuned effective gravity, but the magnetic orientation of the aggregates is also essential. Applying these external forces only during the nucleation step of the aggregation is sufficient to achieve chiral selection. This result shows that an almost instantaneous chiral perturbation can be transferred and amplified in growing supramolecular self-assemblies, and provides evidence that a falsely chiral influence is able to induce absolute enantioselection.

Azaboradibenzo[6]helicene: Carrier Inversion Induced by Helical Homochirality

Abstract: Azaboradibenzo[6]helicene, a new semiconductor material possessing helical chirality, has been synthesized via a tandem bora-Friedel–Crafts-type reaction. Unprecedented carrier inversion between the racemate (displaying p-type semiconductivity) and the single enantiomer (displaying n-type semiconductivity) was observed and can be explained by changes in the molecular packing induced by helical homochirality.

Pasteur's tweezers revisited: on the mechanism of attrition-enhanced deracemization and resolution of chiral conglomerate solids.

Abstract: Insights into the mechanism of attrition-enhanced deracemization and resolution of solid enantiomorphic chiral compounds are obtained by crystal size and solubility measurements and by isotopic labeling experiments. Together these results help to deconvolute the various chemical and physical rate processes contributing to the phenomenon. Crystal size measurements highlight a distinct correlation between the stochastic, transient growth of crystals and the emergence of a single solid enantiomorph under attrition conditions. The rapid mass transfer of molecules between the solution and solid phases under attrition is demonstrated, and the concept of a crystal-size-induced solubility driving force is exploited to overcome the stochastic nature of the crystal growth and dissolution processes. Extension to non-racemizing conditions provides a novel methodology for chiral resolution. Implications both for practical chiral separations and for the origin of biological homochirality are discussed.

Transfer of chirality from molecule to phase in self-assembled chiral block copolymers.

Abstract: Here, we report the mechanisms of chiral transfer at various length scales in the self-assembly of enantiomeric chiral block copolymers (BCPs*). We show the evolution of homochirality from molecular chirality into phase chirality in the self-assembly of the BCPs*. The chirality of the molecule in the BCP* is identified from circular dichroism (CD) spectra, while the handedness of the helical conformation in the BCP* is determined from a split-type Cotton effect in vibrational circular dichroism spectra. Microphase separation of the BCP* is exploited to form a helical (H*) phase, and the handedness of helical nanostructure in the BCP* is directly visualized from transmission electron microscopy tomography. As examined by CD and fluorescence experiments, significant induced CD signals and a bathochromic shift of fluorescence emission for the achiral perylene moiety as a chemical junction of the BCPs* can be found while the concentration of the BCPs* in toluene solution is higher than the critical micelle co...

Chiral J-aggregates of atropo-enantiomeric perylene bisimides and their self-sorting behavior.

Abstract: However, to the best of our knowledge, homo-chiral J-aggregates of intrinsically chiral p-conjugated aro-matic systems, that is, those with chiral core, are not knownto date. To achieve such homochiral J-aggregates, we haveAbstract: Herein we report on structur-al, morphological, and optical proper-ties of homochiral and heterochiral J-aggregates that were created by nuclea-tion–elongation assembly of atropo-enantiomerically pure and racemic per-ylene bisimides (PBIs), respectively.Our detailed studies with conforma-tionally stable biphenoxy-bridgedchiral PBIs by UV/Vis absorption, cir-cular dichroism (CD) spectroscopy, andatomic force microscopy (AFM) re-vealed structurally as well as spectro-scopically quite different kinds of J-ag-gregates for enantiomerically pure andracemic PBIs. AFM investigationsshowed that enantiopure PBIs formhelical nanowires of unique diameterand large length-to-width ratio by self-recognition, while racemic PBIs pro-vide irregular-sized particles by self-dis-crimination of the enantiomers at thestage of nucleation. Steady-state fluo-rescence spectroscopy studies revealedthat the photoluminescence efficiencyof homochiral J-aggregated nanowires(47 3%) is significantly higher thanthat of heterochiral J-aggregated parti-cle-like aggregates (12 3%), which isexplained in terms of highly orderedmolecular stacking in one-dimensionalnanowires of homochiral J-aggregates.Our present results demonstrate thehigh impact of homochirality on theconstruction of well-defined nanostruc-tures with unique optical properties.Keywords: chirality · helical nano-wires · J-aggregates · perylene bisi-mide · self-assembly

Solvent-induced homochirality in surface-confined low-density nanoporous molecular networks.

