Meteoritic C?-Methylated ?-Amino Acids and the Homochirality of Life: Searching for a Link

Supramolecular chirality of coordination polymers of Ag+ with a chiral thiol ligand that bears a β-turn structure

The coordination polymers of Ag⁺ with a β-turn containing chiral thiol ligand exhibit supramolecular chirality showing simultaneously the majority rules effect (MRE) and racemate rules effect (RRE).

Homochirality of β-Peptides: A Significant Biomimetic Property of Unnatural Systems

Homochirality, an interesting phenomenon of life, is mainly an unresolved problem and was thought to be a property of living matter. Herein, we show that artificial β-peptides have the tendency toward homochiral diastereoselective chain elongation. Chain-length-dependent stereochemical discrimination was investigated in the synthesis of foldamers with various side chains and secondary structures. It was found that there is a strong tendency toward the synthesis of homochiral oligomers. The size of the side chain drastically influenced the selectivity of the stereodiscriminative chain-elongation reaction. It is noteworthy that water as the co-solvent increases the selectivity. Such behavior is a novel fundamental biomimetic property of foldamers with a potential of future industrial application.

Helical screw-sense preferences of peptides based on chiral, Cα-tetrasubstituted α-amino acids

The preferred helical screw senses of chiral α-amino acids with a C(α)-tetrasubstituted α-carbon atom, as determined in the crystal state by X-ray diffraction analyses on derivatives and peptides, are reviewed. This survey covers C(α)-methylated and C(α)-ethylated α-amino acids, as well as α-amino acids cyclized on the α-carbon, including those characterized by the combination of lack of chirality at the α-carbon with either side-chain or axial chirality. Although, in general, chiral C(α)-tetrasubstituted α-amino acids show a less pronounced bias toward a single helical screw sense than their proteinogenic (C(α)-trisubstituted) counterparts, our analysis highlights significant differences in terms of magnitude and direction of such a bias among the various sub-families of residues, and between individual amino acids within each sub-family as well. The experimental findings can be rationalized, at least in part, on the basis of steric considerations.

Screw sense alone can govern enantioselective extension of a helical peptide by kinetic resolution of a racemic amino acid

Helical secondary structure alone, even in the absence of local chiral residues, can direct the enantioselectivity of peptide coupling.

Exploring the consequences of a representative "disallowed" conformation of Aib on a 3₁₀-helical fold

The structural effects of a representative "disallowed" conformation of Aib on the 3(10)-helical fold of an octapeptidomimetic are explored. The 1D ((1)H, (13)C) & 2D NMR, FT-IR and CD data reveal that the octapeptide 1, adopts a 3(10)-helical conformation in solution, as it does in its crystal structure. The C-terminal methyl carboxylate (CO2Me) of 1 was modified into an 1,3-oxazine (Oxa) functional group in the peptidomimetic 2. This modification results in the stabilization of the backbone of the C-terminal Aib (Aib*-Oxa) of 2, in a conformation (ϕ, ψ = 180, 0) that is natively disallowed to Aib. Consequent to the presence of this natively disallowed conformation, the 3(10)-helical fold is not disrupted in the body of the peptidomimetic 2. But the structural distortions that do occur in 2 are primarily in residues in the immediate vicinity of the natively disallowed conformation, rather than in the whole peptide body. Non-native electronic effects resulting from modifications in backbone functional groups can be at the origin of stabilizing residues in natively disallowed conformations.

5(4H)-oxazolones as intermediates in the carbodiimide- and cyanamide-promoted peptide activations in aqueous solution

The early days: although considered a species to be avoided in peptide chemistry, the intermediacy of 5(4H)-oxazolones is demonstrated to be essential for the formation of peptides through cyanamide and carbodiimide activation in aqueous solution.

