Effect of Zn²⁺ on the physicochemical characteristics of octacalcium phosphate and its hydrolysis into apatitic phases

Zinc (Zn) ions and OCP crystals have both been reported to be stimulants for bone formation. The present study was designed to investigate how Zn ion affects the precipitation of octacalcium phosphate (OCP) and its hydrolysis process into apatitic phases. Zn-containing octacalcium phosphate preparations (Zn-CaPs) and their hydrolysates (hydrolyzed Zn-CaPs) containing various amounts of Zn (0.06-0.72 mmol/g) were obtained either through a co-precipitation method or by enhancing the OCP hydrolysis in hot water. All of the Zn-CaPs obtained had Ca/P molar ratios similar to that of stoichiometric OCP (133), ranging from 1.23 to 1.37. Zn-CaPs also exhibited similar dissolution behavior to that of original OCP if incubated in a physiological solution (in the 0.1 M HEPES buffer, pH = 7.4, 37 °C). X-ray diffraction analysis of the hydrolyzed samples showed that original OCP converted to the crystalline hydroxyapatite with a specific peak at 2θ = 10.8°, while Zn-CaPs containing Zn above the 0.18 mmol/g retained the amorphous-like structure even after a greater than 25 h hydrolysis period. These results indicate that Zn ions retarded the hydrolysis process of OCP toward apatite, resulting in enhanced formation of the amorphous-like structure.