Showing papers on "Homochirality published in 1998"

Circular polarization in star-formation regions: implications for biomolecular homochirality.

Abstract: Strong infrared circular polarization resulting from dust scattering in reflection nebulae in the Orion OMC-1 star-formation region has been observed. Circular polarization at shorter wavelengths might have been important in inducing chiral asymmetry in interstellar organic molecules that could be subsequently delivered to the early Earth by comets, interplanetary dust particles, or meteors. This could account for the excess of L-amino acids found in the Murchison meteorite and could explain the origin of the homochirality of biological molecules.

Chiral quinolinyl-oxazolines as ligands for copper(I)-catalyzed asymmetric cyclopropanation

Abstract: Chiral quinolinyl-oxazoline compounds have been synthesized from enantiomerically pure amino alcohols and 8-quinoline-carboxylic acid using a convenient procedure. Asymmetric cyclopropanation of styrene with diazoacetates in the presence of 1 mol% of CuOTf and quinolinyl-oxazolines gave 2-phenylcyclopropane carboxylates in moderate enantiomeric excesses.

Homochirality and life.

Abstract: After clarifying the frequently misused term homochirality, the crucial importance of homochirality and chiral purity in the development and maintenance of the essential biopolymers of life--proteins and nucleic acids--is discussed. The harsh and forbidding prebiotic environment during the era of cometary impact after formation of the Earth approximately 4.5 Gyr ago is described, after which the most important abiotic mechanisms proposed historically for the genesis of chiral molecules on the primitive Earth are enumerated. Random and determinate terrestrial mechanisms are each evaluated with regard to the environmental restraints imposed during the impact era, and it is concluded that all such mechanisms would be inapplicable and implausible in the realistic prebiotic environment. To circumvent these limitations, an extended hypothesis is presented describing an extraterrestrial source of homochiral terrestrial molecules. Illustrated in Figure 2, this scenario involves the partial asymmetric photolysis of the racemic constituents of organic mantles on interstellar dust grains by the circularly polarized ultraviolet components of the synchrotron radiation emanating from the neutron star remnants of super-novae. The resulting homochiral constituents with low enanantiomeric excesses (e.e.s) so produced in the organic mantles are subsequently conveyed to Earth either by direct accumulation or, more likely, after coalescence into comets or asteroids, followed by repetitive impingement during the impact era. Finally, the low e.e.s of the extraterrestrial homochiral molecules so introduced are amplified by terrestrial autocatalytic or polymerization mechanisms into a state of chiral purity, then are ultimately concentrated and protected by sequestration in the interiors of spontaneously formed protocellular vesicles--there to await further chemical evolution toward the biomolecules of life. Recent observations of the excess of L-over D-amino acids in the Murchison meteorite are cited as validation for the early stages of the proposed hypothesis.

RNA-directed amino acid homochirality

Abstract: The phenomenon of L-amino acid homochirality was analyzed on the basis that protein synthesis evolved in an environment in which ribose nucleic acids preceded proteins, so that selection of L-amino acids may have arisen as a consequence of the properties of the RNA molecule. Aminoacylation of RNA is the primary mechanism for selection of amino acids for protein synthesis, and models of this reaction with both D- and L-amino acids have been constructed. It was confirmed, as observed by others, that the aminoacylation of RNA by amino acids in free solution is not predictably stereoselective. However, when the RNA molecule is constrained on a surface (mimicking prebiotic surface monolayers), it becomes automatically selective for the L-enantiomers. Conversely, L-ribose RNA would have been selective for the D-isomers. Only the 2′ aminoacylation of surface-bound RNA would have been stereoselective. This finding may explain the origin of the redundant 2′ aminoacylation still undergone by a majority of today's a...

New chiral allylaminosilanes and their use in asymmetric Sakurai reactions

Abstract: The new allylaminosilanes 2a – c , derived from chiral amines, react with benzaldehyde and pivalaldehyde in the presence of SnCl 4 to give homoallylic alcohols 4a – b with enantiomeric excesses of up to 30%.