Activation of carboxyl group with cyanate: peptide bond formation from dicarboxylic acids

The reaction of cyanate with C-terminal carboxyl groups of peptides in aqueous solution was considered as a potential pathway for the abiotic formation of peptide bonds under the condition of the primitive Earth. The catalytic effect of dicarboxylic acids on cyanate hydrolysis was definitely attributed to intramolecular nucleophilic catalysis by the observation of the 1H-NMR signal of succinic anhydride when reacting succinic acid with KOCN in aqueous solution (pH 2.2-5.5). The formation of amide bonds was noticed when adding amino acids or amino acid derivatives into the solution. The reaction of N-acyl aspartic acid derivatives was observed to proceed similarly and the scope of the cyanate-promoted reaction was analyzed from the standpoint of prebiotic peptide formation. The role of cyanate in activating peptide C-terminus constitutes a proof of principle that intramolecular reactions of adducts of peptides C-terminal carboxyl groups with activating agents represent a pathway for peptide activation in aqueous solution, the relevance of which is discussed in connexion with the issue of the emergence of homochirality.

An experimental investigation of the evolution of chirality in a potential dynamic peptide system: N-terminal epimerization and degradation into diketopiperazine

The APED model (activation-polymerization-epimerization-depolymerization) is a unique example of a chemical system that allows symmetry breaking through a dynamic process involving indirect network autocatalysis. In its simplest version, the autocatalytic behavior of this model partly relies on the reproduction of local chiral centers in dipeptides through an epimerization process, with a thermodynamic preference for homochiral chains. We studied the reactivity of di- and tripeptides, containing a N-terminal phenylglycine (Phg) residue, as model compounds for the experimental determination of the kinetic and thermodynamic parameters related to the N-terminal epimerization process. Although the N-terminal residue is prone to spontaneous epimerization, catalysis was required for the epimerization to reach the equilibrium state in reasonable time. Unexpectedly, the observed equilibrium diastereoisomeric excesses have shown a general tendency for more stable heterochiral peptides, especially strong in the case of dipeptides. In parallel to this process, a stereoselective peptide cleavage through diketopiperazine formation was observed. Contrary to the N-terminal epimerization of peptides, the diketopiperazine formation did not need any catalyst, and heterochiral peptides were shown to be dynamically unstabilized, as they were cleaved faster than homochiral peptides. The validity of the extrapolation of these results to other residues and longer peptide chains is discussed, and some directions for future developments of the theoretical model are given.

The organic composition of carbonaceous meteorites: the evolutionary story ahead of biochemistry

Carbon-containing meteorites provide a natural sample of the extraterrestrial organic chemistry that occurred in the solar system ahead of life's origin on the Earth. Analyses of 40 years have shown the organic content of these meteorites to be materials as diverse as kerogen-like macromolecules and simpler soluble compounds such as amino acids and polyols. Many meteoritic molecules have identical counterpart in the biosphere and, in a primitive group of meteorites, represent the majority of their carbon. Most of the compounds in meteorites have isotopic compositions that date their formation to presolar environments and reveal a long and active cosmochemical evolution of the biogenic elements. Whether this evolution resumed on the Earth to foster biogenesis after exogenous delivery of meteoritic and cometary materials is not known, yet, the selective abundance of biomolecule precursors evident in some cosmic environments and the unique L-asymmetry of some meteoritic amino acids are suggestive of their possible contribution to terrestrial molecular evolution.

On-line sample stacking of peptides in capillary electrophoresis for the study of prebiotic reactions between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides

The reaction between alpha,alpha-dialkylated amino acids and amino acid N-carboxyanhydrides is slow leading to low concentrations of products (peptides). The detection by capillary electrophoresis of the analytes contained in such samples is therefore a challenging issue. In this work, on-line sample pre-concentration methods based on field-amplified sample stacking have been implemented and compared. Because of the high ionic strength present in the sample matrix, samples were diluted with an organic solvent prior to analysis to decrease the sample conductivity. Different modes of sample injection (field amplified sample injection (FASI), hydrodynamic normal sample stacking (NSS) or large volume sample stacking (LVSS)) were compared. Pre-concentration factors of 20 for FASI, about 30-40 for NSS and 60 for LVSS were obtained for the analysis of (l,l) dipeptide of valine in a large excess of isovaline and 0.2M of ionic strength. For LVSS application and resolution optimisation, a new non-covalent coating based on the partial modification of the capillary surface was used to tune the electroosmotic flow magnitude and to pump the sample matrix out of the capillary. This on-line sample pre-concentration step allowed confirming that oligopeptides including alpha,alpha-dialkylated amino acids are formed during the reaction between alpha,alpha-dialkylated amino acids and N-carboxyanhydride amino acids.

