Electron Transfer Processes Induced by the Triplet State of Pterins in Aqueous Solutions

M. LAURA DÁNTOLA; MARIANA VIGNONI; CONSTANZA GONZALEZ; CAROLINA LORENTE; PATRICIA VICENDO; ESTHER OLIVEROS; ANDRÉS H. THOMAS

Ferrara

Simposio; XXIII IUPAC Symposium on Photochemistry; 2010

IUPAC and Università degli Studi di Ferrara

Pterins (Pt), heterocyclic compounds widespread in living systems, participate in relevantbiological processes, such as metabolic redox reactions. Under UV-A excitation (320–400 nm),pterins can fluoresce, undergo photooxidation, generate reactive oxygen species (ROS)[1] andphotoinduce oxidation of biomolecules through electron transfer mechanisms.[2,3] We haveinvestigated the electron transfer pathways initiated by excited states of pterin and 6-methylpterin.The experiments were carried out in aqueous solutions under continuous UV-A irradiation andethylenediaminetetraacetic acid (EDTA) was used as an electron donor. The reactions werefollowed by UV/Vis spectrophotometry, HPLC and an enzymatic method for H2O2 determination. The formation of the superoxide anion (O2•‒) was investigated by electron paramagnetic resonance spintrapping.In neutral or slightly acidic aqueous solutions, triplet states of pterins (3Pt*) initiate a seriesof competing reactions that can be summarized as follows:Pt ---1Pt*------ 3Pt*3Pt*------- -----Pt (a)3Pt* + O2 ------Pt + 1O2 (b)3Pt* + D --------Pt•– + D•+ (c)Pt•– + D•+ ------Pt + D (d)Pt•– + O2 -------Pt + O2•– (e)Pt•– + D•+ ------H2Pt + D(-2H) (f)D(-H)• + O2 ----D(-2H) + H+ + O2•‒ (g)O2•– + D•+----- O2 + D (h)2 H+ + 2 O2•–-- H2O2 + O2 (i)Besides intersystem crossing to the ground state (a), 3Pt* can be deactivated by dissolved O2 (b).[1]Alternatively, 3Pt* can react with an electron donor (D) (c), which can be Pt itself or a differentcompound. Three different pathways are possible for the resulting radical anion (Pt•–): back electron transfer to D•+ (Reaction d, main reaction in the absence of O2 and of an electron donor other than Pt itself), electron transfer to O2 (e) or reaction with D•+ to yield a dihydroderivative ((f), main reaction in the presence of the electron donor EDTA under anaerobic conditions). An additional source of O2•‒ is the reduction of O2 by the neutral form of D•+ (g). Finally, O2•‒ can react with D•+ (h) or undergo disproportionation yielding H2O2 and O2 as final products (i).[1] Lorente, C.; Thomas, A. H.; Acc. Chem. Res.2006, 39, 395-402.[2]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.[3] 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.hν ISCN1