Exochirality in the Solar System and Beyond

Abstract: Animals are made of only L-amino acids and not their D mirror images — biology is homochiral, in contrast to non-living systems which are racemic (i.e. contain equal numbers of L and D molecules). A search for extra-terrestrial biology or pre-biotic chemistry can therefore be approached as a Search for Extra-Terrestrial Homochirality, SETH (MacDermott, 1995, MacDermott et al., 1995). Recent discoveries of excesses of L amino acids in the Murchison meteorite represent the first definitive identification of exochirality (chirality outside the Earth) and demonstrate for the first time the operation of a pre-biotic chiral influence.

Archaeal evidence for the role of 2′ aminoacylation of rna in the origin of amino acid homochirality

Abstract: Homochirality, the condition in which all the amino acids have the left-handed, or Lconfiguration, was analyzed based on the evidence that protein synthesis evolved in an environment dominated by ribose nucleic acids. Selection of L-amino acids, therefore, may have arisen as a consequence of the properties of the RNA molecule. Aminoacylation of RNA is the primary mechanism for selection of amino acids for protein synthesis. Models of this reaction with both D and L amino acids, demonstrated that the reaction was automatically selective for the L-enantiomers, but only when the RNA molecule was bound at a surface (mimicking prebiotic surface monolayers). It was hrthermore shown that only the 2’ aminoacylation of RNA would have been stereoselective (1). This finding may explain the origin of the seemingly redundant 2’ aminoacylation still undergone by a majority of today’s amino acids, before conversion (by 2’43’ aminoacyl transferases) t0 the 3’ aminoacylated forms that are used exclusively in protein synthesis. Currently the primary 2’ acylation is carried out by the corresponding class I aminoacyl tRNA synthetases, whereas the 3’ aminoacylated tRNA’s are products of the class I1 synthetases. These observations suggest that 2’ aminoacylation may have evdved before the 3’ process. In this paper, the relationship between the two classes of aminoacyl tRNA synthetases and the codon assignments for their corresponding amino acids is analyzed, in order to obtain more information on the evolutionary sequence of these events. Some recent supportive evidence of synthetase class switching in the Archaeabacteria, is also described. It is generally recognized that the current complement of 20 amino acids evolved in two separate groups, ‘early’ and ‘late’, distinguished largely by the relative numbers of codons assigned. The distribution of ‘early’ and ‘late’ amino acids between the class I and class I1 synthetases was therefore determined. The average number of codons assigned to ‘early’ amino acids is 4.56 +/1.13, and for ‘late’ amino acids, 1.82 +/0.40. The average number of codons assigned to amino acids that are substrates of class I synthetases was found to be 2.82 +/1.78, and for class I1 synthetases 3.33 +/1.41. Thus the group of late amino acids is not overrepresented in the class I1 enzymes, and neither are the early amino acids overrepresented in class I. These results suggest, therefore, that evolution of both early and late amino acids, and their codons, was essentially complete, before 3’ aminoacylation of RNA by the class I1 synthetases arose. The deep primordial origin of the two classes of synthetases, hitherto, has concealed the point of their evolutionary divergence. Recently, however, sequence similarity searches in the genes of two Archaea failed to reveal the expected homolog of a class 11 synthetase for lysyl tRNA. Further examination by Ibba et. al. (2), led to the discovery in a number of Euryarchaeal genomes (Table I), of a synthetase for lysyl tRNA, that closely resembles the class I synthetases, in terms of the characteristic amino acid motifs, molecular structure and mechanism of action. These observations provide the first 6nn evidence that 2’ aminoacylation preceded 3’ aminoacylation, and that class-switching from class I to class II synthetases has occurred during evolution. Table 1. Archrd Evidence for 2’ to 3’ Aminorcylrtion Class-Switching during Evolution of Lysyl tRNA Synthetasea

Salam Phase Transition and the Causal Origin of Homochirality

Abstract: The origin of homochirality has been a basic and challenging problem of life science. Salam’s phase transition hypothesis has been suggested. However, there are three problems implied in this putative phase transition of D- to L-amino acids. In this paper, we present our experiments involving the possible relevance of Salam’s putative phase transition in the origin of homochirality.