Effects of Ti(IV) substitution on protein adsorption behaviors of calcium hydroxyapatite particles

The fundamental experiments on the adsorption behaviors of proteins onto photocatalytic Ti(IV)-doped calcium hydroxyapatite (TiHap) particles with varied amounts of Ti(IV) ions doped (called as original particle) were examined comparing to those onto the calcium hydroxyapatite (CaHap) ones. The heat treated TiHaps and CaHap particles at 650 °C for 1 h were also examined (called as heat treated particle). The Ti/(Ca+Ti) atomic ratio (XTi) of the TiHap particles was varied between 0 and 0.20. Since the surface acidity of the particles was increased by increase in XTi value, the negative zeta potential (zp) of the particles was increased. All the adsorption isotherms of bovine serum albumin (BSA), myoglobin (MGB) and lysozyme (LSZ) from 1 × 10−4 mol/dm3 KCl solution were the pseudo-Langmuirian type. The saturated amounts of adsorbed LSZ ( n S L S Z ) values onto the original particles were increased with increase in the negative zp of the particles. However, the saturated amounts of adsorbed BSA ( n S B S A ) values were decreased by increase in the negative zp except at XTi = 0.05 where n S B S A value exhibited a maximum. In the case of MGB, the saturated amounts of adsorbed MGB ( n s M G B ) values were less dependent on the zp of the particles. These results were explained by changing the electrostatic forces between protein molecules and TiHap particles by doping Ti(IV) ions. On the other hand, n S B S A , n S L S Z and n s M G B values onto the heat treated particles were larger than the original particles in each particle system, though no relationship to the XTi value was recognized in each protein system. This result was interpreted to the formation of β-TCP crystal phase in both the CaHap and TiHap particles by the heat treatment. The Ca2+ ions produced by dissolution from β-TCP phase may exert as binders between BSA and surfaces of the heat treated particles. The weak binder effects of Ca2+ and PO43− ions were observed for the adsorptions of LSZ and MGB.