Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.535l&link_type=abstract
Meteoritics, vol. 30, no. 5, page 535
Computer Science
Aqueous Alteration, Chondrites, Carbonaceous, Isotopic Variation, Organic Compounds, Parent Bodies
Scientific paper
Alpha-hydroxy and alpha-amino carboxylic acids found on the Murchison meteorite are deuterium enriched [1]. It is postulated that they arose from a common interstellar source: the reaction of carbonyl compounds in an aqueous mixture containing HCN and NH3. Carbonyl compounds react with HCN to form alph-hydroxy nitriles, RR'CO + HCN <--> RR'C(OH)CN. If ammonia is also present, the alpha-hydroxy nitriles will exist in equilibirum with the alpha-amino nitriles, RR'C(OH)CN +NH3 .<--> RRCNH2CN + H2O. Both nitriles are hydrolyzed by water to form carboxylic acids: RR'C(OH)CN + H2O --> RR'C(OH)CO2H and RR'C(NH2)CN + H2O --> RR'C(NH2)CO2H. Carbonyl compounds observed in the interstellar medium have been shown to be deuterium enriched [2]. The combined alpha-amino acids found on Murchison have deltaD = +1751 o/oo while the combined alpha-hydroxy acids have deltaD = +573. o/oo [1]. This large discrepancy in deltaD values does not preclude common precursors for the alpha-amino acids and the alpha-hydroxy acids. Different relative amounts of specific alpha-amino and alpha-hydroxy acids could lead to quite different combined D/H ratios. If the alpha-hydroxy acids lose significantly more deuterium during synthesis than the alpha-amino acids or if they have a higher rate of H/D exchange with liquid water than alpha-amino acids, the alpha-hydroxy acids would be isotopically lighter than the alpha-amino acids, because the water responsible for the aqueous alteration of the Murchison parent body was deuterium depleted with deltaD = -100. o/oo [3]. To determine between these alternative mechanisms we measured the rates of hydrogen-deuterium exchange of glycolic acid (the alpha-hydroxy analog of glycine), lactic acid (the alpha-hydroxy analog of alanine), and alpha-hydroxy isobutyric acid have been measured in D2O as a function of pH, temperature and the presence of Allende or Murchison minerals. No detectable H/D exchange was observed. Glycine subjected to similar conditons exchanged as much as 80% of its carbon-bonded deuterium, and alanine 43% [4]. Evidently, the relative deuterium content of the alpha-amino acids and alpha-hydroxy acids found on the Murchison meteorite was not determined by H/D exchange with water. In order to determine if deuterium retention is less for alpha-hydroxy acids than it is for alpha-amino carboxylic acids during synthesis a solution was prepared containing: KCN, 0.005M; NH4Cl, 0.002 M; CD2O, 0.0007M; CD3CDO, 0.0008 M; and (CD3)2CO 0.0006 M, and divided into aliquots which were maintained at 263 K and at 295 K. At the end of one month the carbonyls, alpha-amino acids and alpha-hydroxy acids were separated from the mixture and their deuterium content determined from their mass spectra (see Table 1). The retention of deuterium during synthesis varies significantly between the alpha-hydroxy acids and the alpha-amino acids with common precursors. Only comparisons of D/H ratios of specific acids with a postulated common precursor are meaningful. References: [1] Cronin J. R. et al. (1993) GCA, 57, 4745-4752. [2] Zinner E. (1988) in Meteorites and the Early Solar System (J. R. Kerridge and M. S. Matthews, eds.), 956-983, Univ. of Arizona. [3] Robert F. and Epstein S. (1982) GCA, 46, 81-95. [4] Lerner N. R. (1995) GCA, 59, 1623-1631. Table 1 shows D/(D+H) of carbon-bonded hydrogen in compounds separated after one month from a simulated meteorite solution.
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