4). Perhaps due to their relative instabilities,

neither indigenous cysteine nor methionine has so far been conclusively detected in carbonaceous chondrites (Pizzarello and Shock 2010). Fig. 4 Two possible mechanisms for the prebiotic synthesis of cysteine from glycine via learn more serine or serine hydantoin, which would form dehydroalanine or its hydantoin. PU-H71 cell line Reaction of the latter intermediates with H2S would yield cysteine derivatives. Asterisks represent sulfur-containing compounds detected in this study The presence of homocysteic acid in the samples we have analyzed could be explained by the Strecker degradation of methionine (Schönberg and Moubacher 1952). The Strecker degradation of methionine proceeds via the catalytic decarboxylation and deamination with a carbonyl compound or an inorganic catalyst to produce 3-methylmercaptopropanal (Schönberg and Moubacher 1952), which we did not attempt to detect. However, the Strecker degradation of methionine is Cell Cycle inhibitor also known to produce, among other compounds, homocysteine (Lieberman et al. 1965), which upon oxidation

would yield homocysteic acid. As long as free oxygen was absent in the primitive atmosphere and oceans, methionine could have persisted for significant periods of geologic time (Van Trump and Miller 1972). However, as oxygen began to accumulate in the early atmosphere (Kump 2008), oxidation by metal ions, peroxides, etc. would have likely been important in limiting the concentration of methionine and cysteine present in the primitive oceans and other water bodies (Weber and Miller 1981). Methionine decomposes readily in the presence of oxygen and produces methionine sulfoxide, methionine sulfone, and various sulfides and thiols (Lieberman et al. 1965). It is thus possible that the compounds detected here represent both products synthesized due to the action of electric discharges on an atmosphere of

CH4, H2S, NH3 and CO2 and check the various Strecker and oxidative decomposition products of methionine and cysteine formed during the storage of the extracts. Even though these samples were not preserved under anoxic conditions, the manner in which they were preserved (dry, room temperature, ~50 years) implies that prebiotic methionine may not have been stable once oxygen began to accumulate in the early atmosphere. Conclusions Our findings confirm and extend previous work by Van Trump and Miller (1972) on the prebiotic synthesis of methionine and other sulfur-bearing organic compounds, which could have been formed under primitive Earth conditions. However, the results presented here indicate that in addition to abiotic synthetic processes, degradation of organic compounds of biochemical significance on the primordial Earth could have played a significant role in diversifying the inventory of molecules not readily formed from other endogenous abiotic reactions, or derived from extraterrestrial delivery.