Showing papers on "Homochirality published in 2009"

On the origin of terrestrial homochirality for nucleosides and amino acids

Abstract: Before life could start on earth, it was important that the amino acid building blocks be present in a predominant handedness called the L configuration and that the ribose of RNA be predominantly in the D configuration. Because ordinary chemical processes would produce them in equal L and D amounts, it has long been a puzzle how the needed selectivities could have arisen. Carbonaceous chondrites such as the Murchison meteorite, which landed in Australia in 1969, brought some unusual amino acids with a methyl group replacing their α hydrogen. They cannot racemize and have a small but real excess of those with the L configuration. We have shown that they can partake in a synthesis of normal L amino acids under credible prebiotic conditions. We and others showed that small preferences can be amplified into solutions with very high dominance of the L amino acids because of the higher solubility of the pure L form than of the more stable DL racemic compound crystal. Here, we show that such solubility-based amplification of small excesses of three D nucleosides, uridine, adenosine, and cytidine, can also occur to form solutions with very high D dominance under credible prebiotic conditions. Guanosine crystallizes as a conglomerate and does not amplify in this way. However, under prebiotic conditions it could have been formed from homochiral D ribose from the hydrolysis of amplified adenosine or cytidine.

Racemic β-Sheets as Templates of Relevance to the Origin of Homochirality of Peptides: Lessons from Crystal Chemistry

Abstract: The origin of life is a historical event that has left no relevant fossils; therefore, it is unrealistic to reconstruct the chronology of its occurrence. Instead, by performing laboratory experiments under conditions that resemble the prebiotic world, one might validate feasible reaction pathways and reconstruct model systems of artificial life. Creating such life in a test tube should go a long way toward removing the shroud of mystery over how it began naturally. The riddle of the appearance of natural proteins and nucleic acids--that is, biopolymers wholly consisting of homochiral subunits (L-amino acids and D-sugars, respectively)--from the unanimated racemic prebiotic world is still unsolved. There are two hypotheses concerning the sequence of their emergence: one maintains that long homochiral (isotactic) peptides must have been formed after the appearance of the first living systems, whereas the other presumes that such biopolymers preceded the primeval enzymes. The latter scenario necessitates, however, the operation of nonlinear synthetic routes, because the polymerization of racemates in ideal solutions yields chains composed of residues of either handedness. In this Account, we suggest applying lessons learned from crystal chemistry, in which molecules from isotropic media are converted into crystals with three-dimensional (3D) periodic order, to understand how the generation of homochiral peptides from racemic alpha-amino acids might be achieved, despite its seemingly overwhelming complexity. We describe systems that include the self-assembly of activated alpha-amino acids either in two-dimensional (2D) or in 3D crystals, followed by a partial lattice-controlled polymerization at the crystal-aqueous solution interface. We also discuss the polymerization of mixtures of activated hydrophobic racemic alpha-amino acids in aqueous solutions, as initiated by primary amines or thiols. The distribution of the diastereomeric oligopeptides was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and MS/MS with monomers enantioselectively tagged with deuterium. The reaction performed in aqueous solutions encompasses the following sequential steps: (i) formation of a library of short racemic peptides enriched with isotactic diastereoisomers during the early stages of the polymerization, and (ii) self-assembly of oligopeptides into racemic beta-sheet colloidal-like aggregates that are delineated by enantiotopic sites or rims; these operate as templates (nuclei) for regio-enantioselective growth in the ensuing steps of chain elongation. Desymmetrization of the racemic mixtures of peptides was achieved with enantiopure alpha-amino acid esters as initiators. The enantiomeric excess of the isotactic peptides, not including the initiator, varies with chain length, the result of a cross-enantiomeric impeding mechanism. Our results suggest a feasible scenario in which primitive homochiral peptides might have emerged early in the prebiotic world.

