Paramagnetic intermediates in the photolysis of purine free base in frozen aqueous glasses

Abstract

UV irradiation of purine free base used as a model for the purine bases in deoxyribonucleic acid, in aqueous glassy solutions, produces stable photo-products, as well as reactive paramagnetic intermediates such as triplet states, trapped electrons, purine radical cations and anions and solvent radicals. Electron spin resonance techniques and parameters were used to characterize the intermediates produced during the UV irradiation of purine in aqueous glasses at 77 K at different pH values (8 M NaClO 4, 12 M LiCl, 5 M H 2SO 4, 6 M H 3PO 4 and 8 M NaOH). Biphotonic ionization of the base through a triplet intermediate is one of the principal photodestruction paths in basic and neutral glasses. The quantum yields for the production of trapped electrons in 8 M NaOH and 8 M NaClO 4 glasses are 0.02 and 0.0023 respectively. In 12 M LiCl, 5 M H 2SO 4 and 6 M H 3PO 4 glasses, energy transfer from the purine molecules to the surrounding solvent molecules is observed. This transfer of energy produces Cl − 2, SO − 4, SO − 3 and HPO − 4 radical ions.