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 (ν = 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 Fl^•- receives a proton from another Fl_ox to form FlH^• (and concomitantly produce the deprotonated flavin, Fl^-), although the equilibrium constant for this process favors the back reaction. Any FlH^• formed at the electrode surface immediately undergoes another 1e^- reduction to form FlH^-, which reacts with Fl^- to form 2 molecules of Fl^•-. Further 1e^- reduction of Fl^•- 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^•- (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^•-, 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.