Electrochemical behavior of the Fe(III) complexes of the cyclic hydroxamate siderophores alcaligin and desferrioxamine E

The redox behavior of Fe(III) complexes of the cyclic hydroxamate sideropho res alcaligin and desferrioxamine E was investigated by cyclic voltammetry. The limiting, pH independent redox potential (E-1/2 VS NHE) is -446 mV for alcaligin above pH 9 and -477 mV for ferrioxamine E above pH 7.5. At lower pH values, the redox potential for both complexes shifts positive, with a loss of voltammetric reversibility which is interpreted to be the consequen ce of a secondary dissociation of Fe(II) from the reduced form of the compl exes. These observations are of biological importance, since they suggest t he possibility of a reductive mechanism in microbial cells which utilize th ese siderophores to acquire Fe. For comparison purposes, cyclic voltammogra ms were obtained for Fe(III) complexes with trihydroxamic acids of cyclic ( ferrioxamine E) and linear (ferrioxamine B) structures, with dihydroxamic a cids of cyclic (alcaligin) and linear (rhodotorulic and sebacic acids) stru ctures, and with monohydroxamic acids (acetohydroxamic and N-methylacetohyd roxamic acids) at identical conditions. The observed redox potentials allow us to estimate the overall stability constants for fully coordinated Fe(II ) complexes as log beta(II)(Fe(2)alcaligin(3)) = 24.6 and log beta(II)(ferr ioxamine E) = 12.1. A linear correlation between E-1/2 and pM was found, an d the basis for this relationship is discussed in terms of structural (dent icity and cyclic/acyclic) and electronic differences among the {alkyl-NOH-C O-alkyl} type of hydroxamic acid ligands studied.