Probing the Molecular Interactions of Electrochemically\nReduced Vitamin B<sub>2</sub> with CO<sub>2</sub>

The electrochemical reduction of\nriboflavin (vitamin B<sub>2</sub>) in a dimethyl sulfoxide solvent\nwas examined under a CO<sub>2</sub> atmosphere and compared with results\nunder an argon atmosphere.\nVariable-scan-rate cyclic voltammetry combined with controlled potential\nelectrolysis (CPE) and analysis by UV–vis and EPR spectroscopies\nprovided insights into the nature of interactions of reduced flavins\nwith dissolved CO<sub>2</sub>. Reductive exhaustive CPE experiments\nunder CO<sub>2</sub> indicated an overall two-electron stoichiometry,\ncompared to one-electron reduction under an argon atmosphere, due\nto the lowering of the formal one-electron reduction potential of\nthe flavin radical anion to form the dianion, which can be rationalized\nby riboflavin–CO<sub>2</sub> molecular interactions. UV–vis\nspectroscopic measurements confirmed complete chemical reversibility\nof the redox transformations over extended time scales. Digital simulation\nmodeling of the voltammetric data enabled extraction of thermodynamic\nand kinetic parameters for the proposed mechanism, comprising multiple\nproton-coupled electron transfer steps, diamagnetic anions, radical\nanions, and neutral radical intermediates enroute to the fully reduced\nstate, as well as evidence of a long-lived solution phase complex\nof the reduced riboflavin with CO<sub>2</sub>.