The Carbon Isotopic Distribution of Murchison Amino Acids

The origin of amino acids in meteorites is not yet well understood, in spite of extensive molecular, isotopic, and chiral analyses of the last thirty years that have elucidated many of these compounds' features and distribution. The present general view of meteoritic amino acid formation is that of an interstellar/planetary sequence of Strecker-like reactions between aldehydes/ketones, HCN, water, and ammonia [1]. This type of reaction, however, is non-stereospecific and cannot account for the enantiomeric excesses found in some Murchison and Murray amino acids, at least not directly. Since there are indications that the reaction was, indeed, involved in the formation of amino acids in meteorites [2], the questions arise whether we should consider a concomitant intervention of asymmetric catalyst(s) [3] , and that we may now detect enantiomeric excesses only in amino acids that did not racemize during parent body water alteration, or we should consider a different formation all together for amino acids displaying asymmetry. In fact, recent analyses of the Tagish Lake meteorite have suggested that the synthetic pathways for organics in meteorites, including amino acids, may have been multiple and distinct [4]. Isotopic analyses of meteoritic amino acids, which could help identify their synthetic history(ies), so far have been limited to either very few individual compounds, as in the pioneer study by Engel et al. [5], or groups of amino acids [6], and thus insufficient for detailed comparisons. In the attempt to further our understanding of amino acid formation in meteorites and of the possible differences between their varied subgroups, we have analyzed the carbon isotopic composition of individual Murchison amino acids by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Prior to isotopic analyses, the amino acids were obtained in an hydrolyzed water extract of the meteorite (~ 11.0 g interior fragments) and separated from other meteoritic components following previously tested procedures [7]. In brief, groups of amino acids of similar chain length were isolated by reverse phase HPLC and then derivatized to their O-isopropyl N-TFA derivatives for GC-MS separation and identification. With an approach somewhat different from that used in previous molecular analyses, the HPLC separation aimed to collect the compounds of interest fully in one fraction, to avoid isotopic fractionation which trials with standards have shown to occur (in small extent, ~ 3‰) along the eluting HPLC peaks. Results are summarized in the following Table and Figure. The figure compares graphically the δC of the three homologous series of linear 2-amino 2-H and 2amino 2-methyl amino acids (both referred to as αamino acids), and for the series of linear non-α isomers, i.e., those with the amino group in 3-,4-, 5-, etc. position. It can be seen that all amino acids have high C content, as expected [5]. This abundance differs for the two subgroups of α-amino acids and decreases for both with increasing chain length. This trend appears to be reversed for non-α amino acids , where the lowest value is found in β-alanine while amino acids of longer chain, some of which are still unknown for lack of standards but are identifiable by their mass spectra, reach higher values. An independence between isotopic values and chain length has been observed for dicarboxylic acids [4] and appears to be true also for dicarboxylic amino acids. The δC value measured for 2-methyl glutamic acid, a dicarboxylic six-carbon amino acid, was found not significantly lower than that of five-carbon glutamic acid. Both these sets of values are higher then those of the 5-C and 6-C monocarboxylic α-amino acid measured. The N-substituted amino acid sarcosine gave the highest δC value determined in this study but other compounds of the N-substituted series could not be determined to establish any of the trends observed for the other α-amino acids.