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Non-racemic amino acids in the Murray and Murchison meteorites

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Abstract

Small (1.0–9.2%) l-enantiomer excesses were found in six α-methyl-α-amino alkanoic acids from the Murchison (2.8–9.2%) and Murray (1.0–6.0%) carbonaceous chondrites by gas chromatography-mass spectroscopy of their N-trifluoroacetyl or N-pentafluoropropyl isopropyl esters. These amino acids [2-amino-2,3-dimethylpentanoic acid (both diastereomers), isovaline, α-methyl norvaline, α-methyl valine, and α-methyl norleucine] are either unknown or rare in the terrestrial biosphere. Enantiomeric excesses were either not observed in the four α-H-α-amino alkanoic acids analyzed (α-amino-n-butyric acid, norvaline, alanine, and valine) or were attributed to terrestrial contamination. The substantial excess of l-alanine reported by others was not found in the alanine in fractionated extracts of either meteorite. The enantiomeric excesses reported for the α-methyl amino acids may be the result of partial photoresolution of racemic mixtures caused by ultraviolet circularly polarized light in the presolar cloud. The α-methyl-α-amino alkanoic acids could have been significant in the origin of terrestrial homochirality given their resistance to racemization and the possibility for amplification of their enantiomeric excesses suggested by the strong tendency of their polymers to form chiral secondary structure.

Introduction

Molecular dissymmetry has been sought in meteorite organic matter for many years because of its implications for the origin of this material; that is, whether the organic matter was formed by chemical evolution, by extraterrestrial organisms, or is present only as a consequence of terrestrial contamination. Early attempts to make such determinations by measuring optical rotation, a phenomenon associated with molecular dissymmetry, in meteorite organic compounds gave uniformly negative results. Organic solvent extracts of the Cold Bokkeveld meteorite (Mueller, 1953) and the Mokoia and Haripura meteorites (Briggs and Mamikunian, 1963) failed to show optical rotation, as did both organic solvent and aqueous extracts from eight carbonaceous chondrites of various petrologic types (Kaplan et al., 1963).

Subsequent polarimetric analyses, carried out during a controversy over the occurrence of supposed cellular remains in carbonaceous chondrites, gave conflicting results. Whereas optical activity was reported in saponified organic extracts of the Orgueil meteorite Nagy et al 1964, Nagy 1965, Nagy 1966, it was not observed by others in similar, purified extracts, and the original observation was attributed to impurities and analytical artifacts Hayatsu 1965, Hayatsu 1966, Meinschein et al 1966. In a comprehensive review of this work, Hayes (1967) concluded that dissymmetry in the optically active compounds in Orgueil had not been proven.

By 1969, the development of sensitive gas chromatographic (GC) methods allowed the direct analysis of individual amino acid enantiomers as diastereomeric N-trifluoroacetyl esters of optically active alcohols. Using this approach, Kvenvolden et al. (1970) found that alanine extracted from the recently fallen Murchison meteorite was racemic and that glutamic acid, proline, and valine were nearly so. Additional work showed that “all the amino acids that have asymmetric carbon atoms and whose diastereomeric derivatives could be separated by the GC method used appeared to consist of approximately equal amounts of the d and l isomers” (Kvenvolden, et al., 1971). In a separate study, isovaline was reported to exist in Murchison as a racemic mixture (Pollock et al., 1975). These findings, obtained with the presumably pristine Murchison meteorite, clearly established that amino acids were indigenous to the meteorite and were products of an abiotic chemical process. Concurrent analyses of the Murray meteorite using the same methods gave very similar results, i.e., nearly equal amounts of the d- and l-isomers were found for alanine, glutamic acid, proline, valine, α-amino-n-butyric acid, pipecolic acid, and β-aminoisobutyric acid (Lawless et al., 1971).

