Showing papers on "Homochirality published in 2010"
59 citations
TL;DR: It is shown that reactant precipitation observed in the earlier reported experiments does not affect the asymmetric autocatalysis in the aldol reaction the authors studied and the phenomenon of spontaneous mirror symmetry breaking in such organocatalytic homogenous systems qualitatively is explained.
Abstract: The origin of the single chirality of most biomolecules is still a great puzzle. Carbohydrates could form in the formose reaction, which is proposed to be autocatalytic and contains aldol reaction steps. Based on our earlier observation of organoautocatalysis and spontaneous enantioenrichment in absence of deliberate chiral influences in the aldol reaction of acetone and p-nitrobenzaldehyde we suggest that a similar effect might be present also in the aldol reactions involved in gluconeogenesis. Herein we show that reactant precipitation observed in our earlier reported experiments does not affect the asymmetric autocatalysis in the aldol reaction we studied. We explain the phenomenon of spontaneous mirror symmetry breaking in such organocatalytic homogenous systems qualitatively by non-linear reaction network kinetics and classical transition state theory.
48 citations
TL;DR: Symmetry-breaking phenomena in two-dimensional crystallization at surfaces are reviewed and the potential impact to chiral amplification in three-dimensional systems in connection with the origin of homochirality in the biomolecular world is discussed.
Abstract: Symmetry-breaking phenomena in two-dimensional crystallization at surfaces are reviewed and the potential impact to chiral amplification in three-dimensional systems in connection with the origin of homochirality in the biomolecular world is discussed. Adsorption of prochiral molecules leads to two-dimensional conglomerates, i.e., on a local scale spontaneously to homochiral crystal structures. Small enantiomeric excess or chiral impurities in this environment install homochirality on a global scale, that is, on the entire surface.
42 citations
TL;DR: Acceptance of the possibility of “two equal runners” leads to the importance of research investigations on routes to formation of ensembles of racemic mixtures of isotactic biologically interesting polymers, supramolecular entities and aggregates.
Abstract: Strong arguments can be found in the literature addressed to the question of the origin of homochirality in life, supporting the hypothesis that primordial life could have evolved in both homochiral forms and that early on when life was still rarely found, random events led to the survival of only one of these living mirror images. This proposal is an alternative to the generally accepted view that small enantiomeric excesses of biologically important molecules were amplified to homochirality prior to life’s origin. Acceptance of the possibility of “two equal runners” leads to the importance of research investigations on routes to formation of ensembles of racemic mixtures of isotactic biologically interesting polymers, supramolecular entities and aggregates.
37 citations
TL;DR: In this paper, the ratio between the saturated vapor pressures of the racemate and the pure enantiomers of three enantiomerically enriched amino acids was found to explain the sublimation of the enantioms.
36 citations
TL;DR: The origin of homochirality of organic compounds such as L-amino acids and D-sugars have intrigued many scientists, and several hypotheses regarding its homochIRality have been proposed as discussed by the authors.
Abstract: The origin of homochirality of organic compounds such as L-amino acids and D-sugars have intrigued many scientists, and several hypotheses regarding its homochirality have been proposed. According to the statistical theory, small fluctuations in the ratio of the two enantiomers are present in a racemic mixture obtained from the reaction of achiral molecules.
35 citations
TL;DR: The Mannich reactions previously extensively investigated with organocatalysis of l-proline and other related small molecules were reinvestigated with detailed stereochemical analysis of their autoc atalysis pathways, through employment of both the products themselves and their close structural mimics as the catalysts.
Abstract: The Mannich reactions previously extensively investigated with organocatalysis of l-proline and other related small molecules were reinvestigated with detailed stereochemical analysis of their autocatalysis pathways, through employment of both the products themselves and their close structural mimics as the catalysts. These organo-autocatalytic processes function as meaningful molecular models toward understanding the origin and maintenance of homochirality under biologically relevant conditions.
34 citations
TL;DR: In this article, the authors described the amplification of ee in asymmetric autocatalysis of 5-pyrimidyl alkanol in the reaction between diisopropylzinc and pyrimidine-5-carbaldehdye.
31 citations
TL;DR: It is shown that the homochiral compound has two effects on the solubility of the racemate, which explains why the amplification, while large, is not as large as the simple theoretical equation predicts.