Detection of the chirality of C alpha-methylated alpha-amino acids with a dynamic helical poly(phenylacetylene) bearing aza-18-crown-6 ether pendants

A stereoregular poly(phenylacetylene) bearing the aza-18-crown-6 ether pendants (poly-1) was found to form a predominantly one-handed helix upon complexation with optically active C(alpha)-methylated alpha-amino acids and their amide derivatives including typical meteoritic C(alpha)-methylated alpha-amino acids such as C(alpha)-methyl norvaline and C(alpha)-methyl valine. The complexes exhibited an induced circular dichroism (ICD) in the UV-visible region of the polymer backbone. Therefore, poly-1 can be used as a novel probe for detection of the chirality of C(alpha)-methylated alpha-amino acids. The effect of the enantiomeric excess (ee) of C(alpha)-methylated alpha-amino acids on the helicity induction in poly-1 was also investigated.

The chemistry that preceded life's origin: a study guide from meteorites

Carbonaceous meteorites are rare fragments of asteroids that contain organic carbon of diverse composition, various complexity, and whose lineage can in several instances be traced back to pre-solar environments. Their analyses offer a unique glimpse into the chemistry of the solar system that preceded life and may have been available to its emergence on the early Earth. While the heterogeneity of the organic materials of meteorites is indicative of random synthetic processes for their formation, some of their components have identical counterparts in the biosphere, and a group of meteoritic amino acids were found to display chiral asymmetry, a property known since the time of Pasteur to be inextricably linked to life's processes. The ability of these amino acids to act as asymmetric catalysts, as well as indications that molecular asymmetry in meteorites may not be limited to these compounds, encourage the suggestion of possible involvement of meteoritic material in the induction of selective traits in molecular evolution.

Serine octamers: cluster formation, reactions, and implications for biomolecule homochirality

The emergence of homochirality continues to be one of the most challenging topics associated with the origin of life. One possible scenario is that aggregates of amino acids might have been involved in a sequence of chemical events that led to chiral biomolecules in self-replicating systems, that is, to homochirogenesis. Serine is the amino acid of principal interest, since it forms "magic-number" ionic clusters composed of eight amino acid units, and the clusters have a remarkable preference for homochirality. These serine octamer clusters (Ser8) can be generated under simulated prebiotic conditions and react selectively with other biomolecules. These observations led to the hypothesis that serine reactions were responsible for the first chiral selection in nature which was then passed through chemical reactions to other amino acids, saccharides, and peptides. This Review evaluates the chemistry of Ser8 clusters and the experimental evidence that supports their possible role in homochirogenesis.

Stereoselective acylation of a racemic amine with Cα-methyl phenylglycine-based dipeptide 5(4H)-oxazolones

Reactions of a racemic amine with chiral, N(alpha)-acetylated, C(alpha)-methyl l-phenylglycine-based dipeptide 5(4H)-oxazolones proceed diastereoselectively to give predominantly dipeptide alkylamides comprising d-alpha-phenylethylamine. Diastereoselectivity is remarkably sensitive to solvent polarity and reaction temperature but not significantly to the nature of the C(alpha)-tetrasubstituted alpha-amino acid at position 1 of the dipeptide. The beta-turn 3D structures of the aminolysis products were established in CDCl(3) solution by FT-IR absorption and in one case in the crystal state by X-ray diffraction as well.