Phytic acid:divalent cation interactions. V. titrimetric, calorimetric, and binding studies with cobalt(ii) and nickel(ii) and their comparison with ot

Abstract

The ionization characteristics and accompanying heat changes of phytic acid ( myo-inositol hexaphosphate) in the presence of varying concentrations of Co(II) and Ni(II) have been studied over the pH range 2.5–10.5 in the presence of 0.20 M KNO 3 at 25°C. The effect of the binding of these metal ions on the ionization constants of the ionizable phosphate groups is qualitatively similar to that seen with the cations of Zn(II), Cu(II), and Ca(II), viz., displacement of the phytic acid titration curve to more acid pH values. Quantitatively, the overall effect of Co(II) and Ni(II) binding on the ionization constants of phytic acid is greater than that induced by the interaction with Ca(II) but less than that observed by the binding of Zn(II) or Cu(II) to phytate. The effect of pH on the binding of Ni(II) to phytate has been determined spectrophotometrically and compared with results obtained for the binding of Co(II), Cu(II), and Ca(II) to phytate. The binding of Ni(II) occurs over the pH range from ca. 5 to 8.2 and with the formation of insoluble complexes. Evidence for the formation of soluble complexes was not observed. This contrasts with the results obtained with Cu(II) and Co(II), where soluble complexes with phytate are found at pH 2 to ca. 3. However, in conformity with the results obtained with these two metal ions, it appears that 6 mois Ni(II) can bind to 1 mol phytate. From previous studies, the maximal binding ratio of Ca(II): phytate was found to be 5; for Zn(II), it appears to be only from 3.5 to 4. On the basis of pH binding profiles, it is proposed that the binding of Cu(II), Co(II), and possibly Zn(II) occurs to the equatorial configuration of phytate. The axial form, existing in solution above pH 5, is probably the form to which both Ca(II) and Ni(II) bind. A comparison of the heat changes associated with the deprotonation of phytic acid with those measured in the presence of divalent cations permits evaluation of an apparent heat of binding term For the systems studied, this endothermic heat change (22–27 kcal mol −1) appears to be little affected by either the type of metal ion or the extent of binding Although there are multiple contributors to these heat of binding values, it is suggested that among them the heat of precipitation is probably the largest and effectively blankets heat changes due to the specific binding effects The titration behavior of the aqud metal ions and the heat changes that arise as a function of pH during their conversion to their hydroxides have also been studied Their acidity constants (p K *) were found to be in the increasing order of Cu(II) > Zn(II) > Ni(II) > Co(II) with the exothermic ΔH° values varying from − 7 5 kcal mol −1 for Ni(II) to − 12 8 kcal mol −1 for the titration of Cu(II) Values of ΔG° calculated from the acidity constants (p K * can be equated to −log K) permitted determination of ΔS° Changes in the entropy varied from − 62 to − 72 e.u. As has been found for the ionization of many compounds (phenols, carboxylic acids, etc ) a linear relationship was found to exist between ΔH° and ΔG° and therefore between ΔH° and ΔS°. The solubility of metal lon-phytate complexes as a function of pH at various mol ratios has been studied by the measurements of the apparent absorbance at 400 nm Under conditions wherein pH hysteresis effects are observed, time-dependent changes in “absorbance” also occur rhe two phenomena are not always synchronous processes. At metal ion phytate mol ratios of 6, kinetics of the pH hysteresis effects were such that the biphasic rate curves could be fitted to an equation descriptive of the simultaneous occurrence of two first order reactions yielding a common product The rate constant for the faster reactions ranged from 1.5 to 2.5 min 1 The slower reaction exhibited a greater variability (0.07 to O.23 min −1) In the absence of phytate the rate curves were not biphasic and could be fitted to a simple first-order rate equation At metal ion phytate mol ratios of ≦ 2, the rate curves were no longer biphasic and were first order for both the time dependent changes in pH and “absorbance” Furthermore, above a pH of ca. 8 the insoluble phytate complexes formed with either Ca(II) or Zn(II) became soluble This resolubilization phenomenon was not seen with Cu(II):phytate complexes