A thermodynamic description of the binding of iron to ferrioxamine B in aqueous solutions

The iron exchange reaction between nitrilotriacetate (NTA) and ferrioxamine B (DFO) has been investigated with titration calorimetry. Using characterization models for the initial and final states of the reaction solution based on the determination of the mole fractions of the individual species as a function of pH, the concentration of each participating species was calculated (B. L. Gould, 1980, Masters Thesis, Utah State University, Logan). The contribution of each component reaction to the overall exchange at various pH values was used to determine the enthalpy change for each of these individual reactions. The values determined for the enthalpy and entropy changes are shown below. An analysis of the thermodynamic parameters of the iron exchange system indicated the following interactions among the hydroxamate groups of the ferrioxamine B: (1) The ionization of the individual hydroxamate protons occurs independently with the same enthalpy change for each ionization, and (2) the heat of binding a hydroxamate group to the iron is dependent on the number of previously bound hydroxamates.