Nucleotide Catalyzed Conversion of Racemic Zeolite-Type Zincophosphate into Enantioenriched Crystals

Abstract: Homochirality is a ubiquitous feature in living systems and plays a crucial role in biological processes. In the biosphere, many naturally occurring biochemical molecules such as proteins, nucleic acids, and carbohydrates are homochiral.[1] In comparison, homochirality in the lithosphere and the inorganic world is a quite rare phenomenon. For example, even though quartz is chiral, it is found in nature in both right- and left-handed forms.[2] Also, unlike homochiral organic molecules,[3] chirality in the inorganic system often depends on the crystallization process which allows the generation of chirality through the organization of achiral inorganic units into the enantiomorphous space group. This crystallization process, however, almost always generates a mixture of right- and left-handed crystals, even though statistical fluctuations may occasionally result in enantioenriched bulk samples.[4]

Mechanism of Chirality Conversion by Grinding Crystals -Ostwald Ripening vs Crystallization of Chiral Clusters-

Abstract: It has been found that the simple grinding of crystals in a solution causes the conversion of the chiral structure of crystals and, in the case of organic molecules, the resultant conversion of molecular chirality. In order to clarify the mechanism of the chirality conversion, we study the change in the size distribution of chiral clusters in a Becker–Doring type model, in which chiral dimer reaction and grinding are implemented. In the absence of grinding, Ostwald ripening may cause chirality conversion if the initial size distribution is appropriate. With grinding crystals, the crystallization of chiral clusters causes an exponential amplification of the initial chiral imbalance resulting in a homochiral state. During chirality conversion, the total size distribution does not change.

Chirality Emergence in Thin Solid Films of Amino Acids by Polarized Light from Synchrotron Radiation and Free Electron Laser

Abstract: One of the most attractive hypothesis for the origin of homochirality in terrestrial bioorganic compounds is that a kind of “chiral impulse” as an asymmetric excitation source induced asymmetric reactions on the surfaces of such materials such as meteorites or interstellar dusts prior to the existence of terrestrial life (Cosmic Scenario). To experimentally introduce chiral structure into racemic films of amino acids (alanine, phenylalanine, isovaline, etc.), we irradiated them with linearly polarized light (LPL) from synchrotron radiation and circularly polarized light (CPL) from a free electron laser. After the irradiation, we evaluated optical anisotropy by measuring the circular dichroism (CD) spectra and verified that new Cotton peaks appeared at almost the same peak position as those of the corresponding non-racemic amino acid films. With LPL irradiation, two-dimensional anisotropic structure expressed as linear dichroism and/or linear birefringence was introduced into the racemic films. With CPL irradiation, the signs of the Cotton peaks exhibit symmetrical structure corresponding to the direction of CPL rotation. This indicates that some kinds of chiral structure were introduced into the racemic film. The CD spectra after CPL irradiation suggest the chiral structure should be derived from not only preferential photolysis but also from photolysis-induced molecular structural change. These results suggest that circularly polarized light sources in space could be associated with the origin of terrestrial homochirality; that is, they would be effective asymmetric exciting sources introducing chiral structures into bio-organic molecules or complex organic compounds.

Spontaneous Transition toward Chirality in the NaClO3 Crystallization in Boiling Solutions

Abstract: The crystallization of NaClO3 from supersaturated boiling solutions leads to a strong bias of enantiomorphic crystals of the same chiral sign, which in the range of the experimental errors cannot be distinguished from that of a homochiral crystal mixture. The crystallization reactor is a closed system but with a temperature gradient between the walls of the reactor and the air/liquid interface that entails an intense recycling of the subcritical nuclei formed during the induction period of the primary nucleation in the bulk. During this period, the evolution of the population of subcritical nuclei takes place without any other noticeable crystal growth process. In these experimental conditions, the fast evolution of a myriad of supercritical nuclei and the immediate separation of the crystals strongly suggest that the evolution toward homochirality should occur during the primary nucleation process. The formation of stationary chiral compositions in the closed system by recycling through the irreversible ...

Origin of Homochirality in Biosystems

Abstract: Experimental data for a series of central and simple molecules in biosystems show that some amino acids and a simple sugar molecule have a chiral discrimination in favor of homochirality. Models for segregation of racemic mixtures of chiral amphiphiles and lipophiles in aqueous solutions show that the amphiphiles with an active isomerization kinetics can perform a spontaneous break of symmetry during the segregation and self-assembly to homochiral matter. Based on this observation it is argued that biomolecules with a sufficiently strong chiral discrimination could be the origin of homochirality in biological systems.

