ENDOR study of anO−ion observed in x-ray-irradiated carbonated hydroxyapatite powders

A carbonated hydroxyapatite powder synthesized at high temperatures has been examined with electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). In the EPR spectra, an intense signal assigned to an ${\mathrm{O}}^{-}$ radical was observed. The interactions of the ${\mathrm{O}}^{-}$ radical with three inequivalent sets of ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ nuclei as well as with one set of protons were resolved in the ENDOR powder spectra. By a careful analysis of the ENDOR powder spectra using computer simulations based on the "orientation-selection principle," a detailed model for the ${\mathrm{O}}^{-}$ radical and its surroundings could be derived. In this way, it was established unambiguously that the ${\mathrm{O}}^{-}$ ion is located on a hydroxyl site. In addition, experimental evidence was found that the precursor of the ${\mathrm{O}}^{-}$ radical is a carbonate group, and not a hydroxyl group. Next to the ${\mathrm{O}}^{-}$ radical, EPR and ENDOR resonances of another paramagnetic center were observed. After analysis of the spectra and by comparing the results with previous work, this radical was assigned to a C${\mathrm{O}}_3^{3-}$ ion located at a phosphate site.