Interaction of excited states of pterin with nucleotides

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

Cubatao, San Pablo, Brasil

Congreso; IX Encuentro Latinoamericano de Fotoquimica y fotobiología; 2008

CEPEMA-USP

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 previously demonstrated that pterin (PT) (Figure1), under UV-A irradiation, is able to photoinduce oxidation of 2´-deoxyguanosine 5´-monophosphate (dGMP) and 2´-deoxyadenosine 5´-monophosphate (dAMP) in aqueous solution. The mechanism involved in these processes consists in an electron transfer from the nucleotides to excited pterin In this work we present a study aimed to evaluate the interaction of excited states of pterin derivatives with nucleotides. In order to study the deactivation of the lowest singlet excited states, experiments of fluorescence quenching of PT by nucleotides were performed. The nucleotides used as quenchers were (dGMP, dAMP and 2´-deoxycitidine 5´-monophosphate (dCMP)). The purine nucleotides (dAMP and dGMP) are able to quench the fluorescence of PT through both collisional and static quenching. In contrast, the quenching of the fluorescence of the PT by dCMP is purely dynamic. The bimolecular quenching rate constants were determined. On the other hand, experiments of quenching of 1O2 emission at 1270 nm were carried out to study the interaction between triplet excited state of pterin and nucleotides. We compare the Stern-Volmer constant calculated for the quenching of 1O2 by nucleotides using references (Rose Bengal or Phenalenone) or PT as 1O2 sensitizers.             A detailed kinetic analysis performed on these results suggests that nucleotides interact with both singlet and triplet excited state, but the latter is that involved in the photosensitized oxidations.