Electron-transfer processes induced by the triplet state of pterins in aqueous solutions

Pterins (Pt) are heterocyclic compounds widespread in living systems. They participate in relevant biological processes, such as metabolic redox reactions, and can photoinduce the oxidation of biomolecules through electron-transfer mechanisms. We have investigated the electron-transfer pathways initiated by excited states of pterin (Ptr) and 6-methylpterin (Mep), selected as model compounds. The experiments were carried out in aqueous solutions under continuous UV-A irradiation, in the presence and in the absence of ethylenediaminetetraacetic acid (EDTA), used as an electron donor. The reactions were followed 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-spin trapping. The triplet excited states of Ptr and Mep are efficient electron acceptors, able to oxidize a Pt molecule in its ground state. The resulting radical anion (Pt
- −) reacts with dissolved O2 to yield O2
- −, regenerating the pterin. In the presence of EDTA, this reaction competes efficiently with the anaerobic reaction between Pt
- − and EDTA
- +, yielding the corresponding stable dihydroderivatives H2Pt. The effects of EDTA and dissolved O2 concentrations on the efficiencies of the different competing pathways were analyzed.