Reactions of aqueous iron–DFOB (desferrioxamine B) complexes with flavin mononucleotide in the absence of strong iron(II) chelators

The mechanisms controlling microbial uptake of Fe III–siderophore complexes and subsequent release of the metal for cellular use have been extensively studied in recent years. Reduction of the Fe III center is believed to be necessary to labilize the coordinated Fe and facilitate exchange with cellular ligands. Previous studies report reduction of Fe III–DFOB by various reducing agents in solutions containing Fe II-chelating colorimetric agents for monitoring reaction progress, but the importance of these findings is unclear because the colorimetric agents themselves stabilize and enhance the reactions being monitored. This study examines the reduction of Fe III complexes with DFOB (desferrioxamine B), a trihydroxamate siderophore, by the fully reduced hydroquinone form of flavin mononucleotide (FMN HQ) in the absence of strong Fe II-chelating agents, and Fe redox cycling in solutions containing DFOB and oxidized and reduced FMN species. Experimental results demonstrate that the rate and extent of Fe III–DFOB reduction is strongly dependent on pH and FMN HQ concentration. At pH ⩾ 5, incomplete Fe III reduction is observed due to two processes that re-oxidize Fe II, namely, the autodecomposition of Fe II–DFOB complexes (Fe II oxidation is coupled with reduction of a protonated hydroxamate moiety) and reaction of Fe II–DFOB complexes with the fully oxidized flavin mononucleotide product (FMN OX). Chemical speciation-dependent kinetic models for the forward reduction process and both reverse Fe II oxidation processes are developed, and coupling kinetic models for all three Fe redox processes leads to successful predictions of steady-state Fe II concentrations observed over a range of pH conditions in the presence of excess FMN HQ and FMN OX. The observed redox reactions are also in agreement with thermodynamic constraints imposed by the combination of Fe III/Fe II and FMN OX/FMN HQ redox couples. Quantitative comparison between kinetic trends and changing Fe speciation reveals that FMN species react predominantly with diprotonated Fe III–DFOB and Fe II–DFOB complexes, where protonation of one hydroxamate group opens up two Fe coordination positions. This finding suggests that ternary complex formation (FMN–Fe–DFOB) facilitates inner-sphere electron transfer reactions between the flavin and Fe center.