Electrochemically Induced Chemically Reversible Proton-Coupled Electron Transfer Reactions of Riboflavin (Vitamin B₂)

Abstract

The electrochemical behavior of the naturally occurring vitamin B(2), riboflavin (Fl(ox)), was examined in detail in dimethyl sulfoxide solutions using variable scan rate cyclic voltammetry (ν = 0.1 - 20 V s(-1)) and has been found to undergo a series of proton-coupled electron transfer reactions. At a scan rate of 0.1 V s(-1), riboflavin is initially reduced by one electron to form the radical anion (Fl(rad)(•-)) at E(0)(f) = -1.22 V versus Fc/Fc(+) (E(0)(f) = formal reduction potential and Fc = ferrocene). Fl(rad)(•-) undergoes a homogeneous proton transfer reaction with the starting material (Fl(ox)) to produce Fl(rad)H(•) and Fl(ox)(-), which are both able to undergo further reduction at the electrode surface to form Fl(red)H(-) (E(0)(f) = -1.05 V vs Fc/Fc(+)) and Fl(rad)(•2-) (E(0)(f) = -1.62 V vs Fc/Fc(+)), respectively. At faster voltammetric scan rates, the homogeneous reaction between Fl(rad)(•-) and Fl(ox) begins to be outrun, which leads to the detection of a voltammetric peak at more negative potentials associated with the one-electron reduction of Fl(rad)(•-) to form Fl(red)(2-) (E(0)(f) = -1.98 V vs Fc/Fc(+)). The variable scan rate voltammetric data were modeled quantitatively using digital simulation techniques based on an interconnecting "scheme of squares" mechanism, which enabled the four formal potentials as well as the equilibrium and rate constants associated with four homogeneous reactions to be determined. Extended time-scale controlled potential electrolysis (t > hours) and spectroscopic (EPR and in situ UV-vis) experiments confirmed that the chemical reactions were completely chemically reversible.

A single-molecule multicolor electrochromic device generated through medium engineering

“…These findings indicate that the corresponding TM1-enolate is most likely obtained through electron-coupled proton transfer, 43,44 in which radical anions of nitro and carbonyl groups serve as the 'electro-base' 26,28 to extract the proton from the methylene unit of TM1 to induce its enolization.…”

Biologically inspired pteridine redox centres for rechargeable batteries

“…During photosynthesis and cellular respiration, for example, flavin cofactors serve as a key redox centre facilitating protoncoupled electron transfer reactions for energy transduction. During the reaction, redox cycling occurs at the diazabutadiene of the isoalloxazine moiety 21,22 . On the basis of this electrochemical cellular metabolism, we recently proposed that natural flavin cofactors can be applied to lithium-storing electrodes in rechargeable batteries 23 .…”

Flavin mononucleotide-exfoliated graphene flakes as electrodes for the electrochemical determination of uric acid in the presence of ascorbic acid

“…It is widely accepted that, in aqueous media and when the riboflavin is present in solution, fully oxidized flavin (FMN) is reduced to flavin-hydroquinone (FMNH 2 ) in "one-step" in a 2e -/2H + reduction (scheme 1) [26]. These processes occur at around -0.4 V vs Ag/AgCl, with the peaks referred to as B-B´ being related to these faradaic reactions [27].…”

“…Moreover, the presence of Pt nanoparticles on the graphene support also produces a slight improvement, showing high currents for lower UA concentrations, due to the high electroactivity of Pt towards several reactions. Moreover, the surface of the electrode contains FMN molecules, which can act as intermediates by accepting electrons from many functional groups and donating electrons to other molecules [26][27][28]. Then, FMN can act as a mediator of the oxidation of these compounds, thus producing some interactions between the analytes and the surface.…”