Abstract: We show how the amino acids needed on prebiotic earth in their homochiral L form can be produced by a reaction of L-alpha-methyl amino acids—that have been identified in the Murchison meteorite—with alpha-keto acids under credible prebiotic conditions. When they are simply heated together they perform a process of decarboxylative transamination but with almost no chiral transfer, and that in the wrong direction, producing D-amino acids from the L-alpha-methyl amino acids. With copper ion a square planar complex with two of the reaction intermediates is formed, and now there is the desired L to L transformation, producing small enantioexcesses of the normal L-amino acids. We also show how these can be amplified, not by making more of the L form but by increasing its concentration in water solution. The process can start with a miniscule excess and in one step generate water solutions with L/D ratios in the over 90% region. Kinetic processes can exceed the results from equilibria. We have also examined such amplifications with ribonucleosides, and have shown that initial modest excesses of the D-nucleosides can be amplified to afford water solutions with D to L ratios in the high 90’s. We have shown that the homochiral compound has two effects on the solubility of the racemate. On one hand it decreases the solubility of the racemate by its role in the solubility product, as a theoretical equation predicts. On the other hand, it increases the solubility of the racemate by changing the nature of the solvent, acting as a cosolvent with the water. This explains why the amplification, while large, is not as large as the simple theoretical equation predicts. Thus when credible examples are produced where small enantioexcesses of D-ribose are created under credible prebiotic conditions, the prerequisites for the RNA world will have been exemplified.
30 citations
TL;DR: Recent studies on enantioselective photolysis of racemic amino acids by circularly polarized light and experiments on the asymmetric photochemical synthesis of amino acids from only one C and one N containing molecules by simulating interstellar environments are highlighted.
Abstract: Current research focuses on a better understanding of the origin of biomolecular asymmetry by the identification and detection of the possibly first chiral molecules that were involved in the appearance and evolution of life on Earth. We have reasons to assume that these molecules were specific chiral amino acids. Chiral amino acids have been identified in both chondritic meteorites and simulated interstellar ices. Present research reasons that circularly polarized electromagnetic radiation was identified in interstellar environments and an asymmetric interstellar photon-molecule interaction might have triggered biomolecular symmetry breaking. We review on the possible prebiotic interaction of ‘chiral photons’ in the form of circularly polarized light, with early chiral organic molecules. We will highlight recent studies on enantioselective photolysis of racemic amino acids by circularly polarized light and experiments on the asymmetric photochemical synthesis of amino acids from only one C and one N containing molecules by simulating interstellar environments. Both approaches are based on circular dichroic transitions of amino acids that will be presented as well.
29 citations
TL;DR: The emergence of phosphoryl amino acid 5′-nucleosides having a P–N bond is described as a model of the origin of amino acid homochirality and Genetic Code and it is proposed that the intramolecular interaction between the nucleotide base and the amino acid side-chain influences the stability of particular amino acid5′- nucleotides, and the interaction also selects for the chirality of amino acids.
Abstract: Phosphorylation has to have been one of the key events in prebiotic evolution on earth. In this article, the emergence of phosphoryl amino acid 5′-nucleosides having a P–N bond is described as a model of the origin of amino acid homochirality and Genetic Code. It is proposed that the intramolecular interaction between the nucleotide base and the amino acid side-chain influences the stability of particular amino acid 5′-nucleotides, and the interaction also selects for the chirality of amino acids. The differences between l- and d-conformation energies (ΔE
conf) are evaluated by DFT methods at the B3LYP/6-31G(d) level. Although, as expected, these ΔE
conf values are not large, they do give differences in energy that can distinguish the chirality of amino acids. Based on our calculations, the chiral selection of the earliest amino acids for l-enantiomers seems to be determined by a clear stereochemical/physicochemical relationship. As later amino acids developed from the earliest amino acids, we deduce that the chirality of these late amino acids was inherited from that of the early amino acids. This idea reaches far back into evolution, and we hope that it will guide further experiments in this area.
TL;DR: The discovery of asymmetric organoautocatalysis, the process in which the chiral product of a purely organic reaction acts as an organocatalyst for its own formation under conservation of its absolute configuration, and more recent reports on related systems are discussed.
TL;DR: The essay is dedicated to the relation of symmetry and asymmetry-chirality in Nature, and the analysis of very special cases of homomers revealing non-crystallographic symmetry in crystals.
