Phytic acid: divalent cation interactions: III. A calorimetric and titrimetric study of the pH dependence of copper(II) binding |
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| Affiliation: | 1. Department of Biological Chemistry and Structure, The Chicago Medical School, University of Health Sciences, North Chicago, IllinoisU.S.A.;2. Southern Regional Research Center, U.S. Department of Agriculture, New Orleans, LouisianaU.S.A.;1. Department of Urology, Royal Surrey County Hospital, Guildford, UK;2. Department of Urology, Fundació Puigvert, Universidad Autónoma de Barcelona, Barcelona, Spain;3. Headquarters, European Organisation for Research and Treatment of Cancer, Brussels, Belgium;4. Department of Urology, Radboud UMC, Nijmegen, The Netherlands;1. Centre for Translational Bone, Joint and Soft Tissue Research, University Hospital Carl Gustav Carus and Faculty of Medicine of Technische Universität Dresden, Fetscherstraße 74, 01307 Dresden, Germany;2. University Centre for Orthopaedics and Trauma Surgery, University Hospital Carl Gustav Carus Dresden, Fetscherstraße 74, 01307 Dresden, Germany;3. University of Cooperative Education Riesa, Am Kutzschenstein 6, 01591 Riesa, Germany;4. Institute of Physical Chemistry, Justus Liebig University of Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany;5. Department of Trauma Surgery, University Hospital Giessen-Marburg GmbH, Campus Giessen, Rudolf Buchheim Straße 7, 35385 Giessen, Germany;6. Department for Functional Materials in Medicine and Dentistry, University of Würzburg, Pleicherwall 2, 97070 Würzburg, Germany;1. College of Medical Technology and Engineering, Henan University of Science and Technology, Luo Yang 471023, Henan, PR China;2. College of Material Science and Engineering, Henan University of Science and Technology, Luo Yang 471023, Henan, PR China;3. Key Laboratory of Biologic Resources Protection and Utilization of Hubei Province (Hubei Institute for Nationalities) and Department of Chemistry, Enshi 445000, Hubei, PR China;1. ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Technical Unit Brindisi Technologies of Materials, PO Box 51-Br4, 72100 Brindisi, Italy;2. LEnviroS srl, Sspin-off from University of Bari, Via E. Orabona, 4, 70126 Bari, Italy;3. University of Bari, Department of Chemistry, Via E. Orabona, 4, 70126 Bari, Italy |
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| Abstract: | The interaction of the cupric ion with phytic acid as a function of pH has been studied by potentiometric and thermal titration and by the determination of ligand binding. As has been found for the reaction of zinc and calcium cations with phytate, the presence of the Cu(II) ion results in a displacement of the titration curves to more acid values. Evaluation of the parameters that describe such changes in ionization behavior by curve-fit analysis showed that as the Cu(II):phytate mol ratio was increased from one to eight, the pK' values of the ionizable group sets of phytic acid (ranging from 1.59 to 9.79) were consolidated into just two sets with curve-fit (CP) values ca. 1.5 and 3.7. Marked pH hysteresis effects are seen in such systems because of the pronounced acid strength of the Cu(II):aqua ion and the Cu(II) ligand aqua ion complex. The combined heat of binding and precipitation (plus solvation changes, etc.) of Cu(II) to phytate is endothermic (21.8–22.2 kcal mol−1). This is similar in magnitude to that reported for the binding of either Zn(II) or Ca(II) to phytate. In the titration of Cu(NO3)2 with KOH, presumably to form Cu(OH)2, ΔH° was exothermic (−12.5 kcal mol−1). From measurements of free Cu(II) cation concentration in the presence of phytate the binding reaction was found to be stoichiometric with 6 mols Cu(II) bound at pH 6. Binding occurs within the pH range 2–6. An apparent necessary requirement for binding is the availability of the oxo dianion structure formed from the second dissociation step of a phosphoryl group. Curve-fit analysis of the binding data as a function of pH showed that a group or group set with CP value ca. 4 governs the binding reaction(s) at all mol ratios of Cu(II) to phytate examined. It is suggested that the binding of cupric ions to phytate may occur to the equatorial rather than the axial configuration as suggested for Ca(II) binding. A space-filling molecular model to illustrate this has been constructed. Soluble Cu(II):phytate complexes are formed within the pH range from 2 to ca. 3.4. This is supported by the results of difference absorption spectrometry. |
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