Tryptophan photosensitization by pterin

VIRGINIE RAHAL; MARIANA PAULA SERRANO; PATRICIA VICENDO; ESTHER OLIVEROS; ANDRÉS H. THOMAS; CAROLINA LORENTE

Córdoba

Congreso; Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2012

Encuentro Latinoamericano de Fotoquímica y Fotobiología

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 Pterin (Ptr), the parent compound of oxidized or aromatic pterins, acts as photosensitizer through both type I (electron abstraction) and/or type II (production of singlet molecular oxygen (1O2)) mechanisms. Moreover, Ptr photoinduces DNA damage and oxidizes 2’-deoxyguanosine and 2’-deoxyadenine 5’-monophosphates (dGMP, dAMP)2,3 via electron transfer processes. Tryptophan (Trp), an esencial aminoacid,it is known as a target for oxidation by 1O2.4 But given their structural and reactivity similarity with guanine and its low redox potential, Trp may be also a potential target for pterin photosensitized mediated oxidation. To evaluate the capability of Ptr to photosensitize tryptophan, aqueous solutions containing both compounds were exposed to UV-A irradiation (320-400 nm) under different experimental conditions. The photochemical reactions were followed by UV/VIS spectrophotometry, HPLC, and an enzymatic method for H2O2 determination. In addition, fluorescence quenching and electronic paramagnetic resonance experiments were performed. Mechanistic analysis indicates that the Ptr-sensitized oxygenation/oxidation of Trp does not involve exclusively 1O2 as oxidation agent. By contrast, an electron transfer process plays a fundamental role in the photodegradation of Trp. In this mechanism, the excitation of Ptr is followed by an electron transfer from Trp molecule to the Ptr triplet excited state, leading to the formation of the corresponding ion radicals (Ptr•– and Trp•+). In the following step, the electron transfer from Ptr•– to O2 regenerates Ptr and forms the superoxide anion. The latter, may disproportionate with its conjugated acid (HO2•) to form H2O2 or react with Trp•+ to regenerate Trp. References 1 Lorente, C.; Thomas, A. H.; Acc. Chem. Res. 2006, 39, 395-402 2 Petroselli, G.; Dántola, M. L.; Cabrerizo, F. M.; Capparelli, A. L.; Lorente, C.; Oliveros, E.; Thomas, A. H.; J. Am. Chem. Soc. 2008, 130, 3001–3011. 3 Petroselli, G.; Erra-Balsells, R.; Cabrerizo, F. M.; Lorente, C.; Capparelli, A. L.; Braun, A. M.; Oliveros, E.; Thomas, A. H.; Org. Biomol.Chem. 2007, 5, 2792–2799 4 Pattison, D. I., Suryo Rahmanto A., Davies M. J., Photochem. Photobiol. Sci., 2012, 11, 38-53