The mechanisms controlling microbial uptake of FeIII-siderophore complexes and subsequent release of the metal for cellular use have been extensively studied in recent years. Reduction of the FeIII center is believed to be necessary to labilize the coordinated Fe and facilitate exchange with cellular ligands. Previous studies report reduction of FeIII-DFOB by various reducing agents in solutions containing FeII-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 FeIII complexes with DFOB (desferrioxamine B), a trihydroxamate siderophore, by the fully reduced hydroquinone form of flavin mononucleotide (FMNHQ) in the absence of strong FeII-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 FeIII-DFOB reduction is strongly dependent on pH and FMNHQ concentration. At pH ≥ 5, incomplete FeIII reduction is observed due to two processes that re-oxidize FeII, namely, the autodecomposition of FeII-DFOB complexes (FeII oxidation is coupled with reduction of a protonated hydroxamate moiety) and reaction of FeII-DFOB complexes with the fully oxidized flavin mononucleotide product (FMNOX). Chemical speciation-dependent kinetic models for the forward reduction process and both reverse FeII oxidation processes are developed, and coupling kinetic models for all three Fe redox processes leads to successful predictions of steady-state FeII concentrations observed over a range of pH conditions in the presence of excess FMNHQ and FMNOX. The observed redox reactions are also in agreement with thermodynamic constraints imposed by the combination of FeIII/FeII and FMNOX/FMNHQ redox couples. Quantitative comparison between kinetic trends and changing Fe speciation reveals that FMN species react predominantly with diprotonated FeIII-DFOB and FeII-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.
ASJC Scopus subject areas
- Geochemistry and Petrology