Redox Reactions of Reduced Flavin Mononucleotide (FMN), Riboflavin (RBF), and Anthraquinone-2,6-disulfonate (AQDS) with Ferrihydrite and Lepidocrocite

Flavins are secreted by the dissimilatory iron-reducing bacterium Shewanella and can function as endogenous electron transfer mediators. To assess the potential importance of flavins in Fe­(III) bioreduction, we investigated the redox reaction kinetics of reduced flavin mononucleotide (i.e., FMNH₂) and reduced riboflavin (i.e., RBFH₂) with ferrihydrite and lepidocrocite. The organic reductants rapidly reduced and dissolved ferrihydrite and lepidocrocite in the pH range 4–8. The rate constant k for 2-line ferrihydrite reductive dissolution by FMNH₂ was 87.5 ± 3.5 M^–1·s^–1 at pH 7.0 in batch reactors, and k was similar for RBFH₂. For lepidocrocite, k was 500 ± 61 M^–1·s^–1 for FMNH₂ and 236 ± 22 M^–1·s^–1 for RBFH₂. The surface area normalized initial reaction rates (r_a) were between 0.08 and 77 μmol·m^–2·s^–1 for various conditions in stopped-flow experiments. Initial rates (r_o) were first-order with respect to iron­(III) oxide concentration, and r_a increased with decreasing pH. Poorly crystalline 2-line ferrihydrite yielded the highest r_a, followed by more crystalline 6-line ferrihydrite and crystalline lepidocrocite. Compared to a previous whole-cell study with Shewanella oneidensis strain MR-1, our findings suggest that the reduction of electron transfer mediators by the Mtr (i.e., metal-reducing) pathway coupled to lactate oxidation is rate limiting, rather than heterogeneous electron transfer to the iron­(III) oxide.