Study of Conformation of γ-Irradiated Calcium Gluconate by EPR

One of the most popular drugs used in disorders of calcium metabolism in the body is calcium gluconate (CaGluc2·H2O). The new mechanically activated modified nanodispersed amorphous form of calcium gluconate (MACG) significantly increases the bioavailability and efficacy of treatment. The crystal structure of calcium gluconate remained unknown for a long time. This is due to the fact that it was not possible to grow a single crystal of calcium gluconate to a size sufficient for studies by single-crystal X-ray diffractometry. It was possible to grow a relatively large single crystal using a complex technology with careful selection of growth conditions. A single crystal with dimensions of 0.05 mm × 0.02 mm × 0.01 mm was studied using synchrotron radiation. However, mechanical activation is often accompanied by amorphization, which makes it impossible to use diffraction methods. In this regard, it becomes necessary to use alternative research methods. In particular, this work considers the possibility of using EPR to determine the conformation of calcium gluconate. Since no EPR signal is observed for the original CaGluc2·H2O, it is possible to introduce artificial defects into it to obtain information about the system; in this work, we used ionizing radiation to produce artificial defects. The spectra were analyzed using the Easy Spin program. The data obtained with the help of modeling can be used to calculate torsion angles corresponding to the hyperfine interaction constants. Thus, in the study of CaGluc2·H2O samples, exact values of the torsion angles between the C–H bonds and the axis of the p-radical were obtained. The position of the paramagnetic centers on different carbon atoms with the components of the spectrum was compared. As a result, two possible conformations of the calcium gluconate molecule were revealed. The reliability of the results of conformational analysis is confirmed by EPR studies in two frequency bands (X- and Q-bands).