Carbon isotope composition of individual amino acids in the Murchison meteorite

Abstract

A significant portion of prebiotic organic matter on the early Earth may have been introduced by carbonaceous asteroids and comets. The distribution and stable-isotope composition of individual organic compounds in carbonaceous meteorites, which are thought to be derived from asteroidal parent bodies, may therefore provide important information concerning mechanistic pathways for prebiotic synthesis and the composition of organic matter on Earth before living systems developed. Previous studies have shown that meteorite amino acids are enriched in 13C relative to their terrestrial counterparts, but individual species were not distinguished. Here we report the 13C contents of individual amino acids in the Murchison meteorite. The amino acids are enriched in 13C, indicating an extraterrestrial origin. Alanine is not racemic, and the 13C enrichment of its D- and L-enantiomers implies that the excess of the L-enantiomer is indigenous rather than terrestrial contamination, suggesting that optically active materials were present in the early Solar System before life began.

Organic and volatile elements in the solar system

“…Nevertheless, the progress in GC-MS and protocols for analysis of enantiomeric excess in the 80's allowed the report of small excess of the L-form of some amino acids in Murchison [33], though doubts about contaminations raised [34]. But compound specific isotopic measurements of 13 C and 15 N show that these excesses are not due to contamination [35,36]. This was further confirmed by the discovery of L-excess in non-protein chiral amino acids and in other meteorites [37,38].…”

Freudian abstracts

“…For example, the signal for black bars is not the same as that for white bars, and simple detectors are sensitive to slight changes in the location of the object 7 . In the cat, more robust disparity signals are produced by so-called complex cells at a later stage of processing.…”

“…These authors had previously 7 used the carbon isotope ratio 13 C/ 12 C of alanine in Murchison to argue for an excess of L-alanine over D-alanine, but reservations about these measurements persisted 8 . In their present letter 1 , they instead examine the 15 N/ 14 N ratio of individual Murchison amino-acid enantiomers.…”

Molecular, isotopic and in situ analytical approaches to the study of meteoritic organic material

“…However, with the exception of some terrestrial contamination, there is overwhelming evidence from isotopic and other studies that these organic species are the result of abiotic chemical syntheses (e.g. Smith & Kaplan 1970 ;Becker & Epstein 1982 ;Yang & Epstein 1983 ;Engel et al 1990, Pizzarello et al 1994. The carbonaceous chondrites, therefore, provide natural examples of pre-biotic chemical evolution ; understanding the sources of organic molecules in meteorites and the chemical processes that lead to their formation is the primary goal of research into extraterrestrial organic material.…”

“…Anders et al 1973;Mullie & Reisse 1987;Cronin & Chang 1993). More recently, molecular level isotopic analysis (Yuen et al 1984;Engel et al 1990;Gilmour & Pillinger 1994 ;Sephton et al 1998; has been employed to constrain the origin of the meteoritic organic phases.…”