Effects of Low to Intermediate Water Concentrations on Proton-Coupled Electron Transfer (PCET) Reactions of Flavins in Aprotic Solvents and a Comparison with the PCET Reactions of Quinones

The electrochemical reduction mechanisms of 2 synthesized flavins (Fl(ox)) were examined in detail in deoxygenated solutions of DMSO containing varying amounts of water, utilizing variable scan rate cyclic voltammetry (nu = 0.1-20 V s(-1)), controlled-potential bulk electrolysis, and UV-vis spectroscopy. Flavin 1, which contains a hydrogen atom at N(3), is capable of donating its proton to other reduced flavin species. After 1e(-) reduction, the initially formed F1(center dot-) receives a proton from another Fl(ox) to form FlH(center dot) (and concomitantly produce the deprotonated flavin, Fl(-)), although the equilibrium constant for this process favors the back reaction. Any FlH(center dot) formed at the electrode surface immediately undergoes another 1e(-) reduction to form FlH(-), which reacts with Fl(-) to form 2 molecules of Fl(center dot-). Further 1e(-) reduction of Fl(center dot-) at more negative potentials produces the dianion, Fl(2-), which can also be protonated by another Fl(ox) to form FlH(-) and Fl(-). Flavin 2, which is methylated at N(3) (and therefore has no acidic proton), undergoes a simple chemically reversible 1e(-) reduction process in DMSO provided the water content is low (<100 mM). Further 1e(-) reduction of Fl(center dot-) (from flavin 2) at more negative potentials leads to the dianion, Fl(2-), which is protonated by trace water in solution to form FlH(-), similar to the mechanism of flavin 1 at high scan rates. Addition of sufficient amounts of water to nonaqueous solvents results in protonation of the anion radical species, Fl(center dot-), for both flavins, causing an increase in the amount of FlH(-) in solution. This behavior contrasts with what is observed for quinones, which are also reduced in two 1e(-) steps in aprotic organic solvents to form the radical anions and dianions, but are able to exist in hydrogen-bonded forms (with trace or added water) without undergoing protonation.