Complexation of 1-Hydroxyethylidene-1,1-Diphosphonic Acid (HEDP) and Manganese(II) in Aqueous Solution
Based on pH-potentiometric titration, it was found that the system Mn<sup>2+</sup> - 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 (CRE<sub>2</sub>) of the complexes in the spin-spin relaxation time ( T <sub>2</sub>) 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: MnHL<sub>2</sub><sup>5-</sup>, MnH<sub>2</sub>L<sub>2</sub><sup>4-</sup>, and MnL<sup>2-</sup>. When modeling the injection of such complexes in the human body, which is accompanied by about 1000-fold dilution, it was found that MnL<sup>2-</sup> 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.