Phytic acid-zinc ion interactions: a calorimetric and titrimetric study

Abstract

Abstract Using an isoperibolic titration microcalorimeter, the ionization characteristics and associated heat changes of phytic acid (myo-inositol hexaphosphate) and phytic acid in the presence of varying Zn(II) concentrations have been examined over the pH range 2.5–11 at 25°C in 0.2 M KCl. In the absence of Zn(II), ca. 7 of the 12 ionizable protons in phytic acid are titrated in this pH range with ionization heats varying from ca. 2 to −3 kcal-mol-1. At Zn(II): phytate mol ratios of 4:1 and greater, the dissociation of all protons and complex formation of phytate with Zn(II) occurs below pH 6. From the difference titration curves of phytic acid plus Zn(II) versus Zn(II) alone, ca. 3.5 mol Zn(II) bind per mol phytate. Since Zn(II):phytate complexes are insoluble, the observed heat changes contain contributions not only from heats of precipitation but also from binding, ionization, neutralization, and hydration effects. From the heat change for the titration of (a) phytic acid, pH 2.6–10.4; (b) phytic acid + Zn(II), pH 2.6–6.1; and (c) Zn(II), pH 2.6–6.1 at Zn(II): phytate ratios of 4 to 10, the value of 24.7 ± 0.5 kcal mol−1 phytate has been obtained for the binding of 3.5 mols Zn(II). This figure also includes the heat of precipitation of the complex. In pH-drop experiments, with the initial pH at 8.65, the value of 23.9 kcal mol-1 was obtained for ΔH°. Hysteresis effects are prevalent in these reaction solutions. Time-dependent changes in pH occur with a change in pH. For the phytate-Zn(II) reactions, the time-course curves are biphasic and fit a rate equation for two simultaneous first order reactions. Hysteresis effects seen in the titration of Zn(II) fit simple first-order kinetics. These effects most probably arise from the ejection of a proton from the aqua ion or aqua ion ligand complex(es).