Interactions of Mg(II), Co(II), Ni(II), and Zn(II) with phytic acid. VIII. A calorimetric study.

Abstract

The interactions of Mg(II), Co(II), Ni(II), and Zn(II) with phytic acid to form soluble complexes have been studied by calorimetry. The reactions were examined at metal ion:phytate mol ratios ranging from 1–6. The heats of reaction were endothermic over this range of mol ratios. Enthalpies were calculated in terms of cal mol −1 phytate. These enthalpies are a composite of a number of factors, among which are the heat of complex formation, the heat of dehyrdation of both the phytate and the various metal ions, the heats of ionization of the phytate, and the heat of hydration of protons released from the phytate. The enthalpies of complex formation of these metal cations with phytic acid are compared to those of two other metal cations, Mn(II) and Cu(II), which were studied under similar conditions. Based on the enthalpies, it is suggested that the order of affinity of these various metal cations for phytic acid is Cu(II) ≥ Zn(II) > Mn(II) > Mg(II) > Co(II) > Ni(II) In one instance, Mg(II), the heat of precipitation (which includes binding, solvation changes, etc.) to form the insoluble Mg(II)-phytate complex was determined. The reaction was endothermic and had an enthalpy of 12.9 kcal mol −1. This quantity is markedly different from the reported heats of precipitation reactions involving Cu(II), Zn(II), Mn(II), Ca(II), Co(II), and Ni(II) with phytate. From measurements of the Mg(II) concentration, it was calculated that 5.2 mol Mg(II) binds per mol of precipitated phytate.