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The article presents a study on the mechanism of carbon dioxide (CO2) radical formation in biological and synthetic apatites. The study used tooth enamel from human molars, pig molars and incisors, and cortical bones of cows which were irradiated with cobalt (Co) γ-rays, and examined by electron paramagnetic resonance (EPR). A Varian E12 X-band EPR spectrometer was used to measure EPR at room temperature. The study observed that the higher the characteristic energy of the radiation, the larger the contribution of orthorhombic radicals were.

Biological apatites (tooth enamel, bone) and their synthetic analogues were exposed to gamma rays, UV light, or thermal treatment and studied by electron paramagnetic resonance (EPR). The thermal generation of CO2- radicals in synthetic apatite was observed for the first time. It was shown that the experimental EPR spectra of all of the above-mentioned materials are caused by the contribution of two types of CO2- radicals: axial and orthorhombic. The ratio of their concentrations depends on the characteristic energy of the external influence (i.e., the energy of quantum for radiation or kT for thermal treatment) and also on the quality of the initial material (defectiveness). Based on the analysis of EPR spectra recorded immediately after gamma-irradiation, the authors conclude that the main short-lived radical in bioapatites is CO3(3)- . The unified mechanism of CO2- radical formation in hydroxyapatites at different external influences is proposed; the main stages of transformation are CO3(2)- + e --> CO3(3)- --> CO2-, where the electron (e) originates from the ionization of impurities by radiation/temperature.