Solubility of calcium salts, enamel, and hydroxyapatite in aqueous solutions of simple carbohydrates

Abstract

The solubility of various Ca(II) salts, hydroxyapatite, and powdered human dental enamel in the presence of simple carbohydrates was studied by determining the complex strength between Ca(II) and the carbohydrates. In 1.0 M CaSO4, the following simplified sequence of complex strengths was obtained for the more common carbohydrates: Na-citrate greater than D-sorbitol greater than xylitol greater than D-mannitol greater than D-fructose greater than D-glucose greater than D-xylose. Whereas the more soluble Ca(II) compounds (like CaSO4) exerted measurable complexation with xylitol, no such complexation was found with hydroxyapatite and enamel powder. This also concerned other alditols. Calculation of the stability constants (K) showed sorbitol (K = 0.81 M) and xylitol (K = 0.67 M) to form stronger complexes in saturated CaSO4 than other alditols. The most suitable coordination site appeared to be a vicinal cis-cis-triol. Precipitation studies showed that 0.5 M xylitol and 0.5 M sorbitol significantly retarded the formation of calcium phosphate precipitates from a solution of Ca(II) and phosphate, compared with the effect caused by glucose, sorbose, or xylose. The effect caused by xylitol and sorbitol was explained in terms of partial displacement of water molecules in the primary hydration layer of Ca(II) ions, caused by competition between polyol and water molecules. In the presence of aldoses and ketoses, virtually instantaneous precipitation occurred. These results suggest that open-chain alditols may influence the chemical reactions of Ca(II) in plaque, saliva, and caries lesions. Alditols do not function as demineralizing agents of the teeth, however. Through the retarding effect on calcium phosphate precipitation, alditols may favorably govern remineralization of carious lesions.