Phytic acid: divalent cation interactions. IV. a spectroscopic determination of Co(II) and Cu(II) binding

Abstract

The interaction of phytate with the aqua ions of Co(II) and Cu(II) has been studied as a function of pH using absorption spectrophotometry. At pH 2.45, the molar absorptivity ( A M) and λ max (510 nm) were identical for solutions of 6 mM Co(II) with or without 1 mM phytate. In the absence of phytate, A M was constant to ca. pH 8. With phytate present, however, A M decreased with an increase in pH over a range (2.45 to ~ 4.8) wherein no precipitation occurred. From this it was inferred that Co(II) can bind to phytate to form soluble complexes. Above a pH of 5, insoluble complexes are formed with the decrease in A M being probably due to the diminished Co(II), and presumably phytate, concentration left in solution as a consequence of increased precipitation. From such data, the mols of Co(II) bound per mol phytate as a function of pH has been calculated with the assumption that the decrease in A M of Co(II): phytate solutions represents the binding of Co(II) to phytate in a final 6:1 mol ratio. The binding curve, extending over the pH range 2.5 to ca. 9, can be fitted to the curve-fit parameter 5.33 for five groups and 7.48 for one group-values that closely describe the titration curve for phytic acid above pH 2.50 in the presence of a sixfold concentration of Co(II). The spectrum of the Cu(II) aqua ion shows a maximum at 785 nm with A M (11.5 M −1 cm −1) constant from pH 2.0 to about pH 5. In the presence of phytate (6:1 mol ratio, Cu(II):phytate) and in the pH range 2 to ca. 3.3, wherein soluble complexes are formed, λ max is shifted slightly to the red end of the spectrum (785 to 792 nm) and A M is increased from 12.1 to 13.1 M −1 cm −1. Above a pH of ca. 3.3, Cu(II):phytate complexes become insoluble and, in this case also, the decrease in A M most probably reflects the decrease in Cu(II) ion and phytate concentration. In the pH region wherein comparisons can be made, the mols of Cu(II) bound per mol phytate as estimated from free ion measurements coincides with the decrease in A M . Both the Co(II) and Cu(II) aqua ions are in octahedral coordination. Evidence for a significant coordination change as a consequence of binding to phytate was not seen.