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

Abstract

The electrochemical reduction mechanisms of 2 synthesized flavins (Flox) 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 Flox 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 Flox 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.

A biomimetic redox flow battery based on flavin mononucleotide

“…2a. At pH 8.6 and 10.0, only one redox peak is observed, centred at ∼−0.5 V versus Ag/AgCl, which suggests that the two-electron reaction proceeds in a concerted manner303132. Furthermore, this result shows that FMN 2– is stable in this pH range, as the CVs do not change from 8.6 to 10.0.…”

“…The electronic structure of FMN has been previously studied in regard to biochemical applications, using ultraviolet–vis (ultraviolet–visible) spectroscopy and electron paramagnetic resonance29303132. Its electron-transfer mechanism in water has also been reported33.…”

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

“…83,104,105 The latter two applications of MER/MEO are indirect measures of the target properties and therefore require careful attention to the assumptions of the method in order to avoid misleading results. 106 Aprotic solvents have long been used in organic electrochemistry, [107][108][109][110][111][112][113][114][115] and also have only recently been shown to be useful for characterization of NOM redox properties. 98,116,117 This approach was rooted in recognition that NOM under aqueous conditions develops tertiary structure, which could result in protection of redox active functional groups from direct interaction with electrodes.…”

A Biomimetic Redox Flow Battery Based on a Flavin Mononucleotide

“…2a. At pH 8.6 and 10.0, only one redox peak is observed, centred at B À 0.5 V versus Ag/AgCl, which suggests that the two-electron reaction proceeds in a concerted manner [30][31][32] . Furthermore, this result shows that FMN 2is stable in this pH range, as the CVs do not change from 8.6 to 10.0.…”

“…The electrochemistry of FMN-Na aqueous electrolyte was examined using cyclic voltammetry at various pH levels. The electronic structure of FMN has been previously studied in regard to biochemical applications, using ultraviolet-vis (ultraviolet-visible) spectroscopy and electron paramagnetic resonance [29][30][31][32] . Its electron-transfer mechanism in water has also been reported 33 .…”