Chiral discrimination in the confined environment of biological nanospace: reactions and interactions involving amino acids and peptides

Abstract: Although chiral discrimination in biological systems is overwhelmingly present, its molecular mechanism remained a puzzle. Why the basic blocks of life like L-amino acid and D-sugar are not being scrambled and retain enantiomeric purity since evolution is an unresolved question. In the present review we focus on the recent experimental and computational studies on the chiral discrimination in reactions such as peptide synthesis and aminoacylation. Experimental studies have shown that a clear homochiral preference exists favouring L-amino acid. Recent combined quantum mechanical/molecular mechanical studies explain the high level of stereospecificity of the processes and revealed multiple factors responsible for the discrimination and concomitant retention of the biological homochirality. Chirality of the relevant molecular segments and the intricate interaction between them as well as with the surrounding residues are important. The confinement of the chiral reactants within the biological nanospaces like...

Highly enantioselective asymmetric autocatalysis using chiral ruthenium complex-ion-exchanged synthetic hectorite as a chiral initiator.

Abstract: The synthetic hectorite containing intercalated chiral Δ- and Λ-tris(1,10-phenanthroline)ruthenium(II) ions acts as a heterogeneous chiral catalyst in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde to afford, in combination with asymmetric autocatalytic amplification of enantiomeric excess, 5-pyrimidyl alkanol with high enantiomeric excess.

Asymmetric autocatalysis : Automultiplication of chiral molecules

Abstract: Asymmetric autocatalysis (i.e. enantioselective 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. Pyrimidyl alkanol with extremely low enantiomeric excess automultiplies during three consecutive asymmetric autocatalyses with significant amplification of enantiomeric excess up to > 99.5 percent ee. The asymmetric outocotolysis has been applied to chiral discrimination of cryptochiral compounds. It also provides clues for clarifying the origin of homochirality. Circularly polarized light, inorganic chiral crystals, chiral crystals of achiral organic compounds and chiral compounds resulting from carbon isotope substitution act as chiral initiators in asymmetric autocatalysis affording highly enantioenriched products. Asymmetric autocatalysis in the reaction between pyrimidine-5-carboldehyde and diisopropylzinc has enormous power to amplify the statistically induced tiny enantiomeric imbalance (fluctuation of ee), thus, it is possible to form large amount of optically active compounds without the use of any chiral material. Spontaneous absolute asymmetric synthesis has been realized for the first time.

Selection of Crystal Chirality: Equilibrium or Nonequilibrium?

Abstract: To study the solution growth of crystals composed of chiral organic molecules, a spin-one Ising lattice gas model is proposed. The model turns out to be equivalent to the Blume-Emery-Griffiths model, which shows an equilibrium chiral symmetry breaking at low temperatures. The kinetic Monte Carlo simulation of crystal growth, however, demonstrates that Ostwald ripening is a very slow process with a characteristic time proportional to the system size: The dynamics is nonergodic. It is then argued that by incorporating grinding dynamics, homochirality is achieved in a short time, independent of the system size. Grinding limits cluster sizes to a certain range independent of system size and at the same time keeps the supersaturation so high that population numbers of average-sized clusters grow. If numbers of clusters for two types of enantiomers differ by chance, the difference is amplified exponentially and the system rapidly approaches the homochiral state. Relaxation time to the final homochiral state is determined by the average cluster size. We conclude that the system should be driven and kept in a nonequilibrium state to achieve homochirality.

Selection of Crystal Chirality: Equilibrium or Nonequilibrium?

Abstract: To study the solution growth of crystals composed of chiral organic molecules, a spin-one Ising lattice gas model is proposed. The model turns out to be equivalent to the Blume–Emery–Griffiths model, which shows an equilibrium chiral symmetry breaking at low temperatures. The kinetic Monte Carlo simulation of crystal growth, however, demonstrates that Ostwald ripening is a very slow process with a characteristic time proportional to the system size: The dynamics is nonergodic. It is then argued that by incorporating grinding dynamics, homochirality is achieved in a short time, independent of the system size. Grinding limits cluster sizes to a certain range independent of system size and at the same time keeps the supersaturation so high that population numbers of average-sized clusters grow. If numbers of clusters for two types of enantiomers differ by chance, the difference is amplified exponentially and the system rapidly approaches the homochiral state. Relaxation time to the final homochiral state is ...

