Singlet oxygen (O2(1Δg)) quenching by dihydropterins

Pterins 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. Dihydropterins are one of the biologically active forms of pterins. The photoinduced production and quenching of singlet oxygen (O-1(2)) by a series of dihydropterins (7,8-dihydrobiopterin (DHBPT), 7,8-dihydroneopterin (DHNPT), 6-formyl-7,8-dihydropterin (FDHPT), sepiapterin (SPT), 7,8-dihydrofolic acid (DHFA), and 7,8-dihydroxanthopterin (DHXPT)) in aqueous solution at physiological pH (similar to 7) were investigated, and the quantum yields of O-1(2) production (Phi(Delta)) and rate constants of total quenching (k(t)) of O-1(2) were determined. Studied compounds do not produce O-1(2) under UV-A irradiation and are very efficient O-1(2) quenchers. The chemical reactions between O-1(2) and dihydropterin derivatives were investigated, and the corresponding rate constants (k(r)) were found to be particularly high. The oxidized pterin derivatives, biopterin (BPT), neopterin (NPT), 6-formylpterin (FPT), and folic acid (FA), were identified and quantified during the reaction of O-1(2) with DHBPT, DHNPT, FDHPT, and DHFA, respectively. Besides the oxidation of the dihydropyrazine ring to yield the corresponding oxidized pterins, a second oxidation pathway, leading to fragmentation of the dihydropterin and formation of non-pterinic products, was identified. Mechanisms and biological implications are discussed.