Abstract: Induction of chirality in achiral monolayers has garnered considerable attention in the recent past not only due to its importance in chiral resolutions and enantioselective heterogeneous catalysis but also because of its relevance to the origin of homochirality in life. In this contribution, we demonstrate the emergence of macroscopic chirality in multicomponent supramolecular networks formed by achiral molecules at the interface of a chiral solvent and an achiral substrate. The solvent-mediated chiral induction provides a simple, efficient, and versatile approach for the fabrication of homochiral surfaces using achiral building blocks.

Asymmetric Autocatalysis Triggered by Chiral Crystals Formed from Achiral Compounds and Chiral Isotopomers

Abstract: Asymmetric autocatalysis with amplification of enantiomeric excess is found in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde using pyrimidyl alkanol as an asymmetric autocatalyst. Asymmetric autocatalysis has been employed as a method for clarifying the origin of homochirality. Circularly polarized light, inorganic chiral crystals and statistical fluctuation of enantiomeric imbalance act as chiral initiators in asymmetric autocatalysis to afford highly enantioenriched products. We have investigated asymmetric autocatalysis using chiral crystals formed from achiral and racemic compounds as an origin of chirality. Absolute control of the crystal chirality of cytosine was achieved by the removal of crystal water. Enantioselective carbon-carbon bond formation at the enantiotopic crystal face of aldehydes was established using diisopropylzinc vapor. In addition, asymmetric autocatalysis triggered by chiral compounds arising from H, C and O isotope substitution has been achieved.

Autocatalytic replication and homochirality in biopolymers: is homochirality a requirement of life or a result of it?

Abstract: A key step in the origin of life is the establishment of autocatalytic cycles controlled by biopolymer catalysts. These catalysts (either ribozymes or proteins) are composed of homochiral monomers. Homochirality in living systems is maintained because biopolymers are asymmetric in their catalysis and synthesize molecules of their own handedness. Asymmetric autocatalysis is also possible with small molecules, as demonstrated by the Soai reaction, but it is rare. As far as we know, single nucleotides and amino acids are not autocatalytic. The observation that organic molecules in meteorites can have an enantiomeric excess of a few percent suggests that the prebiotic mixture may have had a partial chiral bias that was caused by external physical influences. Here, we consider the way that such a partial prebiotic bias would influence the origin of ribozymes in an RNA world scenario. We have previously shown how a transition to a living state can occur in a model for RNA polymerization. Here, we add chirality to the problem by considering simultaneous synthesis and polymerization of left- and right-handed monomers. The two chemical synthesis rates may be equal or unequal, due to physical or chemical effects prior to the origin of life. We determine the stationary states of this reaction system. The nonliving state is racemic, or slightly biased. There are two living states that are almost completely homochiral, whether or not the nonliving state is biased. It is a feature of our model that, for some regions of parameter space, living and nonliving states are both found to be stable under the same conditions. The origin of life therefore involves a stochastic transition between the nonliving and living states. Our model extends previous theories by treating the origin of life and the origin of chirality as aspects of the same model.

Self-Assembly of Dendritic Dipeptides as a Model of Chiral Selection in Primitive Biological Systems

Abstract: Biological macromolecules are homochiral, composed of sequences of stereocenters possessing the same repeated absolute configuration. This chapter addresses the mechanism of homochiral selection in polypeptides. In particular, the relationship between the stereochemistry (l or d) of structurally distinct α-amino acids is explored. Through functionalization of Tyr–Xaa dipeptides with self-assembling dendrons, the effect of stereochemical sequence of the dipeptide on the thermodynamics of self-assembly and the resulting structural features can be quantified. The dendritic dipeptide approach effectively isolates the stereochemical information of the shortest sequence of stereochemical information possible in polypeptide, while simultaneously allowing for dendron driven tertiary and quaternary structure formation and subsequent transfer of chiral information from the dipeptide to the dendritic sheath. This approach elucidates a mechanism of selecting a homochiral relationship between dissimilar but neighboring α-amino acids through thermodynamic preference for homochirality in solution-phase and bulk supramolecular helical polymerization.

