Murchison Meteorite - Organic Matter

Organic Matter

Murchison contains common amino acids such as glycine, alanine and glutamic acid as well as unusual ones like isovaline and pseudoleucine. The initial report stated that the amino acids were racemic (that is, the chirality of their enantiomers are equally left- and right-handed), indicating that they are not present due to terrestrial contamination. A complex mixture of alkanes was isolated as well, similar to that found in the Miller-Urey experiment. Serine and threonine, usually considered to be earthly contaminants, were conspicuously absent in the samples. A specific family of amino acids called diamino acids was identified in the Murchison meteorite as well.

More research found that some amino acids were present in enantiomeric excess, leading some to suspect terrestrial contamination, since it would be "unusual for an abiotic stereoselective decomposition or synthesis of amino acids to occur with protein amino acids but not with non-protein amino acids." In 1997 research showed that individual amino-acid enantiomers from Murchison were enriched in the nitrogen isotope 15N relative to their terrestrial counterparts, which confirmed an extraterrestrial source for an -enantiomer excess in the Solar System. The list of organic materials identified in the meteorite was extended to polyols by 2001.

Building on the idea that homochirality (existence of only left-handed amino acids and right-handed sugars) is triggered by deposition of chiral molecules on meteorites, research in 2005 demonstrated that an amino acid like -proline is capable of catalyzing the formation of chiral sugars. The catalysis is non-linear, that is proline with an enantiomeric excess of 20% yields an allose with enantiomeric excess of 55% starting from a benzyloxyacetaldehyde in a sequential aldol type reaction in an organic solvent like DMF. In other words a small enantiomeric excess of left-handed amino acids may explain terrestrial life's preference for right-handed sugars.

Several lines of evidence indicate that the interior portions of well-preserved fragments from Murchison are pristine. A 2010 study using high resolution analytical tools including spectroscopy, identified 14,000 molecular compounds including 70 amino acids in a sample of the meteorite. The limited scope of the analysis by mass spectrometry provides for a potential 50,000 or more unique molecular compositions, with the team estimating the possibility of millions of distinct organic compounds in the meteorite.