Michael Joseph Mobley

19F MAS-NMR and solution chemical characterization of the reactions of fluoride with hydroxyapatite and powdered enamel

Solution chemical and 19F magic angle spinning-nuclear magnetic resonance (MAS-NMR) methods have been utilized to study the effects of fluoride dose, fluoridating pH, and mineral surface area on the dynamics of fluoride reactivity with hydroxyapatite and powdered human dental enamel in vitro. Both solution chemical fluoride uptake and NMR measurements demonstrated that the reaction products of ionic fluoride with apatite include mixtures of FAP, FHAP, and CaF2, with increased amounts of CaF2 promoted by increased F concentration or decreased pH. NMR analysis showed FAP or FHAP as a reaction product of fluoride uptake under all conditions, regardless of whether CaF2 was formed, unambiguously demonstrating fluorite as an additive rather than substitute form of F reactivity. pH stat measurements demonstrated the release of OH- during F reactivity with apatites corresponding to ion exchange formation of FAP/FHAP or dissolution/reprecipitation formation of CaF2. Phosphate release into solution accompanied fluoride uptake under all conditions, including regions where ion exchange predominated. Whereas powdered dental enamel demonstrated fluoride uptake behavior similar to that of synthetic apatite, the resulting reaction products differed as analyzed by 19F MAS-NMR.

New NMR Methods for the Study of Hydroxyapatite Surfaces

Improved surface-characterization techniques are needed to study the adsorption of molecules and ions from aqueous solutions onto microcrystals of the biological mineral hydroxyapatite, the prime constituent of bone and teeth. The continuing development of techniques for obtaining high-resolution nuclear magnetic resonance (NMR) spectra from solids indicates that NMR could provide a valuable spectroscopic characterization of hydroxyapatite surfaces. We report here the successful application of new NMR techniques to two areas: (1) the adsorption onto the surface of hydroxyapatite of diphosphonates, used both as inhibitors of biological mineralization and as bone-scanning agents; (2) the reactions of hydroxyapatite with fluoride ion, which are important in the anti-caries benefits provided through fluoridation of dental enamel.