Part I. Photochemical and photophysical studies of guanine derivatives: Intermediates contributing to its photodestruction mechanism in aqueous solution and the participation of the electron adduct

The low-intensity steady-state (254 nm), microsecond hash and nanosecond (2 66 nm) laser photolysis of some guanine (Gua) derivatives in aqueous soluti on were studied. A photodestruction yield between 10(-3) and 10(-2) at a ba se concentration of 75 mu M was determined for 254 nm irradiation at room t emperature using high-performance liquid chromatography, This yield decreas es with increasing purine concentration. For a similar concentration of the purine bases (2 +/- 1) x 10(-5) M, the yield increases as folIows: Gua app roximate to 9-ethylguanine < deoxyguanosine approximate to guanosine (Guo) < guanosine 5'-monophosphate, At concentrations higher than 2 x 10(-4) M th e Gua derivatives' photodestruction yield seems to converge to a limiting v alue of the order of 10(-4), This behavior is explained in terms of self-qu enching and aggregation effects which deactivate the excited states of the bases. The yields of electron photoejection have been determined in the nan osecond laser photolysis (0.083) and in the low-intensity steady-state (5.8 x 10-3) for Guo, Competition experiments using electron scavengers suggest that the electron adducts of the bases are one of the principal species pa rticipating in the photodestruction mechanism of these monomeric Gua. Close to 75% of the total destruction yield has contributions from initial react ions of the photojected electron at neutral pH, The quantum yield of photod estruction of Guo increases when the pH is increased as follows: 4.7 x 10(- 3) (pH 1.1), 6.5 x 10(-3) (pH 2.9), 7.7 x 10(-3) (pH 7.5) and 8.1 x 10(-3) (pH 11.9), This dependence on pH and the electron scavenger experiments pro ,ide further evidence for the radical anion or its protonated form as one o f the principal species involved in the photodestruction of the bases at th e different pH, Under oxygen saturated conditions a 22% increase in the des truction yield is observed for Guo, However, for the dinucleotides adenylyl (3' --> 5')-guanosine and thymidylyl (3' --> 5')2'-deoxyguanosine, the par ticipation of the electron is 41 and 36%, respectively, suggesting that goi ng into a more DNA or RNA-like structure, the participation of the electron adducts species in the photodamage of DNA and RNA decreases. A mechanism o f photodestruction for the Gua derivatives is proposed which takes into acc ount these findings.