Complexation of 1-Hydroxyethylidene-1,1-Diphosphonic Acid (HEDP) and Manganese(II) in Aqueous Solution

О.V. Bogatyrev*, А.F. Yamaltdinova**, F.V. Devyatov***

Kazan Federal University, Kazan, 420008 Russia

E-mail: *olbogatyrev@gmail.com, **yamaltdinovaa1993@mail.ru, ***fedor.devyatov@kpfu.ru

Received December 7, 2015

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Abstract

Based on pH-potentiometric titration, it was found that the system Mn2+ – HEDP accumulates mono- and binuclear complexes of 1 : 1 and 2 : 2 composition, as well as mononuclear biscomplexes. Equilibrium and stability constants were obtained. Fractional distribution of the forms was studied. The high coefficient of relaxation efficiency (CRE2) of the complexes in the spin-spin relaxation time (T2) and their considerable stability makes it possible to regard them as potential MRI-contrast reagents. In the test system at pH ≈ 7.4 (the pH of the blood), the following complex forms can be identified: MnHL25–, MnH2L24–, and MnL2–. When modeling the injection of such complexes in the human body, which is accompanied by about 1000-fold dilution, it was found that MnL2– is the most stable of them. Other complexes under these conditions are absent, and the concentration of “free” manganese(II) in the blood does not exceed the maximum concentration limit for drinking water.

Keywords: bisphosphonates, contrast agents, self-association equilibria, acid-base properties

Figure captions

Fig. 1. Experimentally obtained curves: a) dependence of the Bierrum function on pH, b) dependence of the spin-spin coefficient of relaxation efficiency (CRE2) on рН for the concentrations given in Table 1.

Fig. 2. Theoretical and experimental curves showing dependence of the Bierrum function (in relation to metal) on pH under the conditions given in Table 1.

Fig. 3. Dependence of the shares of accumulation of forms (in relation to metal) on pH of the environment at CMn(II) = 0.0114 mol/L and СHEDP = 0.02603 mol/L.

Fig. 4. Dependence of the shares of accumulation of forms (in relation to metal) on pH on the environment at CMn(II) = 0.0190 mol/L and СHEDP = 0.0195 mol/L.

Fig. 5. Dependence of the shares of accumulation of forms (in relation to metal) on pH on the environment at CMn(II) = 0.0114 mol/L and СHEDP = 0.0521 mol/L.

Fig. 6. Dependence of the shares of accumulation of forms (in relation to metal) on pH on the environment at CMn(II) = 0.0190 mol/L and СHEDP = 0.0586 mol/L.

Fig. 7. Complexation scheme at considerable ligand excess.

Fig. 8. Complexation scheme in the cases of identical metal : ligand ratios at different absolute concentrations.

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For citation: Bogatyrev О.V., Yamaltdinova А.F., Devyatov F.V. Complexation of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and manganese(II) in aqueous solution. Uchenye Zapiski Kazanskogo Universiteta. Seriya Estestvennye Nauki, 2016, vol. 158, no. 1, pp. 44–54. (In Russian)

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