Part II. Mechanism of formation of guanine as one of the major products inthe 254 nm photolysis of guanine derivatives: Concentration and pH effects

The quantum yield of formation of guanine (Gua), one of the major products Formed in the 254 nm steady-state photolysis of the following Gua derivativ es (9-ethylguanine [9Et-Gua], deoxyguanosine [dGuo], guanosine [Guo], guano sine 5'-monophosphate [GMP) was determined under different conditions, The formation yield increases in the following order: 9Et-Gua < dGuo approximat e to Guo < GMP. Electron scavengers or triplet quenchers were incorporated into the irradiated solutions with the purpose of reacting specifically wit h postulated or previously identified intermediates in the photolysis of Gu a derivatives. a decrease in the yield of formation of Gua is observed with increasing electron scavenger concentration or with pH. These results sugg est a major contribution from Gua-derivative electron adducts on the proces s of N(9)-R bond breakage. At an acidic pH a tautomer of the radical cation (E) of Guo is proposed as the precursor for Gua formation. The relative ef ficiency of the radical cation for initiating the release of free Gua depen ds on the pH of the solution, being less than 39% in neutral pH. Reactions from OH radicals may also result in base release as shown using N2O as addi tive. Finally, the formation of aggregates by the bases at concentrations u sed, plays an important role in the deactivation of the excited states and also in the probability of formation of the free base.