Scavenging of riboflavin-photogenerated oxidative species by uric acid, xanthine or hypoxanthine: A kinetic study

Abstract

Air-equilibrated aqueous solutions of the purine bases (PBs) uric acid (URA), xanthine (XAN) and hypoxanthine (HXA) have been irradiated with visible light in the presence of riboflavin (Rf) as the only light-absorbing compound, to study to what extent the reactive oxygenated species (ROSs) singlet molecular oxygen, O 2( 1Δ g), and superoxide radical anion, O 2 −, are involved in the observed photodegradation processes. Both reactive species are generated from triplet excited riboflavin ( 3Rf*) by well-known processes: energy transfer from 3Rf* to ground state oxygen to yield O 2( 1Δ g), and electron transfer from each PB to 3Rf* to produce the radical anion Rf − that, after another electron transfer step to ground state oxygen, yields O 2 − and Rf. The kinetics of the involved processes have been studied by polarographic detection of oxygen uptake and time-resolved phosphorescence detection of O 2( 1Δ g), at concentrations in the range 0.05–0.5 mM in aqueous solutions at pH values 5, 7, 9, and 12, to compare the reactivity of the neutral and ionic species of each purine base. By means of Rose Bengal photosensitized reactions, it could be shown that O 2( 1Δ g) is quenched by the three PBs with an overall rate constant decreasing in the order URA > XAN > HXA, with values ranging from 1.16 × 10 9 M −1 s −1 (URA at pH 12) to 5 × 10 6 M −1 s −1 (HXA at pH 7). When Rf is used instead, the quenching process takes place mostly by a chemical mechanism that prevents the oxidation of Rf by O 2( 1Δ g). Experiments with Rf and superoxide dismutase indicate that the PBs also react with O 2 − by a mechanism which is not simple. With URA, largely the most efficient O 2( 1Δ g)-quencher, the major ROS generated is O 2 −, and the process is characterized by the largest rate of oxygen uptake, indicating efficient overall-scavenging and URA oxidation. With HXA, the less efficient O 2( 1Δ g)-quencher, the process shows a relatively high rate of oxygen consumption, especially in alkaline solution, likely as a result of the efficient generation of O 2 − from Rf − and its concomitant reaction with HXA. Finally, with XAN the quenching efficiency takes intermediate values. At physiological pH, URA is the most efficient antioxidant, showing the highest overall rate constant for O 2( 1Δ g) quenching, the lowest efficiency for the reaction with O 2( 1Δ g), and the highest relative rate of oxygen uptake.