The Chiral Amines of the Murchison Meteorite: a Preliminary

L-enantiomeric excesses have been determined for some amino acids of the Murchison and Murray meteorites [1-4]. The finding has taken a particular significance in view of the homochiral distribution of biochemical molecules and the possible contribution of asymmetry-carrying organics to the early Earth by meteorite and cometary delivery. So far, evidence of nonracemic compounds in meteorites has been limited to a series of α-substituted amino acids, of which isovaline (2-amino 2-methyl butanoic acid) is the first homologue. This amino acid has been found in Murchison with varying L-enantiomeric excesses of up to15% [5], the largest value determined to date. Amines are an abundant group of water-soluble organics in CM chondrites where they are found as a series of alkyl compounds of one to at least five-carbon chain length. Meteoritic amines show isotopic enrichments in deuterium, C, and N which are comparable to those displayed by amino acids with δN values, in particular, that are very similar in both classes of compounds (+90 to +94‰) [6]. In view of these similarities it has been proposed that at least a fraction of meteoritic amines could have derived from amino acids via decarboxylation, which is a common pathway of thermal decomposition for α-amino acids. The molecular distribution of Murchison amines would be compatible with this proposal, since sixteen of the twenty amines identified in the meteorite could have derived from known Murchison α-amino acids. We report here an investigation of the chiral distribution of Murchison amines, obtained from a 20 g stone of the meteorite (Center for Meteorite Studies, Arizona State University). Amines were collected from the water extracts as volatile bases (at pH ~ 12) by cryogenic transfer and subsequently separated by HPLC (BIORAD, Aminex HPX-72-O column) into fractions of individual or small groups of compounds of similar carbon chain length. Amines were again recovered from these samples by cryogenic transfer and then dried as hydrochloride salts. Chiral analyses were performed by Gas Chromatography-Mass Spectrometry (GC-MS) of the amine volatile N-trifluoro acetyl (TFA) or N-pentafluoro propionyl PFP) derivatives [6], using a fused silica column coated with a cyclodextrine bonded phase (CHROMPACK, CP-Chirasil-Dex CP, 25m x 0.25mm, 0.25μm df). The identification of individual Murchison amines was achieved by comparison of their mass spectra and chromatographic retention times with those of derivatized standards. This is a common compound recognition process which became particularly important for amines since these compounds share very low ionization energy and a simple α-cleavage fragmentation which is not always accompanied by M, M-1, or other diagnostic low abundance ions (e.g. all α-H, and α-methyl compounds have base peaks at m/z 126 and 140, respectively). Also the GC phase employed is unpredictable as to the order of elution of the two enantiomers of a chiral compound, whose identification always requires confirmation with individual enantiomer standards. Being chiral amines less numerous than amino acids, of the twenty four alkyl amines with oneto five-carbon chain that can be derivatized with TFA (i.e. excluding the N-disubstituted species) only five have one chiral center. These amines are: secbutyl; 1-, and 2-methyl butyl; 1, 2-dimethyl propyl; and N-methyl sec-butyl. A positive L-enantiomeric excess has been determined for Murchison sec-butyl amine that has the value of 17.8%, with a σ=0.8 for six data. Figure 1 shows the mass spectrum of the meteoritic amine (top) and its chromatographic separation (center) compared with that of a standard.