to a factor of 10 lower than at pH = pAl(H2P04-) -1.02, proving that H+ determines the overall rate of disappearance at all temperatures.At ambient temperature and up to 100 °C reaction 6 is rate determining with an activation energy of 7.6 ± 0.8 kJ mol-1 (1.8 ± 0.2 kcal mol-1) and a rate constant k6 = (9.6 ± 1.0) X 107 dm3 mol-1 s-1 at 20 °C. These figures are in agreement with reported data.6 78Above 100 °C the rate constant calculated as k6 (Figure 5) increases drastically, and the linearity of the Arrhenius plot in the temperature region °C suggests that a new reaction becomes rate determining. The activation energy determined at these high temperatures is very large, EA = 80 ± 12 kJ mol-1 (19 ± 3 kcal mol-1). From eq V an overall decay rate constant of /c7 = 1.0 X 1010 mol-1 s-1 may be calculated at pH 5.64 and at 285 °C (reactor conditions). At higher pH's the rate constant decreases linearly with the H+ concentration. As a tentative

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The photo-stability of simulated free and bound amino acids exposed to photon sources of 1.2-1.3 MeV γ -rays from a 60 C• source and ultra-violet (UV) irradiation from a 10 eV deuterium lamp was examined. The free amino acids in aqueous solution were drastically decreased while the bound amino acids in aqueous solution were slightly decreased with the duration of γ -rays and UV irradiation. The apparent half-lives (t 1/2 ) of bound serine and threonine under γ -rays irradiation were more than 10.6 and 27.9 times as stable as the free amino acids, respectively. The photo-stability of bound amino acids was greater than that of free amino acids under irradiation by high-energy photons. These results strongly suggest that bound, but not free, amino acids could be found in extraterrestrial environments. Radio-or photo-alteration processes are mainly decomposition pathways, with the production of secondary amino acids via decomposition of the α-carboxylic group; β-alanine and γ -aminobutyric acids, α-decarboxylation products of aspartic acid and glutamic acid, respectively, were detected.

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