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PHYSICAL SCIENCES / CHEMISTRY
Molecular asymmetry in extraterrestrial chemistry: Insights from a pristine meteorite
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Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85018-1604; and Department of Geological Science, Brown University, Providence, RI 02920
Edited by Jack W. Szostak, Massachusetts General Hospital, Boston, MA, and approved January 3, 2008 (received for review October 18, 2007)
The nonracemic amino acids of meteorites provide the only natural example of molecular asymmetry measured so far outside the biosphere. Because extant life depends on chiral homogeneity for the structure and function of biopolymers, the study of these meteoritic compounds may offer insights into the establishment of prebiotic attributes in chemical evolution as well as the origin of terrestrial homochirality. However, all efforts to understand the origin, distribution, and scope of these amino acids' enantiomeric excesses (ee) have been frustrated by the ready exposure of meteorites to terrestrial contaminants and the ubiquitous homochirality of such contamination. We have analyzed the soluble organic composition of a carbonaceous meteorite from Antarctica that was collected and stored under controlled conditions, largely escaped terrestrial contamination and offers an exceptionally pristine sample of prebiotic material. Analyses of the meteorite diastereomeric amino acids alloisoleucine and isoleucine allowed us to show that their likely precursor molecules, the aldehydes, also carried a sizable molecular asymmetry of up to 14% in the asteroidal parent body. Aldehydes are widespread and abundant interstellar molecules; that they came to be present, survived, and evolved in the solar system carrying ee gives support to the idea that biomolecular traits such as chiral asymmetry could have been seeded in abiotic chemistry ahead of life.
carbonaceous chondrites | chemical evolution | chiral asymmetry | diastereomer amino acids
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0709909105/DC1.
¶ Diastereomers allow for a special case of racemization, called epimerization, when only one chiral carbon racemizes. This is the case for ile and other 2-H amino acids in water at C2; this carbon, which is weakly acidic by its proximity to the electron-withdrawing carboxyl group, easily loses/reacquires a proton, leading to racemization. Epimerization leads not to a molecule's enantiomer but to its diasteromer of opposite configuration (e.g., from L-ile to D-allo).
To whom correspondence should be addressed. E-mail: pizzar{at}asu.edu
© 2008 by The National Academy of Sciences of the USA
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