title: 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.
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University , 637371, Singapore.
authors.forename: Serena L J
authors.initials: SL
authors.lastname: Tan
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University , 637371, Singapore.
authors.forename: Maria L
authors.initials: ML
authors.lastname: Novianti
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University , 637371, Singapore.
authors.forename: Richard D
authors.initials: RD
authors.lastname: Webster
mesh_terms: D055664:Electrochemical Techniques; D004579:Electron Transport; D005415:Flavins / Q000737:chemistry*; D015394:Molecular Structure; D011522:Protons; D011809:Quinones / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D014867:Water / Q000032:analysis*
chemical_list: D005415:Flavins; D011522:Protons; D011809:Quinones; D012997:Solvents; D014867:Water