Metal-catalyzed degradation and racemization of amino acids in iron sulfate brines under simulated martian surface conditions

Abstract

Recent lines of evidence indicate that Mars may have been substantially warmer and wetter, raising questions about whether the planet’s surface was once more conducive to the possibility of life. To better constrain the extent of preservation of ancient amino acid biosignatures, the effects of metal ions and UV radiation were investigated with respect to the rates of diagenesis of amino acids in metal-rich sulfate brines. Individual experimental results indicate that the oxidation rates of amino acids loosely followed trends characteristic of a radiolytic oxidation mechanism that is dependent on variations in side-chain constituents. Relative rates of racemization only show a limited dependence on the presence of metals; overall, rates of racemization in the presence of metals in solution are an order of magnitude slower than oxidation and both are several orders of magnitude faster than previously reported. This increase in rate constant is indicative of an extremely efficient catalytic enhancement of this destructive diagenetic pathway. These findings imply that chiral life-detection strategies should focus on specific geochemical environments that exhibit relatively slow rates of both degradation and racemization. Episodic liquid water during periods of increased water activity on the martian surface may have significant implications for the oxidation of organic matter through secondary diagenetic reactions and may act as a leveling mechanism for organic compounds.