A UNIQUE PROTONATED PHOSPHATE CROUP IN BONE-MINERAL NOT PRESENT IN SYNTHETIC CALCIUM PHOSPHATES - IDENTIFICATION BY P-31 SOLID-STATE NMR-SPECTROSCOPY

The detailed chemical composition and microstructure of freshly deposi ted bone mineral, and haw these properties change with maturation of t he mineral, have been studied intensively and still remain controversi al. For example, current analytical technology is inadequate for the u nambiguous characterization of the monohydrogen phosphate ions in bone mineral. Using a differential cross polarization/magic angle spinning solid state nuclear magnetic resonance spectroscopy technique, we sup press the dominant orthophosphate (PO4-3) signal to reveal the spectra of the minor phosphate constituents. This method depends upon differe nces in the cross polarization time constants for phosphorus-31 nuclei in protonated and non-protonated phosphate ions. It is now possible f or the first time to directly measure both the proportion of acid phos phate (HPO4-2) as well as the parameters which characterize its isotro pic arid anisotropic chemical shift. In bone from three species at sev eral developmental stages, we have found a single type of acid phospha te species, identical in all of the specimens examined. The phosphorus -31 isotropic chemical shift of this acid phosphate group in bone mine ral. corresponds precisely with that of acid phosphate in octacalcium phosphate, and not with that of brushite. In contrast, the bone acid p hosphate anisotropic chemical shift parameters are close to those of b rushite, and differ significantly from those of octacalcium phosphate. The orthophosphate resonances of bone mineral, synthetic hydroxyapati te and synthetic octacalcium phosphate share identical chemical isotro pic shifts, and similar chemical shift anisotropies. The implication o f these results is that the intimate structure of the acid phosphate g roup in bone mineral is unique, and that none of the common synthetic calcium phosphates accounts well for all of the observed solid state p hosphorus-31 NMR properties of bone mineral.