Abstract: The essay is dedicated to the relation of symmetry and asymmetry-chirality in Nature. The Introduction defines symmetry and its impact on basic definitions in science and human activities. The following section Chirality of molecules reveals breifly development of notion of chirality and its significance in living organisms and science. Homochirality is a characteristic hallmark of life and its significance is presented in the section Homochirality of Life. Proteins, important constituents of living cells performing versatile functions are chiral macromolecules composed of L-amino acids. In particular, the protein assemblies are of a great importance in functions of a cell. Therefore, they have attracted researches to examine them from different points of view. Among proteins of known three-dimensional structures about 50–80% of them exist as homomeric protein complexes. Protein monomers lack any intrinsic, underlying symmetry, i.e. enantiomorphic protein molecules involve left-handed amino acids but their asymmetry does not appear to extend to the level of quaternary structures (homomeric complexes) as observed by Chothia in 1991. In the section Homomeric assemblies we performed our analysis of very special cases of homomers revealing non-crystallographic symmetry in crystals. Homochiral proteins can crystallize only in enantiomorphic space groups. Among 230 existing space groups 65 are enantiomorphic containing limited symmetry elements that are rotation and screw-rotation axes. Any axis of rotation symmetry of a crystal lattice must be two-fold, three-fold, four-fold, or six-fold. Five-fold, seven-fold, and higher-fold rotation symmetry axes are incompatible with the symmetry under spatial displacement of the three-dimensional crystal lattice.
TL;DR: In order to predict whether the interaction of circularly polarized light with various racemic amino acids can induce an enantiomeric excess, this work investigated the electronic and chiroptical properties of the amino acids valine and isovaline by a molecular modelling approach based on quantum chemistry (Density Functional Theory).
Abstract: Amino acids are involved in many scientific theories elucidating possible origins of life on Earth. One of the challenges when discussing the evolutionary origin of biopolymers such as proteins and oligonucleotides in living organisms is the phenomenon that these polymers implement monomers of exclusively one handedness, a feature called biomolecular homochirality. Many attempts have been made to understand this process of racemic symmetry breaking. Assuming an extraterrestrial origin of the molecular building blocks of living organisms, their susceptibility to asymmetric photolysis by the absorption of circularly polarized electromagnetic radiation in interstellar space was proposed. In order to predict whether the interaction of circularly polarized light with various racemic amino acids can induce an enantiomeric excess, we investigated the electronic and chiroptical properties of the amino acids valine and isovaline by a molecular modelling approach based on quantum chemistry (Density Functional Theory). The average spectra of both L-valine and L-isovaline have been produced on the basis of Boltzmann population analysis using computed spectra for the various conformations of each amino acid.
TL;DR: It is proposed that a mixture of racemic amino acids in the prebiotic sea caused spontaneous and effective optical resolution through self crystallization, even if asymmetric synthesis of a single amino acid has never occurred without the aid of an optically active molecule.
Abstract: Discussions are made concerning realistic mechanisms for the origin of L-amino acids in the biosphere. As the most plausible mechanism, it is proposed that a mixture of racemic amino acids in the prebiotic sea caused spontaneous and effective optical resolution through self crystallization, even if asymmetric synthesis of a single amino acid has never occurred without the aid of an optically active molecule. This hypothesis is based on recrystallization of a mixture of D,L-amino acids in the presence of excess of D,L-asparagine (Asn). The enantiomeric excess (ee) of each amino acid in the resulting crystals indicates that crystallization of co-existing amino acids with the configuration same as that of Asn took place, although it was incidental whether the enrichment occurred in L- or D-amino acids. In addition, the resulting ee was sufficiently high (up to 100%) to account for the predominance of L-amino acids on the earth.
TL;DR: In this article, an aldol reaction catalyzed by a proline-thiourea host-guest complex in a nonpolar solvent shows excellent nonlinear effects.
Abstract: An aldol reaction catalyzed by a proline–thiourea host–guest complex in a nonpolar solvent shows excellent nonlinear effects. This proline–thiourea system has the ability to form a hydrogen-bonding network. The enantiomeric excess of proline in a solution can be significantly enhanced by its incorporation with a urea molecule into its solid racemate. This suggests a general and facile route to homochirality, which may be involved in the origin of chirality on earth.
TL;DR: In this paper, a simple irreversible growth model of a lattice-gas with four possible states on a site: occupied by an achiral molecule A, or by a chiral enantiomer R or S, or empty.
Abstract: To explore the mechanism of chiral symmetry breaking in a process of crystal growth under grinding, we propose a simple irreversible growth model of a lattice-gas with four possible states on a site: occupied by an achiral molecule A, or by a chiral enantiomer R or S, or empty. After two A molecules on neighboring sites form a chiral dimer R 2 or S 2 , clusters grow by incorporating A's at cluster periphery, irreversibly. Only the grinding recycles products R or S back to A. It is then demonstrated in kinetic Monte Carlo (KMC) simulations that chirality selection takes place in the presence of the grinding. The cause for this realization is attributed to mutual antagonistic inhibition: that is, clusters of opposite enantiomeric types are brought into contact through stirring, and they block crystallization sites on cluster peripheries each other. The density evolution obtained by time integration of the rate equations with this antagonistic inhibition fits well with results of KMC simulations.
