Sequestering Ability of Phytate toward Biologically and Environmentally Relevant Trivalent Metal Cations

Abstract

Quantitative parameters for the interactions between phytate (Phy) and Al(3+), Fe(3+), and Cr(3+) were determined potentiometrically in NaNO(3) aqueous solutions at I = 0.10 mol L(-1) and T = 298.15 K. Different complex species were found in a wide pH range. The various species are partially protonated, depending on the pH in which they are formed, and are indicated with the general formula MH(q)Phy (with 0 ≤ q ≤ 6). In all cases, the stability of the FeH(q)Phy species is several log K units higher than that of the analogous AlH(q)Phy and CrH(q)Phy species. For example, for the MH(2)Phy species, the stability trend is log K(2) = 15.81, 20.61, and 16.70 for Al(3+), Fe(3+), and Cr(3+), respectively. The sequestering ability of phytate toward the considered metal cations was evaluated by calculating the pL(0.5) values (i.e., the total ligand concentration necessary to bind 50% of the cation present in trace in solution) at different pH values. In general, phytate results in a quite good sequestering agent toward all three cations in the whole investigated pH range, but the order of pL(0.5) depends on it. For example, at pH 5.0 it is pL(0.5) = 5.33, 5.44, and 5.75 for Fe(3+), Cr(3+), and Al(3+), respectively (Fe(3+) < Cr(3+) < Al(3+)); at pH 7.4 it is pL(0.5) = 9.94, 9.23, and 8.71 (Al(3+) < Cr(3+) < Fe(3+)), whereas at pH 9.0 it is pL(0.5) = 10.42, 10.87, and 8.34 (Al(3+) < Fe(3+) < Cr(3+)). All of the pL(0.5) values, and therefore the sequestering ability, regularly increase with increasing pH, and the dependence of pL(0.5) on pH was modeled using some empirical equations.

Voltammetric Investigation of Iron(III) Interactions with Phytate

“…This is in good agreement with the work of Bretti et al [21] and Torres et al [24], who both potentiometrically determined different partially protonated iron(III) phytate species, indicated with general formula [FeH i L] z-(with 0 ≤ i ≤ 6 and 2 ≤ i ≤ 5, respectively). Iron phytate species written in the reactions (12-13) below were predicted on the basis of the latter, hence the most protonated (less stable) species being [FeH 5 L] 4-.…”

“…Fe(II) phytates were investigated by many authors [9,20,21,24,47]. Torres et al [24] reported that the highest protonation degree of iron(III) phytate species formed in acidic media is i = 5 and log of the stability constant …”

“…Iron phytate species written in the reactions (12-13) below were predicted on the basis of the latter, hence the most protonated (less stable) species being [FeH 5 L] 4-. As known from the literature [21,24] in acidic media various protonated iron(III)-phytate species coexist, but due to limited resolution of the cyclic voltammetry minor species cannot be detected and distinguished from major phytate complexes C1 and C2 prevailing at given pH in the solution. The evidence for the presence of other species can also be seen in Fig.…”

“…One of the most important properties of phytate is its ability to form strong complexes with divalent and trivalent cations as well as with the alkali cations, and there are numerous reports on the determination of stability constants of phytate with metallic cations [19][20][21][22]. Coordination of Fe(III) with phytate does not influence only mineral bioavailability but also exhibits antioxidant properties [7].…”

Potentiometric Determination of Phytic Acid and Investigations of Phytate Interactions with Some Metal Ions

“…39 Phytates interact strongly also with iron(III) ions and play an important role in biologically-relevant processes as described in numerous papers. 8,35,[40][41][42][43][44] Complexation of iron(III) ions with phytate show different behavior than alkaline or alkaline earth cations. Three distinct EPs are detected on titration curves of phytic acid in the presence of various amounts of Fe 3+ ions (Fig.…”