KINETICS AND MECHANISM OF IRON EXCHANGE IN HYDROXAMATE SIDEROPHORES -CATALYSIS OF THE IRON(III) TRANSFER FROM FERRIOXAMINE-B TO ETHYLENEDIAMINETETRAACETIC ACID

The oxalate catalyzed iron(III) transfer from a trihydroxamate siderop hore 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 maintai ned by NaClO4. The results reveal that oxalate is a more efficient cat alyst than the so far studied synthetic monohydroxamic acids. Any role oi. reduction of Pe(Hdfb)(+) by oxalate in the catalysis has been rej ected by the experimentally observed preservation of the oxalate conce ntration during the reaction time. Therefore, catalysis has been propo sed to be a substitution based process. Under our experimental conditi ons Fe(Hdfb)(+) is hexacoordinated and addition of oxalate results in the formation of Fe(H(2)dfb)(C2O4), Fe(H(3)dfb)(C2O4)(2)(-) and Fe(C2O 4)(3)(3-). Therefore, catalysis was proposed to be accomplished by the intermediate formation of the ternary and tris(oxalato) complexes. Al l three complexes react with H(2)edta(2-) to form thermodynamically st able Fe(edta)(-) as a final reaction product. Whereas the formation of the ternary complexes is fast enough to feature a pre-equilibrium pro cess to the iron exchange reaction, the formation of Fe(C2O4)(3)(3-) i s slow and is directly involved in rhs rate determining step of the Fe (edta)(-) formation. Nonlinear dependencies of the rate constant on th e oxalate and the proton concentrations have been observed and a four parallel path mechanism is proposed for the exchange reaction. The rat e and equilibrium constants for the various reaction paths were determ ined from the kinetic and equilibrium study involving the desferrioxam ine 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 catal ysis of iron dissociation from the siderophore has been discussed in v iew of its significance with respect to in vivo microbial iron transpo rt. (C) 1998 Elsevier Science Inc. All rights reserved.