Protein adsorption behaviors onto Mn(II)-doped calcium hydroxyapatite particles with different morphologies

Fundamental experiments on the adsorption behaviors of proteins onto plate-like and rod-like manganese-doped calcium hydroxyapatite particle (abbreviated as MnHAp) were examined. All of the obtained adsorption isotherms of bovine serum albumin (BSA) and lysozyme (LSZ) in a 1 × 10^-4 mol/dm³ KCl solution were of the Langmuirian type. We found that the saturated amounts of the adsorbed BSA (n_s^BSA) increased with the increase in Mn/(Ca + Mn) atomic ratio (abbreviated as X_Mn) of the plate-like MnHAp, while the saturated amounts of adsorbed LSZ (n_s^LCZ) decreased. This result is explained by plate shape of the particles; the large fraction of positively charged adsorbing sites produced on the ac and bc faces (C sites) of these particles is advantageous to the adsorption of negatively charged BSA. In this case, however, the fraction of negatively charged adsorbing sites produced on the ab faces (P sites) decreased, and the (n_s^LCZ) values therefore decreased. In the case of the rod-like MnHAp, (n_s^BSA) decreased until X_Mn = 0.08, while the (n_s^LCZ) values were almost constant (ca. 0.2 mg/m²) over the whole range of X_Mn. This decrease in (n_s^BSA) values is explained by the increase in the specific surface area of rod-like particles as X_Mn increased. However, since the fraction of P sites on the ab faces does not depend on the particle length, the (n_s^LCZ) values were nearly constant. The binding effect of the Mn²⁺ and Ca²⁺ ions dissolved from rod-like particles caused the increase in (n_s^BSA) at X_Mn ≥ 0.1. The adsorption behavior of proteins onto MnHAp is therefore strongly dependent on the morphology of these particles.