Stability constants of the fungal siderophore rhizoferrin with various microelements and calcium

Stability constants of metal-ligand complexes determine their speciation in solution at equilibrium. Therefore, stability constants of siderophore-metal complexes are of crucial importance in studies of their efficiency as Fe mediators. In this study, potentiometric titrations were used to determine protonation and stability constants of Fe 3+ , Fe 2+ , Cu 2+ , Ca 2+ , and Zn 2+ with rhizoferrin, a siderophore produced by Rhizopus arrhizus, which has been shown in previous studies to be an effective carrier of Fe to plants. Rhizoferrin was found to be a specific Fe 3+ chelator but with a stability constant lower than most microbial siderophores. The ferric complex of rhizoferrin is anionic throughout the pH range prevailing in soils, and therefore, it is expected to be mobile in the rhizosphere. The following log K app values at 25°C, pH 7.0, and 0.1 M ionic strength were determined: 19.1, 7.5, 6.2, 6.0, and 4.4 for Fe 3+ , Fe 2+ , Cu 2+ , Ca 2+ , and Zn 2+ , respectively. The apparent stability constant (K app ) value for the ferric complex of rhizoferrin suggests that ligand exchange with mugineic acid is the mechanism facilitating the efficiency of rhizoferrin as an Fe carrier to graminaceous plants.