Nikola Kujundžić

Electrochemical behavior of iron(III) complexes with aminohydroxamic acids

Abstract The electrochemical reduction of various aminohydroxamate complexes of iron(III), such as alanine-, serine-, lysine-, histidine- and glutamo-γ-hydroxamate, has been investigated in aqueous solution by cyclic voltammetry on hanging mercury drop electrodes to determine the mechanism involved in the electron transfer processes. In all the studied cases the iron(III) complexes, with the exception of histidinehydroxamate, have been found to undergo reversible reductions followed by irreversible chemical reactions (EC mechanism). Rate constants for the irreversible dissociation of iron(II) complexes were calculated. The typical quasi-reversible pattern for the reduction of histidinehydroxamate was attributed to the different coordination mode. The observed differences in redox potentials between the investigated complexes suggest that the electronic effect of the substituent on the carbonyl group, involved in the coordination to the iron center, may modify the donor properties of the oxygen atoms of the hydroxamate moiety. The potentials determined at physiological pH are in the range of biological reducing agents, which makes these compounds potential siderophore models.

Kinetics and mechanism of iron exchange in hydroxamate siderophores: Catalysis of the iron(III) transfer from ferrioxamine B to ethylenediaminetetraacetic acid

Abstract The oxalate catalyzed iron(III) transfer from a trihydroxamate siderophore ferrioxamine B, [Fe(Hdfb) + ], to ethylenediaminetetraacetic acid (H 4 edta) has been studied spectro-photometrically in weakly acidic aqueous solutions at 298 K and a constant 2.0 M ionic strength maintained by NaClO 4 . The results reveal that oxalate is a more efficient catalyst than the so far studied synthetic monohydroxamic acids. Any role of reduction of Fe(Hdfb) + by oxalate in the catalysis has been rejected by the experimentally observed preservation of the oxalate concentration during the reaction time. Therefore, catalysis has been proposed to be a substitution based process. Under our experimental conditions Fe(Hdfb) + is hexacoordinated and addition of oxalate results in the formation of Fe(H 2 dfb)(C 2 O 4 ), Fe(H 3 dfb)(C 2 O 4 ) − 2 and Fe(C 2 O 4 ) 3− 3 . Therefore, catalysis was proposed to be accomplished by the intermediate formation of the ternary and tris(oxalato) complexes. All three complexes react with H 2 edta 2− to form thermodynamically stable Fe(edta) − as a final reaction product. Whereas the formation of the ternary complexes is fast enough to feature a pre-equilibrium process to the iron exchange reaction, the formation of Fe(C 2 O 4 ) 3− 3 is slow and is directly involved in the rate determining step of the Fe(edta) − formation. Nonlinear dependencies of the rate constant on the oxalate and the proton concentrations have been observed and a four parallel path mechanism is proposed for the exchange reaction. The rate and equilibrium constants for the various reaction paths were determined from the kinetic and equilibrium study involving the desferrioxamine B- (H 4 dfb + ), oxalate- and proton-concentration variations. The observed proton catalysis was attributed to the fast monoprotonation of ferrioxamine B as well as of the oxalate ligand. The observed catalysis of iron dissociation from the siderophore has been discussed in view of its significance with respect to in vivo microbial iron transport.

The reactivity of different iron(III) species in the formation of monoacethydroxamatoiron(III) complex

Abstract The dependence of the rate constant of the monoacethydroxamatoiron(III) complex formation on ionic strength in acid solution 25 °C has been studied. The observed ‘secondary salt effect’ strongly supports the assumption that an iron(III) hydroxo complex is the reactive species. Evidence is presented showing that the iron(III) hydroxo dimer (Fe4(OH02(H2O)4+8 exhibits approximately equal reactivity toward acethydroxamic acid as the monomer does. This fact, and lack of evidence for ‘primary salt effect’ strongly confirms the previous findings that the reactive form of ligand is the undissociated rather than dissociated hydroxamic acid. The rate of the reaction of hydroxamic acid with iron(III) hydroxo ion is estimated to be of the same order of magnitude as the rate of water exchange of the same ion.