Abstract
As a ligand exchange kinetic model of an Fe(III)-fulvic acid complex and a siderophore, desferrioxamine B (DFOB) was developed. The model includes two pathways of Fe(III)-DFOB formation, (1) a disjunctive pathway where DFOB binds to unchelated Fe(III) following the dissociation of Fe(III) from Fe(III)-fulvic acid and (2) an adjunctive pathway where Fe(III)-DFOB is formed via the direct association of DFOB with an Fe(III)-fulvic acid complex. Iron binding sites in fulvic acid were classified into two ligand classes on the basis of binding strength. Fitting the kinetic model to experimental observation with a spectrophotometer, the rate constant of ligand exchange for a strong binding site [k1 (M-1 · s-1)] was determined to be k1 = 4.7 × 103 / [FA]T + 5.5 × 102, where [FA]T (mg · l-1) denotes the total fulvic acid concentration. The rate constant for a weak binding site [k2 (M-1 · s-1)] was estimated to be greater than 1 × 105. The model calculation provided profiles of ligand exchange rate and the amount of Fe(III) bound to each ligand class (relative ratio to total Fe), which are functions of fulvic acid concentration and both of fulvic acid and iron concentrations, respectively.
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