Electron transfer from EDTA to triplet states of pterins

MARÍA L. DÁNTOLA; MARIANA VIGNONI; COSNTANZA GONZÁLEZ; CAROLINA LORENTE; ANDRÉS H. THOMAS

La Serena

Congreso; X Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2010

Solar radiation induces modifications to genomic DNA and is implicated in the generation of human skin cancers.[i] Although nucleobases absorb very weakly above 300 nm, both UV-B (280-320 nm) and UV-A (320-400 nm) radiations can induce modifications to DNA through photosensitized reactions.[ii] This indirect action can take place through different mechanisms: Energy transfer from the triplet state of the photosensitizer to nucleobases,[iii] generation of radicals (type I mechanism), e.g. via electron transfer or hydrogen abstraction, and/or the production of singlet molecular oxygen (1O2) (type II mechanism).[iv] 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.[v] 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.[vi] Although, a general mechanism for the photoreduction of pterins has not been suggested, obviously an electron transfer process must be involved.             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. 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.              Mep in the presence of EDTA in argon atmosphere 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. [i]       Van der Leun J. C.; Gruijl de F. R. Photochem. Photobiol. Sci. 2002, 1, 324. [ii]       Cadet J.; Sage E.; Douki T. Mutat. Res. 2005, 571, 3. [iii]      Delatour T.; Douki T.; D’Ham C.; Cadet J. J. Photochem. Photobiol. B: Biol. 1998, 44, 191. [iv]      Foote C. S. Photochem. Photobiol. 1991, 54, 659. [v]       Kappock T. J. and Caradonna J. P. Chemical Reviews, 1996, 96, 2659. [vi]      Kritsky, M. S.; Lyudnikova, T. A.; Mironov, E. A.; Moskaleva, I. V. J. Photochem. Photobiol. B 1997, 39, 43.