Phytic acid-metal ion interactions. II. The effect of pH on Ca(II) binding.

The interaction of phytic acid with Ca(II) has been studied by potentiometric titration and by measurement of free Ca(II) concentrations using an ion Selective electrode. With increasing Ca(II) concentration, the titration curve of phytic acid is displaced to regions of lower pH. In the binding of calcium ions to phytic acid, there is no evidence that significant binding occurs below approximately pH 5. Above this pH, the extent of binding is dependent upon both pH and the calcium to phytic acid ratios. Maximum binding obtains at a Ca(II):phytate ratio of 6 with 4.8 mol of Ca(II) bound per mol of phytate above pH ca. 8. Binding constants are apparently very large since binding isotherms at any Ca(II):phytate ratio are a linear function of the total calcium ion concentration. In all cases, binding occurs only when one or more phosphate groups have been converted to the oxo dianion form. The apparent pK' values (curve-fit parameters) that describe the potentiometric titration data are in good agreement with the constants evaluated from the binding of Ca(II) to phytate as a function of pH. Using CPK space-filling models, structures containing six metal ions in coordinate linkage to pairs of oxo dianions have been constructed and discussed within the framework of the axial conformation of phytic acid and the order of proton removal with an increase in pH based upon NMR studies.