Tryptophane photosensitization by pterin

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

Mendoza

Congreso; 21st Inter-American Photochemical Society Conference; 2011

Inter-American Photochemical Society

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 act as photosensitizer through type I (electron abstraction) and/or type II (the production of singlet molecular oxygen (1O2)) mechanisms. Moreover, pterin photoinduces DNA damage and oxidizes 2´-deoxyguanosine and 2´-deoxyadenine 5´-monophosphates (dGMP, dAMP)2,3 via an electron transfer process. Due to the structural similarity with guanine and its low redox potential, tryptophan may be also a potential target for pterin mediated oxidation.To evaluate the capability of pterin (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 adition, fluorescence quenching experiments were performed. During irradiation of solutions containing Ptr and Tryp, an increase of the absorbance at 340 nm was observed. HPLC analysis clearly showed that this photoreaction occurred only in the presence of oxygen and led to a high consumption of Tryp, whereas the amount of Ptr remained stable. These results suggest that Tryp may be oxydized by pterin via a mechanism involving, at least in part, an electron transfer process. This reaction yields different photoproducts as shown by HPLC analysis. These preliminary results point out for the first time that pterins may not only photosensitize nucleic acids but also proteins via a complex reaction mechanism. Investigations are under progress to better characterize this original photoreaction. 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