doi:10.1016/S0016-7037(00)01283-8 Nonracemic isovaline in the Murchison meteorite: Chiral distribution and mineral association SANDRA PIZZARELLO, 1, *MICHAEL ZOLENSKY, 2 and KENDRA A. TURK 3 1 Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA 2 NASA Johnson Space Center, Houston, TX 77058, USA 3 NASA Ames Research Center, Moffet Field, CA 94035, USA (Received June 5, 2002; accepted in revised form September 27, 2002) Abstract—The enantiomeric and carbon-isotopic composition of the amino acid isovaline have been analyzed in several samples of the Murchison meteorite and one sample of the Murray meteorite. L-Enantiomeric excesses of the amino acid were found to range from 0 to 15.2%, varying significantly both between meteorite stones and at short distances within a single stone. The upper limit of this range is the largest enantiomeric excess measured to date for a biologically rare meteoritic amino acid and raises doubts that circularly polarized light irradiation could have been the sole cause of amino acids chiral asymmetry in meteorites. Individual D- and L-isovaline  13 C values ware found to be about +18‰, with no significant differences between the two enantiomers to suggest terrestrial contamination. The amino acid relative abundance also varied between samples, with isovaline/alanine ratios of 0.5 to 6.5. X-ray diffraction analyses of contiguous meteorite fragments suggest a possible correlation between isovaline and hydrous silicates abundances. Copyright © 2003 Elsevier Science Ltd 1. INTRODUCTION The exclusive one-handedness of terrestrial amino acids and sugars is essential to the formation, structure, and function of biopolymers and is a defining molecular trait of terrestrial life. The unknown origin of this “homochirality” has been investi- gated for well over a century, since Pasteur first discovered it. The ensuing debate has paralleled in scope and rationale the more general studies about the origin of life: was biologic homochirality the product of prebiotic processes, or the result of selection brought about by life itself? Was it because of choice or chance? Was it at first broad-scaled, or was it of limited extent? The finding of small but significant L-enantiomeric excesses (ee) in some amino acids of carbonaceous meteorites that are rare or unknown in the biosphere (Cronin and Pizzarello, 1997) appears to indicate that interstellar and planetary chemical evolution (i.e., purely physicochemical abiotic processes) could yield chiral asymmetry. It also suggests a possible prebiotic contribution to the origin of biologic homochirality by impact delivery of meteoritic material to the early Earth, i.e., if the organic compounds found in carbonaceous chondrites are rep- resentative of some fraction of the organic milieu present on the early Earth (Chyba and Sagan, 1992), the small meteoritic enantiomeric excesses could have provided a chiral bias suffi- cient for amplification that culminated in homochirality (Piz- zarello and Cronin, 2000). The -methyl--amino acids, which have shown significant enantiomeric excesses in meteorites, are known to resist racemization, and are helix formers when polymerized (Altman et al., 1988; Formaggio et al., 1995), seem particularly well suited for such a role in prebiotic chem- istry. It has been proposed that the optical activity of amino acids in meteorites could be the result of asymmetric decomposition, brought about by ultraviolet circularly polarized light (UV CPL) irradiation of meteoritic organic compounds during their syntheses (Cronin and Pizzarello, 1997; Engel and Macko, 1997). The likelihood of this assumption was supported by theoretical and experimental work showing that UV CPL can produce enantiomeric excesses within the range so far observed in meteorite amino acids (Balavoine et al., 1974; Cronin and Pizzarello, 1997). To further characterize enantiomeric excesses in meteorites, and to constrain the possible mechanism of their formation in prebiotic environments, we have assessed the extent of chiral asymmetry, relative distribution, and likely petrological asso- ciation of meteoritic isovaline (2-amino-2-methylbutyric acid). This is one of the most abundant optically active amino acids in meteorites, where it is present with the largest L-excess (9%) (Pizzarello and Cronin, 2000). We have used several samples of the Murchison and Murray carbonaceous chondrites, both from interior and near-surface fragments, and from different curatorial facilities. To exclude the possibility of terrestrial contamination, the  13 C of D- and L-isovaline enantiomers were measured individually by gas chromatography-combustion-iso- tope ratio mass spectrometry (GC-C-IRMS) in five representa- tive samples of the two meteorites. 2. MATERIALS AND METHODS 2.1. Samples Murchison analyses listed in Table 1 were obtained from powdered stones of 3 to 20 g weight, although the actual chiral analyses for some were performed with smaller portions of the powders. They are desig- nated by their curatorial source: ASU indicates various specimens from the collections of the Arizona State University (Center for Meteorite Studies, ASU 828), and SMIT indicates specimen USNM 5341 ob- tained from the Smithsonian Institution. Sample ASU ’70 was residual powder that had been stored below 0°C since the initial analyses of Kvenvolden et al. (1970). The I and E letters following these denom- * Author to whom correspondence should be addressed (pizzar@asu.edu). Pergamon Geochimica et Cosmochimica Acta, Vol. 67, No. 8, pp. 1589 –1595, 2003 Copyright © 2003 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/03 $30.00 + .00 1589