Complex formation between phytic acid and divalent metal ions: a solution equilibria and solid state investigation

Abstract

An investigation on the complex formation equilibria between divalent metal ions Me (with Me=Mn, Co, Ni, Cu, Cd, and Pb) and phytic acid (H(12)L) is presented. Experiments were performed through a potentiometric methodology by measuring, at 25 degrees C, the proton and, in some cases (Cu(2+), Cd(2+), and Pb(2+)), also the metal ion activity at equilibrium in solutions containing, besides the metal and the ligand, 3 M NaClO(4) as the ionic medium. Unhydrolyzed solutions of the metal ion at millimolar concentration levels were titrated with solutions of about 10 mM sodium phytate, until the formation of a solid phase took place (always at pH approximately 2.5, except in the case of Cu(2+), which formed soluble complexes up to pH approximately 3.3). Coulometry was employed to produce very dilute solutions of either Cu(2+), Cd(2+), or Pb(2+) of accurately known composition. The emf data were explained by assuming, in the acidity interval explored, the formation of the complexes of general stoichiometry MeH(5)L(5-) and Me(2)H(3)L(5-). Coordination compounds in the solid state were also synthesized and characterized by elemental analysis, thermal analysis, and ICP spectroscopy. The solids had a general stoichiometry Me(6)H (t)LCl (t). x H(2)O, with the following t and x values for each metal investigated: Me ( t; x) = Mn (4; 2); Co (4; 2); Ni (4; 2); Cu (2; 2.5); Zn (2; 1); Sn (6; 6).

Speciation of phytate ion in aqueous solution. Characterisation of Ca-phytate sparingly soluble species

“…Our result is consistent with the data on the thermal decomposition of CaHPO 4 reported in literature (Mesmer and Irani, 1963). One difference in the anionic species in the final compound can be observed in a paper by Vasca et al (2002), in which the final compounds deriving from the thermal decomposition of some transition metal cations, are Me 3 (PO 4 ) 2 (Me = divalent transition cation). Calculations that allow for the final formation, in our case, of Ca 3 (PO 4 ) 2 are inconsistent with the thermogravimetric profile.…”

Speciation of phytate ion in aqueous solution. Protonation constants and copper(II) interactions in NaNO3aq at different ionic strengths

“…Concerning the last two, and most recent works on copper (II)/phytate complexes [31,32], the experimental conditions (of temperature, medium and ionic strength) make comparisons not significant (in the paper by Vasca et al [31], authors worked in NaClO 4 at I = 3 mol L − 1 and t = 25°C, while we reached a maximum of I = 1 mol L − 1 ; Torres et al [32] worked at t = 37°C).…”

“…In this specific case, to our knowledge and at present time, just six papers deal directly with copper(II)-phytate system [27][28][29][30][31][32] (two further papers are concerned with "interactions" [33] between phytate and copper and "binding differences" of phytate toward copper(II) [34], but only from a qualitative and medical point of view). In two of them, by Martin and Evans [27,28], authors evaluated the binding ability of phytate toward copper(II) by potentiometric, calorimetric and spectrophotometric studies at t = 25°C in KNO 3 0.2 mol L − 1 aqueous solution.…”

Solution behaviour of myo-inositol hexakisphosphate in the presence of multivalent cations. Prediction of a neutral pentamagnesium species under cytosolic/nuclear conditions

“…The quantitative studies of Bebot-Brigaud et al [20] and Vasca et al [21], encompassed a range of divalent cations, but excluded Ca(II) and Mg(II), and employed conditions of temperature and ionic strength deviating from the physiologically most relevant. Two main conclusions stem from these two reports.…”