Thermally Induced Changes Of The Carbonate Structure In Biological Hydroxyapatite Studied By Epr And Endor

Changes of the carbonate structure in tooth enamel hydroxyapatite under annealing in the temperature interval T-ann = 20-1000 degrees C are studied by electron paramagnetic resonance ( EPR) and electron nuclear double resonance ( ENDOR). In our studies,CO2- 2 radicals induced by gamma-irradiation after the annealing of specimens are used as a probe. The lineshape of EPR spectra and proton ( 1.) and phosphorus ( 31.) ENDOR spectra, as well as their thermally induced modifications, are analyzed and explained. It is shown that the general tendency under the annealing of specimens is the escape of carbonate from biohydroxyapatites and its simultaneous transformation from type B into type A at T-ann > 700 degrees C. Specific features of carbonate escape from biohydroxyapatite at T-ann = 400 degrees C are explained by the formation of localized CO2 molecules in apatite. The results obtained correlate with the data of IR and NMR studies of tooth enamel, as well as with the EPR and ENDOR spectra of Co-2(-) radicals in synthetic hydroxyapatites.