Electron Transfer from EDTA to Triplet States of Pterins

M. LAURA DÁNTOLA; MARIANA VIGNONI; CONSTANZA GONZALEZ; CAROLINA LORENTE; ANDRÉS H. THOMAS

La Serena

Congreso; X Encuentro Latinoamericano de Fotoquímica y Fotobiología (X ELAFOT); 2010

Universidad de Chile, Universidad de Santiago, Pontificia Universidad Católica y Universidad Andrés Bello

Pterins, a family of heterocyclic compounds, are present in biological systems in multiple forms and play different roles ranging from pigments to enzymatic cofactors for numerous redox and one-carbon transfer reactions.[i]Upon excitation with UV-A radiation, pterins are able to photoinduce DNA damage. The mechanism involved in this process was proposed to be an electron transfer with the subsequent formation of the biomolecule radical cation and a pterin radical anion. In this mechanism the photosensitizer is not consumed. In contrast, it has been suggested that some pterins in the presence of electron donors undergo photoreduction, yielding the corresponding dihydropterin derivative, which in turn is reduced to a tetrahydropterin.[ii]             The present work is aimed to get a better understanding of photoinduced electron transfer mechanisms wherein 6-methylpterin (Mep) act as photosensitizers and ethylenediaminetetraacetate (EDTA) act as electron donor (D). The experiments were performed in slightly acidic aqueous solution under UV-A radiation. We investigated the production of H2O2, the effect of dissolved O2 in the efficiencies of the processes and the experimental conditions needed to achieve photoreduction of Mep. hv   Under anaerobic conditions, Mep in the presence of EDTA undergoes photoreduction, yielding 6-methyl-7,8-dihydropterin (H2Mep). However, in air-saturated solutions this reduction does not take place; instead of that, the Mep radical anion reduces O2 into superoxide radical (O2•–), which eventually dismutates into H2O2. Experiments in the presence of selective quenchers suggested the participation of Mep triplet state as electron acceptor. The specific mechanism proposed for these photosensitized reaction is summarized in Equations 1-7. Mep 1Mep* (1) 1Mep*   3Mep* (2) 3Mep* + O2 Mep + 1O2 (3) 3Mep* + D Mep•– + D•+ (4) Mep•– + D•+ H2Mep + D(-2H) (5) [Mep•– + D•+] [HMep• + D(-H)•] (5a) [HMep• + D(-H)•] H2Mep + D(-2H) (5b) Mep•– + O2 Mep + O2•– (6) 2 H+ + 2 O2•– H2O2 + O2 (7)   [i]       Kappock T. J. and Caradonna J. P. Chemical Reviews, 1996, 96, 2659. [ii]       Kritsky, M. S.; Lyudnikova, T. A.; Mironov, E. A.; Moskaleva, I. V. J. Photochem. Photobiol. B 1997, 39, 43.