Building blocks of life: Growing the seeds of homochirality

Abstract: Shining circularly polarized light into a suspension of racemic amino-acid-derivative crystals in a saturated solution and then grinding them results in enantiomerically pure crystals. This evolution is shown to be directed by an unknown chiral product.

The Double Solubility Rule Holds for Racemizing Enantiomers

Abstract: Chiral amnesia: Experimental results demonstrate that the double solubility rule indeed holds true for the solution-phase enantiomers of a conglomerate-forming solid under fast-racemizing conditions. There is neither experimental evidence for chiral recognition via solution-phase clusters nor a need to invoke them to produce a lucid explanation of the compelling model for the evolution of solid-phase homochirality first communicated by Viedma.

Inherent homochirality of primary particles and meteorite impacts as possible source of prebiotic molecular chirality

Abstract: A new hypothesis is presented that suggests super high velocity impacts (SHVI) of meteorites and collisions of other cosmic bodies as a universal source of nonracemic prebiotic organic matter. The hypothesis combines the notion by Davankov [29] on subatomic particles composing a homochiral pool of primary matter with the results by Managadze [49] on the synthesis of organic molecules in the plasma torch caused by a laser beam action on a solid target. The latter system serves as an adequate model for the plasma torch generated by SHVI of meteorites. New arguments are presented supporting the idea of inherent chirality of electrons and photons. Formation of chiral combinations of strong magnetic and electrostatic fields in the plasma torch is discussed, which could cause a chiral imbalance of molecules that self-assemble in the torch from the homochiral pool of atomic and subatomic species.

Physical and astrophysical aspects of the problem of origin of chiral asymmetry of the biosphere

Abstract: A brief survey of different models of the origin of chiral asymmetry of the biosphere is given Main attention is paid to the discussion of the radiation mechanism of chiral action and the possible role of supernovae in the cosmological scenario of the origin of biological homochirality

Absolute Asymmetric Synthesis

Abstract: Absolute asymmetric synthesis is the synthesis of optically active products from achiral or racemic precursors only. This has generally been regarded as impossible and is relevant in the discussion of the origins of biomolecular homochirality. A possible route to absolute asymmetric synthesis involves total spontaneous resolution, which is possible for stereochemically labile substances which crystallise as conglomerates (i.e. the enantiomers crystallise in separate crystals). Using total spontaneous resolution it was, for the first time, possible to prepare bulk-quantities of configurationally labile five-, seven-, and nine-coordinate enantiomers, containing only achiral ligands. Previously, only four- and six-coordinate complexes have been prepared enantiomerically pure in bulk quantities. Spontaneous resolution of eight-coordinate complexes has also been reported. It was also possible to perform total spontaneous resolution of a diaryl sulphide, an octanuclear organo(oxo)zinc complex, and a diindenylzinc complex. In the case of a helical coordination polymer based on copper(I) chloride and triallylamine, it was found that repeated synthesis always yielded an excess of the same enantiomer, possibly due to the influence of cryptochirality. It has previously been practically impossible to measure enantiomeric excesses in stereochemically labile microcrystalline samples. A method utilising quantitative solid-state CD spectroscopy has been introduced to solve this problem. In the case of the chiral organometallic reagent di(3-picoline)di(1-indenyl)zinc, it was possible to perform reactions with N-chlorosuccinimide in the presence of methanol and p-benzoquinone yielding optically active stereochemically inert 1-chloroindene in high yield and high enantiomeric excess (up to 89% ee). During the cause of theses studies, three cases of concomitant crystallisation of racemic and chiral phases have been discovered. This is a rare phenomenon of considerable interest e.g. in structure prediction. The first synthetic route to well-defined hydridoalkylzincates is also reported.

Enantiomeric excesses in the Murchison meteorite and the origin of homochirality in terrestrial biology

Abstract: Breslow and Cheng have recently proposed a scheme for how the enantiomeric excesses (ee) found for some non-protein amino acids in the Murchison carbonaceous chondrite might have been involved in the origin of biochemical homochirality on Earth (1). However, there is a central problem with their scenario: it would be difficult for any ee in non-protein amino acids such as isovaline to be efficiently delivered to the early Earth in significant amounts.