Chirality and Protein Biosynthesis

Abstract: Chirality is present at all levels of structural hierarchy of protein and plays a significant role in protein biosynthesis. The macromolecules involved in protein biosynthesis such as aminoacyl tRNA synthetase and ribosome have chiral subunits. Despite the omnipresence of chirality in the biosynthetic pathway, its origin, role in current pathway, and importance is far from understood. In this review we first present an introduction to biochirality and its relevance to protein biosynthesis. Major propositions about the prebiotic origin of biomolecules are presented with particular reference to proteins and nucleic acids. The problem of the origin of homochirality is unresolved at present. The chiral discrimination by enzymes involved in protein synthesis is essential for keeping the life process going. However, questions remained pertaining to the mechanism of chiral discrimination and concomitant retention of biochirality. We discuss the experimental evidence which shows that it is virtually impossible to incorporate D-amino acids in protein structures in present biosynthetic pathways via any of the two major steps of protein synthesis, namely aminoacylation and peptide bond formation reactions. Molecular level explanations of the stringent chiral specificity in each step are extended based on computational analysis. A detailed account of the current state of understanding of the mechanism of chiral discrimination during aminoacylation in the active site of aminoacyl tRNA synthetase and peptide bond formation in ribosomal peptidyl transferase center is presented. Finally, it is pointed out that the understanding of the mechanism of retention of enantiopurity has implications in developing novel enzyme mimetic systems and biocatalysts and might be useful in chiral drug design.

Homochiral 1D helical chain based on an achiral Cu(II) complex.

Abstract: Self-assembly of an achiral [Cu(L)] complex produced a homochiral helical chain [Cu(L)]3·2H2O (1) (L = 2-dimethylaminoethyl(oxamato)). Interestingly, complex 1 obtained in our laboratory exhibits only a left-handed helical chain without any chiral source. Single-crystal X-ray analysis revealed the absolute structure and homochirality of its helical chain structure in the space group of P32. Solid-state circular dichroism (CD) spectra confirmed the high enantio excess of the crystals obtained in different synthesis batches. Magnetic susceptibility measurements reveal a relatively strong intrachain antiferromagnetic interaction between Cu(II) centers via an oxamato bridge (J = −74.4 cm–1).

8.2 Perspective and Concepts: Biomolecular Significance of Homochirality: The Origin of the Homochiral Signature of Life

Abstract: ‘Homochirality’ or one-handedness is a characteristic hallmark of terrestrial life, which is based on nucleic acids made of all- D sugars coding for proteins made of all- L amino acids. But why is life made of L -amino acids and not their ‘unnatural’ D mirror images? In this article we consider at what stage in the origin of life homochirality first arose, and whether life's handedness was selected by chance or by a chiral influence. We discuss in detail the two most popular chiral influences – circularly polarized radiation, and parity-violation by the weak force – together with mechanisms to amplify small enantiomeric excesses to homochirality. We focus in particular on the ‘parity-violating energy difference’ or ‘PVED’ between enantiomers, and review calculations of the PVED, which show that in most cases the natural enantiomers are indeed of slightly lower energy than their ‘unnatural’ mirror images. We also review proposed methods to measure the PVED. We conclude with a review of the new field of ‘exochirality’ (chirality outside Earth), and discuss proposed instruments to search for homochirality as a signature of life both in situ on solar system bodies and remotely on extra-solar planets.

Chiral polymerization in open systems from chiral-selective reaction rates.

