Concentration- and pH-Dependent Ammonia Yields in the Reaction of Hydrated Electrons with Glycylglycine

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Abstract
Competition between recombination of the radicals formed in the gamma radiolysis of water (eaq-, OH, H) and the reaction of eaq- with glycylglycine is investigated in glycylglycine solutions of various concentrations and pH. First, the concentration dependence of the ammonia yields is measured in the glycylglycine solutions of between 0.5 mM and 1.0 M. The G values of ammonia appear to reach a plateau of 2.47 +/- 0.05 molecules/100 eV at the low concentrations (1-5 mM) and increase to 3.81 +/- 0.07 molecules/100 eV at the high concentration (1 M). Secondly, the pH dependence of the ammonia yields is measured at low (1 and 5 mM) and high (0.1 and 0.2 M) concentrations of glyclyglycine. For concentrated glyclyglycine solutions, the ammonia yields increase with increasing pH, as expected based on comparison of the probabilities of the competitive reactions. However, the G values of ammonia reach a maximum at pH 9.0 and then decrease as the pH increases for dilute solutions. The lower G values at higher pH for dilute glycylglycine solutions may result from increased interspur recombination since the rate constant for the reaction of eaq- with glycylglycine is decreased and, as a consequence, the scavenging capacity of glycylglycine for eaq- decreases approximately 4.7-fold from low to high pH.