Origin of Homochirality

Abstract: Chirality, the property of an object to be non-superimposable with its mirror image, is a common feature of biological molecules and has intrigued chemists for a long time. Of all the possible combinations, nature uses nearly exclusively L-amino acids and D-sugars and most other chiral molecules in enantiomerically pure form, too. This biological homochirality is regarded as a prerequisite of life as we know it, but how it evolved from a presumably racemic prebiotic Earth remains an open question. It is therefore linked to the general question of the origin of life on Earth itself. An overview is given on this topic focussing on recent experimental results, theoretical models and scenarios that might hold the clues to one of the biggest puzzles of science.

Theory of chiral recognition in DNA condensation

Abstract: Collapse of long chiral A-macromolecules assisted by a chiral B-additive (where B associates with A) is considered theoretically. Based on a simple model we demonstrate that the compaction activity of B molecules can strongly depend on the sense of their chirality. The theory is applied to the DNA compaction in the presence of conformationally rigid divalent enantiomeric cations (SS and RR enantiomers).1 It is shown that both chiral isomers of the dications adsorb well on the DNA surface, but the SS- and RR-binding energies are different, reflecting their unequal stereo-complementarities (the ‘bolt-and-nut’ effect). Unexpectedly, we find that the compaction activity of less strongly bound SS dication is significantly higher than that of its RR enantiomer. Moreover, the RR molecules tend to replace the SS enantiomers adsorbed on the DNA, so that SS compaction ability is significantly reduced in the presence of RR isomers. The theory thus provides a basis for explanation of recently observed dramatic stereoisomeric selectivity and antagonistic effects in DNA compaction.1 The revealed chiral discrimination effects may serve as selection mechanisms leading to natural homochirality.

Fluctuation Induced Homochirality in an Open System

Abstract: We study a possibility for chiral symmetry breaking to take place without autocatalytic process in an open system. Our model consists of an inflow of enantiomers and outflow of heterogeneous dimers from the system, which corresponds to the Frank model without autocatalysis. It is demonstrated analytically as well as numerically that the rate equations for this system does not predict any kind of symmetry breaking but the stochastic master equation predicts that the system approaches asymptotically a homochiral state. This is caused by the fluctuations induced by the discreteness of population numbers of enantiomers in their perpetual flow. It is argued that this scenario can be effective for a very small open system and could be a candidate for an initial chiral imbalance.

On biological homochirality

Abstract: Generalizing Landau's spontaneous symmetry breaking arguments using the standard groupoid approach to stereochemistry allows reconsideration of the origin of biological homochirality. On Earth, limited metabolic free energy density may have served as a low temperature analog to 'freeze' the system into the set of simplest homochiral transitive groupoids representing reproductive chemistries. These engaged in Darwinian competition until a single configuration survived. Subsequent path dependent evolutionary process licked in this initial condition. Astrobiological outcomes, in the presence of higher initial metabolic free energy densities, could well be considerably richer, perhaps of mixed chirality. One result would be a complicated distribution of biological chirality across a statistically large sample of extraterrestrial stereochemistry, in contrast with a recent prediction of a racemic average.

Homochiral Selection in a Non-equilibrium Process: Origin of Life

Abstract: Chemistry and therefore life is a manifestation of the electromagnetic interaction which cannot distinguish rightfrom left, and all chemical reactions in the absence of an external chiral bias produce equal amounts of right and left-handed molecules. Nevertheless, the living matter is chiral biased and generally constituted of only one of the enantiomers, right handed or left handed. The origin of biochemical chirality remains an unresolved problem believed to be concurrent or an essential prerequisite to origion of life. Most hypotheses attempting to understand this issue, attribute homochiral selection to a biasing influence and presupposition that asymmentry can be begotten onlyfrom asymmetry. A system in thermodynamic equilibrium in the absence of a constant external chiral influence is indeed racemic and contain equal proportions of both enantiomers. Living systems and the origin of life itself are not equilibrium situations and could deviate from racemicity. A mathematical model ispresented to show that any enant iomeric selfreplicating molecule created in the racemic prebiotic medium can proliferate exhausting all precursor molecules of both handedness. The homo chirality of life is a continuation of this non-equilibrium process.