Murchison meteorite
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The Murchison meteorite is named after Murchison, Victoria, in Australia. Fragments of the meteorite fell near the village on 28 September 1969. The meteorite belongs to the CM group of carbonaceous chondrites (see meteorite classification). Over 100 kg of material was recovered, in pieces weighing up to ~7 kg, making this the largest (and consequently the most studied) CM chondrite.
[edit] Organic matter
Murchison was found to contain common amino acids such as glycine, alanine and glutamic acid but also unusual ones like isovaline and pseudoleucine.[1] The initial report stated that the amino acids were racemic (that is, the chirality their enantiomers are equally left- and right-handed) supporting a theory that the source is extraterrestrial. A complex mixture of alkanes was isolated as well which was similar to that found in the Miller-Urey experiment. Serine and threonine are usually considered earthly contaminants and these compounds were conspicuously absent in the samples.
More research found that some amino acids were present in enantiomeric excess.[2] Homochirality is considered a unique biological property. The claims were challenged on the grounds that the protein amino acids were non-racemic but the other amino acids were.[3] 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 L-enantiomer excess in the Solar System.[4] The list of organic materials identified on the meteority material was extented to polyols by 2001.[5]
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 L-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 benzyloxy acetaldehyde in a sequential aldol type reaction in an organic solvent like DMF.[6] In other words a small amount of chiral amino acids may explain the evolution of right-handedness of sugars.
According to Engel, several lines of evidence indicate that the interior portions of well-preserved fragments from Murchison are pristine. Engel points to the array of amino acids Murchison contains and to isotope studies to bolster his position. Other scientists are equally convinced that the evidence proves the opposite: that Murchison is now thoroughly contaminated by terrestrial organic material.
Indeed, the results of various experiments performed on Murchison are a bit of a head-scratcher - and a good window into how science works when data are ambiguous.
[edit] References
- ^ Kvenvolden, Keith A.; Lawless, James; Pering, Katherine; Peterson, Etta; Flores, Jose; Ponnamperuma, Cyril, Kaplan, Isaac R.; Moore, Carleton (1970). "Evidence for extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite". Nature 228 (5275): 923–926. doi:.
- ^ Engel, Michael H.; Nagy, Bartholomew (April 29, 1982). "Distribution and enantiomeric composition of amino acids in the Murchison meteorite". Nature 296: 837–840. doi:.
- ^ Bada, Jeffrey L.; Cronin, John R.; Ho, Ming-Shan, Kvenvolden, Keith A.; Lawless, James G.; Miller, Stanley L.; Oro, J.; Steinberg, Spencer (February 10, 1983). "On the reported optical activity of amino acids in the Murchison meteorite". Nature 301: 494–496. doi:.
- ^ Engel, Michael H.; Macko, S. A. (September 1, 1997). "Isotopic evidence for extraterrestrial non-racemic amino acids in the Murchison meteorite". Nature 389: 265–268. doi:.
- ^ Cooper, George; Kimmich, Novelle; Belisle, Warren; Sarinana, Josh; Brabham, Katrina; Garrel, Laurence (December 20, 2001). "Carbonaceous meteorites as a source of sugar-related organic compounds for the early Earth". Nature 414: 879–883. doi:.
- ^ Córdova, Armando; Engqvist, Magnus; Ibrahem, Ismail; Casas, Jesús; Sundén, Henrik (2005). "Plausible origins of homochirality in the amino acid catalyzed neogenesis of carbohydrates". Chem. Commun.: 2047–2049. doi:.
