The 254 nm low intensity and 266 nm laser photochemistry of adenosine. Effect of pH and concentration on the reactive precursors of the principal products, adenine and FAPyAde

The 254 nm low intensity steady-state photolysis of adenosine in aqueous solutions at different pHs and concentrations was studied. The quantum yield of photodestruction of adenosine decreases as the pH is increased: (1.7 ± 0.1) × 10^-2 (pH 2.6), (1.20 ± 0.04) × 10^-2 (pH 6.5) and (0.29 ± 0.01) × 10^-2 (pH 13.3). For the photodestruction of adenosine and the formation of adenine the quantum yield depends on the initial ground-state concentrations of adenosine and converges to a limiting value of the order of 10^-3 and 10^-4, respectively. The effect of different substrates that react selectively with postulated reactive intermediates of adenosine was studied and the short-lived precursors for the formation of the major products are proposed. For adenine and 4, 6-diamino-5-formamidopyrimidine, two of the products formed with the highest yields, their quantum yield of formation were determined under different experimental conditions. Contributions from electron adducts of the base play a major role in N-glycosidic bond cleavage for the nucleoside. Increases in pH and concentration result in a decrease in the yield of formation of adenine. The dependence on pH and the electron scavenger experiments provide further evidence for the radical anion or its protonated form to be one of the principal species involved in the process of base release at neutral pH. At an acidic pH a tautomer of the radical cation of adenosine is proposed as the precursor for adenine formation. The relative efficiency of the radical cation of adenosine for initiating the release of adenine at neutral pH is intrinsically low (35-43%) and this correlates with the efficiency reported for other adenine-containing polymers. Furthermore, the photoionization of adenosine using a 266 nm nanosecond laser excitation occurs through a combination of one- and two-photon processes under the experimental conditions used. Reactions of OH radicals and oxygen reactive species may also result in base release as shown in irradiations done in the presence of N₂O and O₂ as additives. For 4, 6-diamino-5-formamidopyrimidine, the addition N₂O does not affect its yield, implying that the hydroxyl radicals are not involved in its formation. However, the presence of O₂, T1⁺, Ag⁺, OH^-, H⁺ or alcohols inhibits its formation.