Reactivity of conjugated and unconjugated pterins with singlet oxygen (O2(1Δg)):: Physical quenching and chemical reactiont

Pterins (PTs) belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. We have investigated the reactivity of conjugated PTs (folic acid [FA], 10-methylfolic acid [MFA], pteroic acid IPA]) and unconjugated PTs (PT, 6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT]) with singlet oxygen (O-1(2)) in aqueous solutions, and compared the efficiencies of chemical reaction and physical quenching. The chemical reactions between 102, produced by photosensitization, and PT derivatives were followed by UV-visible spectrophotometry and high-performance liquid chromatography, and corresponding rate constants (k(r)) were evaluated. Whenever possible, products were identified and quantified. Rate constants of O-1(2) total quenching by the PT derivatives investigated were obtained from steady-state O-1(2) luminescence measurements. Results show that the behavior of conjugated PTs differs considerably from that of unconjugated derivatives, and the mechanisms of O-1(2) physical quenching by these compounds and of their chemical reaction with O-1(2) are discussed in relation to their structural features.