ADSORPTION BEHAVIOR OF PROTEINS ON Mg(II)-DOPED CALCIUM HYDROXYAPATITE PARTICLES AT VARIOUS Mg(II) CONTENTS

Abstract

In this study, fundamental experiments on the adsorption behavior of proteins on synthesized magnesium-doped calcium hydroxyapatite particles (MgHap) at various Mg/(Ca + Mg) molar ratios (abbreviated as X_Mg : 0–0.3) were examined. Rod-like calcium hydroxyapatite particles, with a width of 24 nm and length of 180 nm, were obtained in the absence of Mg²⁺ (X_Mg = 0). With the increase in X_Mg up to 0.10, the width and length of MgHap particles decreased to ~10 and 50 nm, respectively. Agglomerates of small thin particles were obtained at X_Mg ≥ 0.15. All of the synthesized particles exhibited single phase of calcium hydroxyapatite particles (CaHap). The X_Mg values for the particles precipitated at X_Mg ≥ 0.04 were less than those of the corresponding solutions. Hence, almost all of the MgHap particles are deficient in magnesium. We propose that to compensate for the cation deficiency, PO₄^3- is incorporated as HPO₄^2- and H₂PO₄^- and thus decreases the particle negative charge. In addition, all of the adsorption isotherms of typical acidic and basic proteins such as bovine serum albumin (BSA) and lysozyme (LSZ) were compared. The isotherms obtained from a 1 × 10⁻⁴ mol/dm³ KCl solution were of the Langmuirian type. The saturated amount of adsorbed BSA (n_s^BSA) strongly depended on the particle length rather than on the X_Mg value. At a particle length of greater than 150 nm, n_s^BSA values abruptly increased. This result strongly suggests that the effects of the C sites produced on the ac and bc faces are advantageous for BSA adsorption. In other words, the adsorption of BSA on MgHap particles was considerably dependent on the length at a critical value of greater than 150 nm. However, the saturated amount of adsorbed LSZ (n_s^LSZ) exhibited no remarkable variation with the particle length.