Coordination Chemistry and Redox Processes in Siderophore-Mediated Iron Transport

Abstract

In this mini-review we present an environmental iron mobility/transport scheme consisting of inter-related controls, whereby the first coordination shell of iron modulates the iron redox potential (E1/2), and the oxidation state of iron controls the chemistry of the first coordination sphere and therefore the immediate chemical environment of the iron. Siderophores (microbially generated iron specific chelators) may be viewed as iron redox mediators. Siderophore chelation of environmental iron in a reduced (Fe(II)) oxidation state results in facile air oxidation of iron due to the negative redox potentials observed for Fe-siderophore complexes. This solubilizes the iron and locks it into a specific coordination environment, thereby preventing hydrolysis and precipitation. The high-spin Fe3+ → Fe+ electron transfer process may be viewed as a switch that controls the thermodynamic stability and kinetic lability of the first coordination shell. Reduction of iron(III)-siderophore complexes to iron(II)-siderophore complexes decreases thermodynamic stability, increases the rate of siderophore ligand exchange, and increases the ease of siderophore donor atom protonation, thus facilitating a rapid turnover of the first coordination shell. Results are presented for iron-siderophore pH and oxidation state dependent speciation studies that are relevant to environmental and microbial iron mobility and transport.