Concurrent Binding of Calcium Ion and Chondroitin Sulfate Ion to the Surface of Hydroxyapatite

Abstract

Concurrent adsorption of chondroitin sulfate ion (Chs) and calcium ion (Ca²⁺) to hydroxyapatite (HAP) from an aqueous solution of Na₂Chs mixed with CaCl₂ was studied at 25°C. The adsorbed amount of Chs increased with increasing concentration of Ca²⁺ for the following two reasons : (a) Ca²⁺ adsorbed on the surface of HAP offers an adsorption site for Chs owing to its positive charges, and (2) the binding of Ca²⁺ to anionic sites of Chs depresses the inter- and intramolecular electrostatic repulsion between Chs segments, resulting in the dense adsorption of Chs. The apparent amount of Ca²⁺ adsorbed by HAP is also increased in the presence of Chs, because Ca²⁺ is captured by negative charges of Chs adsorbed on the surface of HAP as well as by adsorption sites for Ca²⁺ on the surface of HAP itself. However, it was concluded that the amount of Ca²⁺ adsorbed directly on the surface of HAP in the presence of Chs is almost the same as that in the absence of Chs when the adsorbed amount of Ca²⁺ is shown as a function of the concentration of free Ca²⁺.