The influence of minerals on decomposition of the n-alkyl-α-amino acid norvaline under hydrothermal conditions

Laboratory experiments were conducted to observe the effect of iron oxide and sulfide minerals on decomposition reactions of norvaline, a representative of a group of alkyl-α-amino acids observed in meteorites and prebiotic synthesis experiments. The primary products observed during heating of aqueous solutions of norvaline at temperatures of 156–186 °C in the presence of minerals included CO2, NH3, butyric acid, and valeric acid. The products indicated that norvaline predominantly decomposed by a combination of pathways that included both decarboxylation followed rapidly by oxidative deamination (norvaline → butanamide + CO2 → butyric acid + NH3) and deamination directly to valeric acid (norvaline → valeric acid + NH3). An experiment performed with alanine under similar conditions showed it decomposed by analogous reactions that produced acetic and propionic acids along with CO2 and NH3. For both amino acids, the presence of minerals accelerated decomposition rates as well as altered the final products of reaction, when compared with decomposition in the absence of mineral substrates. In addition, decomposition of norvaline was found to proceed much faster in the presence of the mineral assemblage hematite–magnetite–pyrite (HMP) than with the assemblage pyrite–pyrrhotite–magnetite (PPM), a trend that has been observed for several other organic compounds. The influence of minerals on decomposition reactions of these amino acids appears to be attributable to a combination of surface catalysis and production of dissolved sulfur compounds. Overall, the results indicate that minerals may exert a substantial influence on amino acid stability in many geologic environments, and emphasize the need to consider the impact of minerals when evaluating the lifetimes and decomposition rates of amino acids in terrestrial and planetary systems. Estimated half-lives for alkyl-α-amino acids based on the experimental results indicate that moderately hot hydrothermal environments (<∼100 °C) would have been the most favorable for accumulation of these amino acids in the early solar system, and that the predominance of alkyl-α-amino acids in some meteorites may only be compatible with temperature remaining below about 60 °C following their formation.