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

摘要

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., FMNH_2) and reduced riboflavin (i.e., RBFH_2) with ferrihydrite and lepidocrocite. The organic reductants rapidly reduced and dissolved ferrihydrite and [epidocrocite in the pH range 4-8. The rate constant k for 2-line ferrihydrite reductive dissolution by FMNH_2 was 87.S ± 3.5 M~(-1)·s~(-1) at pH 7.0 in batch reactors, and k was similar for RBFH_2. For lepidocrocite, k was 500 ± 61 M~(-1)·s~(-1) for FMNH_2 and 236 ± 22 M~(-1)·s~(-1) for RBFH_2. 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_u followed by more crystalline 6-h'ne 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.