Abstract: We investigate the possibility that prebiotic homochirality can be achieved exclusively through chiral-selective reaction rate parameters without any other explicit mechanism for chiral bias. Specifically, we examine an open network of polymerization reactions, where the reaction rates can have chiral-selective values. The reactions are neither autocatalytic nor do they contain explicit enantiomeric cross-inhibition terms. We are thus investigating how rare a set of chiral-selective reaction rates needs to be in order to generate a reasonable amount of chiral bias. We quantify our results adopting a statistical approach: varying both the mean value and the rms dispersion of the relevant reaction rates, we show that moderate to high levels of chiral excess can be achieved with fairly small chiral bias, below 10%. Considering the various unknowns related to prebiotic chemical networks in early Earth and the dependence of reaction rates to environmental properties such as temperature and pressure variations, we argue that homochirality could have been achieved from moderate amounts of chiral selectivity in the reaction rates.

Life's Chirality From Prebiotic Environments

Abstract: A key open question in the study of life is the origin of biomolecular homochirality: almost every life-form on Earth has exclusively levorotary amino acids and dextrorotary sugars. Will the same handedness be preferred if life is found elsewhere? We review some of the pertinent literature and discuss recent results suggesting that life's homochirality resulted from sequential chiral symmetry breaking triggered by environmental events. In one scenario, autocatalytic prebiotic reactions undergo stochastic fluctuations due to environmental disturbances. In another, chiral-selective polymerization reaction rates influenced by environmental effects lead to substantial chiral excess even in the absence of autocatalysis. Applying these arguments to other potentially life-bearing platforms has implications to the search for extraterrestrial life: we predict that a statistically representative sampling of extraterrestrial stereochemistry will be racemic (chirally neutral) on average.

Hydrogen bonding in homochiral dimers of hydroxyesters studied by Raman optical activity spectroscopy

Abstract: Spectroscopic analysis of homochiral dimerization is important for the understanding of the homochirality of life and enantioselective catalysis. in this paper, (s)-methyl lactate and related molecules were studied to provide detailed structural information on hydrogen bonding in homochiral dimers of chiral a-hydroxyesters through the experimental and theoretical study of raman optical activity. different homochiral dimers can be distinguished by comparing their simulated raman optical activity spectra with the experimental results. hydrogen bonding motions are decoded with the aid of vibrational motion analysis, which are apparently involved in vibrational motions below 800?cm1. a common feature related to the chain-bending mode also indicates the absolute configuration of methyl lactate and related molecules. the differing behavior of electric dipoleelectric quadrupole invariants (beta(a)2) compared with the electric dipolemagnetic dipole invariant (beta(g')2), suggests that the intermolecular hydrogen bonding motion behaves differently from the intramolecular one in the asymmetric molecular electric and magnetic fields. these results may help understand hydrogen-bonded self-recognition and other dynamical features in chiral recognition. copyright (c) 2011 john wiley & sons, ltd.

Life's Chirality From Prebiotic Environments

Abstract: A key open question in the study of life is the origin of biomolecular homochirality: almost every life-form on Earth has exclusively levorotary amino acids and dextrorotary sugars. Will the same handedness be preferred if life is found elsewhere? We review some of the pertinent literature and discuss recent results suggesting that life’s homochirality resulted from sequential chiral symmetry breaking triggered by environmental events. In one scenario, autocatalytic prebiotic reactions undergo stochastic uctuations due to environmental disturbances. In another, chiral-selective polymerization reaction rates inuenced by environmental eects lead to substantial chiral excess even in the absence of autocatalysis. Applying these arguments to other potentially life-bearing platforms has implications to the search for extraterrestrial life: we predict that a statistically representative sampling of extraterrestrial stereochemistry will be racemic (chirally neutral) on average.

Effect of enantiomeric ratio and preparation method on proline crystal form

Abstract: Proline enantiomers were crystallized in a range of enantiomeric ratios and characterized by thermal methods, X-ray diffraction, and solid-state NMR spectroscopy to qualitatively and quantitatively characterize the resulting crystal forms. When crystallized by solvent evaporation or in the absence of enantiomeric excess, the observed racemic cocrystal consisted of only the previously published form, DL-proline (form I). However, lyophilization of solutions containing enantiomeric excess resulted in the presence of a new, thermodynamically stable polymorph of the racemic cocrystal, DL-proline form II. Both the enantiomeric ratio and crystallization method influenced the polymorphism of the racemic cocrystal. Due to the prevalence of chiral molecules and cocrystals among pharmaceuticals, this work has implications for current polymorphic screening methods.

