I. Spasojevic et al., Electrochemical behavior of the Fe(III) complexes of the cyclic hydroxamate siderophores alcaligin and desferrioxamine E, INORG CHEM, 38(3), 1999, pp. 449-454
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.