Study on Adsorption Mechanism of Proteins Onto Synthetic Calcium Hydroxyapatites Through Ionic Concentration Measurements

To clarify the adsorption mechanism of proteins onto calcium hydroxyapatite (Hap), the kinetic studies of dissolution and ion-exchange properties of synthetic Hap particles in the absence and presence of proteins were examined at 15°C. In the absence of proteins, Hap particles slightly dissolved to give low amounts of calcium ([Ca²⁺] = 0.09-0.14 μmol m^-2) and phosphate [PO ₄ ^3- ] = 0.01-0.08 μmol m^-2) ions in KCl, CaCl₂, BaCl₂ and AlCl₃ solutions. The [Ca²⁺] increased with increase in the Ca/P ratio of Hap, while the [PO ₄ ^3- ] decreased. The[ Ca²⁺] and [ PO ₄ ^3- ] were independent of the ionic strength. Ba²⁺ and AI³⁺ ions were completely ion-exchanged with calcium ions in Hap lattice within 2 hr. The solution pH was increased by 1.1-1.8 after the dissolution of OH^- ions on the Hap surface. In the presence of bovine serum albumin (BSA), the Hap particles dissolved slightly faster than the systems without protein. This fact was explained by a complexation of dissolved ions to functional groups of BSA. The adsorption of BSA induced a reduction of [Ca²⁺] and [ PO ₄ ^3- ] in the aqueous medium and minima appeared on [Ca²⁺] and [PO ₄ ^3- ] profiles before the BSA adsorption reached a saturation. This result suggests that the adsorption of BSA onto Hap is governed by [Ca²⁺] ions complexing to BSA molecules (binding effect) together with the operation of [Ca²⁺] ions exposing on the Hap surfaces by dissolution of OH^- ions, so-called “C-sites”. The addition of BaCl₂ and AlCl₃ steeply increased the amounts of adsorbed BSA (n^BSA) at the initial adsorption step by the strong binding effect of these di- and tri-valent cations between BSA and Hap. However, after eliminating these cations from the Hap surface by the ion-exchange reaction, the binding effects disappeared and n^BSA decreased. Since the number of functional groups is small, the binding effect of the counter ions was only slightly detected for the systems with di- and trivalent ions on the adsorption systems of lysozyme (LSZ).