Differences in Proton-Coupled Electron-Transfer Reactions of Flavin Mononucleotide (FMN) and Flavin Adenine Dinucleotide (FAD) between Buffered and Unbuffered Aqueous Solutions

Abstract

The electrochemical reduction mechanisms of flavin mononucleotide (FMN) in buffered aqueous solutions at pH 3-11 and unbuffered aqueous solutions at pH 2-11 were examined in detail using variable-scan-rate cyclic voltammetry (ν = 0.1-20 V s(-1)), controlled-potential bulk electrolysis, UV-vis spectroscopy, and rotating-disk-electrode voltammetry. In buffered solutions at pH 3-5, FMN undergoes a two-electron/two-proton (2e(-)/2H(+)) reduction to form FMNH2 at all scan rates. When the buffered pH is increased to 7-9, FMN undergoes a 2e(-) reduction to form FMN(2-), which initially undergoes hydrogen bonding with water molecules, followed by protonation to form FMNH(-). At a low voltammetric scan rate of 0.1 V s(-1), the protonation reaction has sufficient time to take place. However, at a higher scan rate of 20 V s(-1), the proton-transfer reaction is outrun, and upon reversal of the scan direction, less of the FMNH(-) is available for oxidation, causing its oxidation peak to decrease in magnitude. In unbuffered aqueous solutions, three major voltammetric waves were observed in different pH ranges. At low pH in unbuffered solutions, where [H(+)] ≥ [FMN], (FMN)H(-) undergoes a 2e(-)/2H(+) reduction to form (FMNH2)H(-) (wave 1), similar to the mechanism in buffered aqueous solutions at low pH. At midrange pH values (unbuffered), where pH ≤ pKa of the phosphate group and [FMN] ≥ [H(+)], (FMN)H(-) undergoes a 2e(-) reduction to form (FMN(2-))H(-) (wave 2), similar to the mechanism in buffered aqueous solutions at high pH. At high pH (unbuffered), where pH ≥ pKa = 6.2 of the phosphate group, the phosphate group loses its second proton to be fully deprotonated, forming (FMN)(2-), and this species undergoes a 2e(-) reduction to form (FMN(2-))(2-) (wave 3).

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.…”

SERS speciation of the electrochemical oxidation–reduction of riboflavin

“…These findings agree with previous fluorescence and absorbance experiments of riboflavin at different pH values that demonstrate the neutral form is present from pH 3 to 9 and the anionic at a pH greater than 11. 17, 22 …”

“…The expected pathway at each pH examined is indicated by the green (pH 4 and 7) and the blue (pH 12) highlighted species. 17, 18 …”

Electrochemical Control of Bioluminescence by Blocking the Adsorption of the Bacterial Luciferase Using a Mercaptobipyridine Self-assembled Monolayer

“…It can be expected that the bipyridinium unit functions as a mediator for the reduction process of the FMN. The standard redox potential of the bipyridinium compound is about -0.31 V 11 (vs. Ag/AgCl), and that of FMN is about -0.43 V. 12 Although the redox potential of 4BMBP is higher than that of FMN, electron transfer from 4BMBP to FMN is expected to be possible at a potential lower than the FMN reduction:…”