title: Mechanism of N(5)-ethyl-flavinium cation formation upon electrochemical oxidation of N(5)-ethyl-4a-hydroxyflavin pseudobase.
abstract: We investigated the oxidation behavior of 5-ethyl-4a-hydroxy-3-methyl-4a,5-dihydrolumiflavin (pseudobase Et-FlOH) in acetonitrile with the aim of determining if the two-electron oxidized Et-FlOH(2+) undergoes a release of hydroxyl cation and the production of 5-ethyl-3methyllumiflavinium cation (Et-Fl(+)). The focus of this work is to investigate the possibility of using Et-FlOH as a catalyst for water oxidation. The cyclic voltammetry demonstrates that Et-FlOH exhibits two one-electron oxidation potentials at +0.95 and +1.4 V versus normal hydrogen electrode (NHE), with the second oxidation potential being irreversible. The production of Et-Fl(+) is observed in the cyclic voltammetry of Et-FlOH and has been previously assigned to the release of OH(+) from the two-electron oxidized Et-FlOH(2+). The results of our study show that this is not the case: (i) we performed bulk electrolysis of the electrolyte solution at +2 V and then added Et-FlOH to the electrolyzed solution. We found that Et-Fl(+) is produced from this solution, even though Et-FlOH itself was not oxidized; (ii) reactions of Et-FlOH with chemical oxidants (ceric ammonium nitrate, nitrosyl tetrafluoroborate, and tetrabutylammonium persulfate) demonstrate that Et-Fl(+) production occurs only in the presence of strong Lewis acids, such as Ce(4+) and NO(+) ions. On the basis of these results, we propose that the production of Et-Fl(+) in the electrochemistry of Et-FlOH occurs because of the shift in the Et-FlOH/Et-Fl(+) acid-base equilibrium in the presence of protons released during anodic oxidation. We identified two sources of protons: (i) oxidation of traces of water present in the acetonitrile releases oxygen and protons and (ii) two-electron oxidized Et-FlOH(2+) releases protons located on the N(5)-alkyl chain. The release of protons from Et-FlOH(2+) was confirmed by cyclic voltammetry of Et-FlOH in the presence of pyridine as a base. The first oxidation peak of Et-FlOH at +0.95 V is reversible in the absence of pyridine. The addition of pyridine leads to the shift of the oxidation potential to a less positive value, which is consistent with a proton-coupled electron transfer (PCET). Furthermore, the anodic current increases, and the cathodic peak becomes irreversible, giving rise to two additional reduction peaks at -0.2 and -1 V. The same reduction peaks were observed in the high scan rate cyclic voltammogram of Et-FlOH in the absence of pyridine, implying that the release of protons indeed occurs from Et-FlOH(2+). To determine which functional group of Et-FlOH(.+) is the source of protons, we performed DFT calculations at the B3LYP/6-311++G** level of theory for a reaction of Et-FlOH(.+) with pyridine and identified two proton sources: (i) the >N-CH(2)- group of the N(5) alkyl chain and (ii) the -OH group in the 4a-position of the radical cation. Because the appearance of new reduction peaks at -0.2 and -1.0 V occurs in the model compound that lacks -OH protons (Et-FlOMe), we conclude that the proton removal occurs predominantly from the >N-CH(2)- moiety.
authors.affiliation: Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, USA.
authors.forename: Vincent
authors.initials: V
authors.lastname: Sichula
authors.forename: Ying
authors.initials: Y
authors.lastname: Hu
authors.forename: Ekaterina
authors.initials: E
authors.lastname: Mirzakulova
authors.forename: Samuel F
authors.initials: SF
authors.lastname: Manzer
authors.forename: Shubham
authors.initials: S
authors.lastname: Vyas
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Hadad
authors.forename: Ksenija D
authors.initials: KD
authors.lastname: Glusac
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D002412:Cations / Q000737:chemistry*; D055664:Electrochemical Techniques; D004566:Electrodes; D005415:Flavins / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D000097:Acetonitriles; D002412:Cations; D005415:Flavins; C001922:lumiflavin; C064924:N(5)-ethyl-4a-hydroxy-3-methyl-4a,5-dihydrolumiflavin; C032159:acetonitrile