TL;DR: In this article, the enantiomeric composition of different carboxylic acids estimated by the 1H NMR method, based on the integration of the corresponding methine proton signals are in good correlation with that determined using HPLC method.
Abstract: Efficient use of the readily accessible chiral C
2-symmetric acyclic diamines (1–2) as well as macrocyclic amines (3–5) containing trans-1,2-diaminocyclohexyl moiety as chiral solvating agents (CSA) for the determination of enantiomeric excess of representative carboxylic acids (6–7) and an amino acid derivative (8) is illustrated. The enantiomeric composition of different carboxylic acids estimated here by the 1H NMR method, based on the integration of the corresponding methine proton signals are in good correlation with that determined using HPLC method. The data are in accordance with the formation of multimolecular diastereomeric complexes in solution, which render good splitting of NMR signals for the enantiomers of representative carboxylic acids as well as for N-Ts-phenylglycine (up to ΔΔδ = 0.295 ppm, 118 Hz).
TL;DR: A chiral ionic liquid (CIL) based on nicotinium salt has been synthesized and used as an efficient asymmetric chiral catalyst for reduction of acetophenone derivatives with NaBH4 in methanol at room temperature as discussed by the authors.
TL;DR: In this article, the authors 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.
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.
TL;DR: In this paper, the complexation of optically active or racemic amines, amino esters and amino acids with chiral ruthenium Halterman porphyrins is described.
Abstract: The complexation of optically active or racemic amines, amino esters and amino acids with chiral ruthenium Halterman porphyrins is described. For various amino esters, chiral recognition was observed for the complexation of the ligand with up to 60% enantiomeric excess for 1-(1-naphthyl) ethylamine. A water-soluble ruthenium Halterman porphyrin, due to the presence of four sulfonate groups at the para-positions, was also prepared and used for the chiral recognition of amino acids in water.
TL;DR: A series of β-amino alcohols derivatives were synthesized from (R)-2amino-1-butanol and (S)-1,2-propanediol, and they have been used as organocatalaysts in the racemic ring opening of epoxide.
TL;DR: Using a simple, second-order chiral autocatalytic core model, it is shown that all these scenarios lead to similar mathematical description despite the fundamentally different chemical background and practically complete homochirality is reached in all of these cases.
TL;DR: The origin of homochirality in life remains a mystery that some believe is essential for life, and which may result from chiral symmetry breaking interactions with galactic organic material.
Abstract: The origin of homochirality in life remains a mystery that some believe is essential for life, and which may result from chiral symmetry breaking interactions with galactic organic material.
TL;DR: The present study should encourage laboratory experiments to determine rotational constants of higher precision prior to submission of observation proposals, as this report addresses the question of whether some chiral molecules have a probability of being detected in the interstellar medium (ISM).
Abstract: In this report we address the question of whether some chiral molecules have a probability of being detected in the interstellar medium (ISM). To this end we rely on the Minimum Energy Principle which states that the most abundant isomer of a given generic formula should be that of lowest energy. The relative stability of the chiral molecules with respect to the other possible species of the same chemical formula are calculated by means of quantum simulations based on density functional theory (DFT). The result is that no chiral isomer in the C3H6O (acetone), C2H5ON, C3H7ON (amide), C2H5O2N, C3H7O2N (amino acid) families is the most stable species. This is also true of the C2(H2O)2 and C3(H2O)3 species when restricted to the sugar families, but another chiral molecule of the same chemical formula, i.e. lactic acid HOCH(CH3)COOH is the most stable of all structures. Two other molecules with an NH2 group, namely, NH2CH(CH3)CN, the precursor of α-alanine and NH2CH(CH3)OH, the simplest chiral molecule, are also the most stable species in their respective families. These three molecules satisfy the conditions for being detected according to the Minimum Energy Principle. With dipoles moments of 2.3, 2.7 and 1.6 Debye respectively, they make appealing targets. The present study should encourage laboratory experiments to determine rotational constants of higher precision prior to submission of observation proposals.
TL;DR: In this article, the molecular chirality in crystal was used as a source of homochirality for solid-state photoreactions using chiral crystals composed of achiral materials and leading to optically active products.
