Purine and Pyrimidine Nucleotides Photosensitization by Pterin

M.P. SERRANO; C. LORENTE; F.E. MORÁN VIEYRA; C.D. BORSARELLI; A.H. THOMAS

Córdoba

Congreso; 16th INTERNATIONAL CONGRESS ON PHOTOBIOLOGY; 2014

International Union of Photobiology

Pterins belong to a family of heterocyclic compounds present in a wide range of living systems and participate in relevant biological functions. Under UV-A excitation (320?400 nm), pterins can fluoresce, undergo photooxidation and generate reactive oxygen species (ROS) [1]. In the presence of oxygen, pterin (Ptr) act as photosensitizer through type I (electron abstraction) and/or type II (1O2-mediated oxidation) mechanisms [2-3]. The photosensitized degradations of purine and pyrimidine nucleotides by Ptr were studied in neutral aqueous solutions upon UV-A irradiation (350 nm) at room temperature, under different experimental conditions. The photochemical reactions were followed by UV⁄vis spectrophotometry and HPLC, and the photoproducts were analyzed by means of electrospray ionization mass spectrometry. Photophysical properties of the triplet excites states of the sensitizer were characterized using laser flash photolysis. After analysis of the results, reaction mechanisms were proposed. Under anaerobic conditions, for purine nucleotides, a recombination of the radicals occurred, and no consumption of the nucleotide was registered On the other hand, for pyrimidine nucleotides, the formation of an adduct between the substrate and the sensitizer was observed. The fluorescence properties of this adduct are similar to those of Ptr itself. In the presence of O2, it was observed more consumption of the nucleotides than in anaerobic condition. In this case, there exist a competition between different reactions that includes type I and type II mechanisms for guanine nucleotide. For the others, which do not react with singlet oxygen, the reaction is initiated by an electron transfer from the nucleotide to the triplet excited state of the pterin yielding the corresponding pair of radical ions (Pt?− and dNMP?+), with successive photosensitizer recovery by electron transfer from Pt?− to O2. Finally, dNMP?+ participates in subsequent reactions to yield degradation products.