Distribution of Damage in Irradiated 5′-AMP: 8,5′-Cyclo-AMP, 8-Hydroxy-AMP, and Adenine Release

The yields of 8,5′-cycloadenosine-5′-monophosphate, 8-hydroxyadenosine-5′-monophosphate, and adenine release have been measured in aqueous solutions of adenosine-5′-monophosphate (5′-AMP) irradiated under nitrous oxide. The yields are strongly pH dependent in a way which reflects the pKₐ' s of ionizable groups within 5′-AMP. Redistribution of hydroxyl radical attack between base and ribose moieties appears to be occurring in response to changes in the acid-base equilibria of 5′-AMP. Other pH-dependent phenomena which only indirectly reflect the pKₐ' s of 5′-AMP include the result that the yield of the (R) epimer of 8,5′-cycloadenosine-5′-monophosphate predominates at low pH while that of the (S) epimer predominates at high pH.