Iron release mechanism in a trihydroxamate siderophore analogue. Kinetics and effect of pH

Abstract

In view of the potential importance of the siderophore analogue DOTRMAHA as an iron carrier in biological systems, we have decided to study the mechanism and kinetics of iron release from the ferric DOTRMAHA complex to EDTA. The kinetics of the iron(III) exchange was monitored by UV-Vis absorption spectroscopy at the wavelength maximum of the iron(III) trihydroxamate complex. The decay of the complex absorbance with time is described as a sum of two exponentials plus a constant term. The mechanism of the exchange reaction is examined under conditions of varying concentrations of the competing ligand and the hydrogen ion. The kinetics reveal pseudo-first order dependence of rate constant(s) on the EDTA concentration. Furthermore, pH dependence studies show that the exchange reaction is accelerated with increasing acidity of the medium. The results are mechanistically interpreted by a kinetic scheme involving two parallel pathways for the iron exchange: one is a bimolecular process involving the direct attack of the EDTA on the ferric siderophore analogue complex; the other involves initially the protonation of the complex followed by a rapid attack of the competing ligand. The set of kinetic data presented here is further rationalized in terms of known coordination and structural features of the DOTRMAHA ligand as well as the corresponding ferric complex, and compared with available data of some naturally occurring siderophores and synthetic analogues.