Rudko V. The structure and properties of СО2(-) paramagnetic centers in biological and synthetic hydroxyapatite

Dissertation deals with the EPR investigations of structure and properties of paramagnetic defects induced by UV- and gamma-irradiations in a carbonate-containing biological and synthetic apatites, and also their transformations under the influence of thermal annealing. It is shown that СО2(-) radicals - axial and orthorhombic - are the dominating paramagnetic centres that are induced in hydroxyapatite (HAP) under irradiation. The ratio between the amounts of two types of radicals depends on the type of irradiation and, probably, on the quality of initial material. It is shown that annealing of irradiated HAP, both biological and synthetic, leads to the transformation of orthorhombic radicals into axial ones. The conclusion is done that both types of СО2(-) radicals are located in identical structural positions, namely in the positions B (substitution of phosphorus by carbon). The analysis of EPR spectra of UV- and gamma-irradiated tooth enamel HAP has shown that different types of irradiation lead to different ratios between the amounts of axial and orthorhombic radicals; this ratio is a characteristic value for each type of irradiation. A new method of separate estimation of doses of gamma- and UV-irradiation separately in case of mixed (gamma plus UV) irradiation of enamel is proposed. The method is based on the determination of the anisotropy of enamel plates spectra. Based on the experimental data obtained and theoretical calculations it is shown that the formation of a stable paramagnetic defects occurs via metastable radicals, namely, СО3(3-) radicals in the position B. The mechanism of СО2(-) formation in B type HAP is proposed.