The enantiomeric compositions of several Murchison amino acids were redetermined later by Engel and Nagy (1982). Although they found isovaline to be racemic and α-amino-n-butyric acid apparently so, five protein amino acids (alanine, glutamic acid, proline, aspartic acid, and leucine) showed substantial l-excesses, which they suggested were characteristic of these amino acids as native to the meteorite. This proposal was criticized, largely on the grounds that the sampling procedure did not exclude terrestrial contaminants (Bada et al., 1983). Subsequently, Engel et al. (1990) bolstered the case for a significant enantiomeric excess in Murchison alanine using a chiral GC phase to separate enantiomers in a gas chromatography-combustion-isotope-ratio mass spectrometer (GC-C-IRMS). They found an l-alanine enantiomer excess of 8% and δ13C values for the l- and d enantiomers of +27‰ and +30‰, respectively. On this basis, they argued for an indigenous l-excess on the grounds that terrestrial contamination accounting for 8% of the alanine would have lowered the l-enantiomer δ13C value more than was observed. Recently, Engel and Macko (1997) have extended this approach to 15N and, in this case, argued for indigenous l-excesses in alanine and glutamic acid of 33% and 54%, respectively, based on the similar δ15N values obtained for the d- and l-enantiomers of these amino acids.

Recently, we reported small (2.8–9.2%) l-enantiomer excesses in a subset of Murchison amino acids, the α-methyl-α-amino acids (Cronin and Pizzarello, 1997); however, enantiomeric excesses were not observed in α-H-α-amino acids, including alanine (Pizzarello and Cronin, 1998). The latter finding is in agreement with the early results (Kvenvolden et al., 1970), but contrasts with the results of Engel and his coworkers Engel and Nagy 1982, Engel et al 1990, Engel and Macko 1997. In this article, we report additional Murchison amino acids with l-enantiomer excesses and the presence of a similar set of amino acids with l-excesses in the Murray meteorite.

Section snippets

Extraction and isolation of amino acids

Two interior pieces of the Murray meteorite (Center for Meteorite Studies, Arizona State University) weighing 26 g in total were crushed in a steel press, powdered by using a glass mortar and pestle, and extracted with triple-distilled water for 24 hr at 110°C under vacuum. The powder was sedimented by centrifugation and the extract decanted and combined with two water rinses of the insoluble residue. The extract was then concentrated by rotary evaporation, acidified to pH 2 with 1N sulfuric

Enantiomeric analyses of Murchison and Murray amino acids

The results of enantiomeric analyses of ten amino acids from the Murray and Murchison meteorites are given in Table 2. The corrected enantiomeric excesses (ee) are based on mean values calculated from integrated single ion plots and, when justified (see below), total ion (TI) plots of data obtained from multiple chromatographic runs. Corrections were made in each case for the small mean ee obtained from runs of the corresponding racemic amino acid standards under the same conditions and using

Comparison with previous results

The significant enantiomeric excesses in the α-amino acids of the Murchison and Murray carbonaceous chondrites appear to be confined to the α-methyl amino alkanoic acids, a subset of the amino acids that has not been extensively analyzed heretofore. Our finding of a significant l-enantiomer excess in isovaline contrasts with Pollock et al. (1975) who reported this amino acid to be racemic in Murchison; however, it should be noted that the latter result was obtained by GC analysis alone of a

Conclusions

1) Six α-methyl-α-amino acids have been extracted from both the Murchison and Murray meteorites and found to have small excesses of the l-enantiomers. These amino acids are either unknown or of rare occurrence in the biosphere.

2) The enantiomeric excesses range from 2.8% to 9.2% in the amino acids from the Murchison meteorite and from 1.0% to 6.0% in the corresponding amino acids from the Murray meteorite.

3) Thus far, amino acids that lack an α-methyl group in addition to the usual α-alkyl

Acknowledgements

The authors gratefully acknowledge research support from the NASA Exobiology Program (NAG5-4131), meteorite samples provided by Carleton Moore from the collections of the Center for Meteorite Studies, Arizona State University, and constructive reviews by Keith Kvenvolden and Jeffrey Bada.

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