Separation of Chiral Molecules: A Way to Homochirality

Abstract: A mechanism for separating chiral molecules is proposed. The separation of two enantiomers in an aquifer is considered. The molecules are dragged in the aquifer porous medium by a flow of water or of another liquid. The molecule velocity is u=v/R, where v is the liquid velocity, and R is the retardation factor. The aquifer consists of two one-dimensional layers disposed in series. The layers differ by the retardation factor or by the liquid velocity. The enantiomer velocity is a function of the enantiomer concentrations. This function is different in the two layers. For certain values of the model parameters and when the molecules entering the aquifer are enantiomerically enriched or when the medium is chiral, the concentration of one enantiomer increases around the interface between the two layers and in one layer, whereas the concentration of the other enantiomer decreases. Enantiomer synthesis and decay are not taken into account. The needed values of the parameters can be obtained when the enantiomers are moved by an alternating liquid flow, and the retardation factor oscillates in synchronism with the alternating liquid flow. The parameters of the model are then understood as quantities averaged over one oscillation period. The equations that give the values of the stationary concentrations of the enantiomers are found. The evolution of the enantiomer concentrations is determined by numerically solving a system of two nonlinear advection-dispersion equations. The proposed mechanism may have played a role in the emergence of biomolecular homochirality.

Chemical Models for the Origin of Biological Homochirality

Abstract: Chirality is the property of a molecule to be nonsuperimposable with its mirror image. In biology, basically all chiral molecules exist in only one mirror-image form, or enantiomer, a phenomenon called biological homochirality. For example, chiral amino acids occur nearly exclusively in the l-form and carbohydrates in the d-form. In contrast, chiral molecules on the early Earth should have been racemic, i.e., consisting of equal amounts of both enantiomers, before life came into existence. The emergence of biological homochirality is therefore directly linked with the origin of life but remains an intriguing and unanswered puzzle for scientists. To chemists, it poses a challenge to create model systems for the emergence of homochirality from a racemic state. This chapter gives an overview of such chemical models as well as other experiments and observations that might explain how biological homochirality was achieved.

How Have Scientists Explained the Amino Acid Chirality

Abstract: In this chapter several of the models that purport to ­describe how amino acids are produced are discussed. Specifically, the means by which circularly polarized light could create chiral amino acids, both on Earth and in outer space, are detailed. Next, several models that rely on the weak interaction to select chirality are described. And, finally, the possibility that chiral selection might have occurred on solid surfaces is mentioned. In each case, the experimental documentation for the hypothetical model is presented. The chapter then discusses amplification mechanisms, notably, autocatalysis, which could operate either in outer space or on Earth. Finally, experiments that have demonstrated amplification toward homochirality are discussed.

Response to Tencer and Bielski

Abstract: The separation mechanism proposed in Atencio (2012) is not based on mechanical resolution, hydrodynamic separation, hydrodynamic resolution, or mechanical selection in the sense in which these words are commonly used and are used by Tencer and Bielski (2011, 2012), Hirschfelder et al. (1977), or De Gennes (1999). Therefore the remarks of Tencer and Bielski (2012) are not relevant to the mechanism proposed in the cited article. In the mechanism put forward in Atencio (2012), in “achiral chromatography” (“achiralphase chromatography”) Charles and Gil-Av (1984), Cundy and Crooks (1983) and Soloshonok (2006) and in other usual chromatographic methods of separation of chiral molecules, the chiral molecules need not maintain their orientation to be separated. Rotational diffusion does not impede the separation. In “achiral chromatography” and in the mechanism considered in Atencio (2012) two enantiomers may be separated in an achiral environment when their concentrations are different, and thus the entire system (achiral environment + enantiomers) is chiral. The velocities of the two enantiomers are then different provided the chiral molecules interact (in order to feel the chirality of the system), for example, they form dimers.