ENDOR study of an O- 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 O- radical was observed. The interactions of the O- radical with three inequivalent sets of P-31 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 O- radical and its surroundings could be derived. In this way, it was established unambiguously that the O- ion is located on a hydroxyl site. In addition, experimental evidence was found that the precursor of the O- radical is a carbonate group, and not a hydroxyl group. Next to the 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 CO33- ion located at a phosphate site.