Interaction of excited states of pterin with nucleotides

GABRIELA PETROSELLI; M. LAURA DÁNTOLA; FRANCO M. CABRERIZO; CAROLINA LORENTE; ESTHER OLIVEROS; ANDRÉS H. THOMAS

Cubatao/Santos, SP., Brasil,

Congreso; IX Encuentro Latinoamericano de Fotoquímica y Fotobiología (IX ELAFOT); 2008

            UV-A radiation (320-400 nm) induces damages to the DNA molecule and its components through different photosensitized reactions. Among these processes, photosensitized oxidations may occur through electron transfer or hydrogen abstraction (type I) and/or the production of singlet molecular oxygen (1O2) (type II). Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have demonstrated previously that oxidation of 2'-deoxyguanosine 5'-monophosphate (dGMP) by pterin (PT) in aqueous solution under UV-A irradiation occurs through two mechanisms.[1] Each mechanism contributes in different proportions according to the pH. The dominant mechanism in alkaline media involves the reaction of dGMP with 1O2 produced by energy transfer from the PT triplet state to molecular oxygen (type II). In contrast, under acidic pH conditions, where PT and the guanine moiety of dGMP are not ionized, the main pathway for dGMP oxidation involves an initial electron transfer between dGMP and the PT triplet state (type I mechanism). Moreover we have shown that PT is able to oxidize 2'-deoxyadenosine 5'-monophosphate (dAMP) in acidic media via a type I mechanism.[2]             In this work we evaluated which one of the excited states of pterin (singlet or triplet state) participate in those photosensitized reactions. To study the interaction between singlet excited state of pterin and nucleotides we did quenching of fluorescence experiments. We shown that dAMP and dGMP are able to quench the fluorescence of pterin thought both collisional and static quenching and we evaluated the bimolecular quenching rate constant. On the other hand, to study the interaction between triplet excited state of pterin and nucleotides we used quenching of 1O2 emission at 1270 nm. In those experiments we compare the Stern-Volmer constant calculated for the quenching of 1O2 by nucleotides using references (Rose Bengal or Phenalenone) as sensitizer or pterins. Based on these results it can be concluded that the triplet state of pterin is deactivated by both nucleotides.             According to the results, we postulate that the electron transfer reaction occurs between the triplet excited state of pterin and the nucleotides.    [1] Gabriela Petroselli, M. Laura Dántola, Franco M. Cabrerizo, Alberto L. Capparelli, Carolina Lorente, Esther Oliveros and Andrés H. Thomas. Oxidation of 2’-deoxyguanosine 5’-monophosphate photoinduced by pterin: type I versus type II mechanism. J. Am. Chem. Soc, 2008, 130, 3001-3011. [2] Gabriela Petroselli, Rosa Erra-Balsells, Franco Cabrerizo, Carolina Lorente, Alberto L. Capparelli, André M. Braun, Esther Oliveros and Andrés H. Thomas. Photosensitization of 2'-deoxyadenosine-5'-monophosphate by pterin. Org. Biomol. Chem., 2007, 5, 2792-2799.