BACTERIAL SIDEROPHORES - IRON EXCHANGE MECHANISM WITH ETHYLENEDIAMINETETRAACETIC ACID

This article describes the iron exchange kinetics between three bacter ial siderophores (deferriferrioxamine B, azotobactin delta and pyoverd in PaA) and a trivial scavenging agent taken as a model, EDTA. Ferriox amine B is a linear trihydroxamate ligand, the two other ligands being mixed fluorescent siderophores of the pyoverdin family. The two last compounds consist of a peptidic chain containing 8 to 10 amino acids b ound to a catecholate-type group on the chromophore derived from 2,3-d iamino 6,7-dihydroxyquinoline, the two other iron coordination sites b eing two hydroxamic acids for pyoverdin PaA and a hydroxamic and a hyd roxycarboxylic acid for azotobactin delta. They all form strong hexade ntate chelates with iron(III). Iron exchange studies between the ferri c complexes of siderophores and ethylenediaminetetraacetic acid (EDTA) in excess were carried out in a pH range between 3.6 and 5.2 (acetate buffer) using UV-visible spectrophotometric methods. The exchange kin etics show first-order dependence on the concentrations of the siderop hore complexes considered. Variations of the pseudo-first order rate c onstants with EDTA and protons concentrations show saturation kinetics for the three siderophores considered. These results are interpreted with a common three-steps mechanism. The first two fast equilibria inv olve the protonation of the iron siderophore complex followed by the f ormation of a ternary complex between the protonated ferric complex an d EDTA, the rate determining step being the dissociation of the ternar y complex. Ferrioxamine B shows the slowest dissociation rate constant . The differences of reactivity of these siderophores are interpreted in terms of structural features. This study points out the iron exchan ge properties of three different bacterial siderophores which reflect the ability of these natural compounds to prevent the complexed iron(I II) cation from removal by other potent physiological ligands.