Solid state carbon-13 and proton NMR studies of carbonate-containing calcium phosphates and enamel

Carbonate ions which replace phosphates—type B—and hydroxide ions—type A—in bone and synthetic apatites were studied by 13C and 1H solid state NMR spectroscopy. Heated apatites and dipolar suppression experiments have been employed to assign the spectra lines. Two overlapping peaks around 170 ppm which behaved differently on heating were assigned to type B carbontes. Pure type A carbonate gave a sharp signal at 166.5 ppm, and we were able to identify this signal in type AB apatite and in a presumably pure type B carbonate apatite. Amorphous calcium phosphate carbonates gave a broad gaussian peak of about 3 ppm at full width half-maximum with a center of gravity at about 168 ppm. The same resonance line, but with lower intensity, was also observed for apatites containing type A and B carbonates. Magic angle spinning 1H NMR spectra of these apatites showed resolved OH− and H2O signals but were inconclusive in identifying specific structural and adsorbed H2O groups. Implications of the NMR data to the crystal structure of carbonate apatites is discussed.