Abstract: Many examples of solid-state photoreactions using chiral crystals composed of achiral materials and leading to optically active products have been successfully demonstrated and recognized as absolute asymmetric synthesis. Recently, we have discovered a new methodology for asymmetric synthesis using the molecular chirality in crystal as a source of homochirality in fluid media. The chirality can be effectively transferred to optically active products by asymmetric reactions involving nucleophilic reactions and an intermolecular photochemical reaction, and which had so far not been achieved in solid state reactions.
TL;DR: A novel solution to the homochirality problem based on a recently proposed thermodynamic dissipation theory for the origin of life is suggested to have been incorporated gradually into the emerging life as a result of asymmetric right- over left-handed photon-induced denaturation of RNA/DNA occurring when Archean sea surface temperatures became close to the denaturing temperatures ofRNA/DNA.
Abstract: The homochirality of the molecules of life has been a vexing problem with no generally accepted solution to date. Since a racemic mixture of chiral nucleotides frus- trates the extension and replication of RNA and DNA, un- derstanding the origin of homochirality has important impli- cations to the investigation of the origin of life. Theories on the origin of life have generally elected to presume an abi- otic mechanism giving rise to a large prebiotic enantiomer enrichment. Although a number of such mechanism have been suggested, none has enjoyed sufficient plausibility or relevance to be generally accepted. Here we suggest a novel solution to the homochirality problem based on a recently proposed thermodynamic dissipation theory for the origin of life. The ultraviolet absorption and dissipation characteris- tics of RNA/DNA point to their origin as photoautorophs, their replication assisted by UV light and temperature, and acting as catalysts for the global water cycle. Homochi- rality is suggested to have been incorporated gradually into the emerging life as a result of asymmetric right- over left- handed photon-induced denaturation of RNA/DNA occur- ring when Archean sea surface temperatures became close to the denaturing temperatures of RNA/DNA. This differential denaturing success would have been promoted by the some- what right-handed circularly polarized submarine light of the late afternoon when surface water temperatures are high- est, and a negative circular dichroism band extending from 220 nm up to 260 nm for small segments of RNA/DNA. A numerical model is presented demonstrating the efficacy of such a mechanism in procuring 100% homochirality of RNA or DNA from an original racemic solution in less than 500 Archean years assuming a photon absorption threshold for replication representing the hydrogen bonding energies be- tween complimentary strands. Because cholesteric D-nucleic acids have greater affinity for L-amino acids due to a positive structural complementarity, and because D-RNA/DNA+L- amino acid complexes also have a negative circular dichro- ism band between 200 - 300 nm, the homochirality of amino acids can also be explained by the theory.
TL;DR: In this paper, two two-dimensional homochiral Ag (I) metal frameworks constructed from enantiopure camphor-10-sulfonic acid and hexamethylenetetramine have been synthesized at the room temperature.
Posted Content•
TL;DR: In this paper, a thermodynamic dissipation theory for the origin of life is proposed, where homochirality is assumed to have been incorporated gradually into the emerging life as a result of asymmetric right- over left-handed photon-induced denaturation of RNA/DNA occurring when Archean sea surface temperatures became close to the denaturing temperatures of RNA and DNA.
Abstract: The homochirality of the molecules of life has been a vexing problem with no generally accepted solution to date. Since a racemic mixture of chiral nucleotides frustrates the extension and replication of RNA and DNA, understanding the origin of homochirality has important implications to the investigation of the origin of life. Here we suggest a novel solution to the homochirality problem based on a recently proposed thermodynamic dissipation theory for the origin of life. Homochirality is suggested to have been incorporated gradually into the emerging life as a result of asymmetric right- over left-handed photon-induced denaturation of RNA/DNA occurring when Archean sea surface temperatures became close to the denaturing temperatures of RNA/DNA. This differential denaturing success would have been promoted by the somewhat right-handed circularly polarized submarine light of the late afternoon when surface water temperatures are highest, and a negative circular dichroism band extending from 220 nm up to 260 nm for small segments of RNA/DNA. A numerical model is presented demonstrating the efficacy of such a mechanism in procuring 100% homochirality of RNA or DNA from an original racemic solution in less than 500 Archean years assuming a photon absorption threshold for replication representing the hydrogen bonding energies between complimentary strands. Because cholesteric D-nucleic acids have greater affinity for L-amino acids due to a positive structural complementarity, and because D-RNA/DNA+L-amino acid complexes also have a negative circular dichroism band between 200 - 300 nm, the homochirality of amino acids can also be explained by the theory.