Abstract
Thermodynamic parameters have obtained in the present study by measuring the adsorption isotherms of bovine serum albumin (BSA: isoelectric point (iep) = 4.7), myoglobin (MGB: iep = 7.0) and lysozyme (LSZ: iep = 11.1) onto calcium hydroxyapatite (Hap) at different temperatures (15°C and 35°C) in an attempt to clarify the adsorption mechanism of such proteins. The adsorption affinity of proteins strongly depended on the Ca/P molar ratio of the Hap particles. In the case of BSA, the initial slopes of the adsorption isotherms increased remarkably with an increase in the Ca/P ratio. In contrast, the initial slopes of the adsorption isotherms for LSZ changed gradually with increasing Ca/P ratio. Finally, no Ca/P ratio dependence was observed in the MGB system.
These results were explained in terms of the electrostatic interaction between the proteins and Hap. The negative values of ΔGads for the BSA system increased from −34.0 kJ/mol to −36.1 kJ/mol with increasing Ca/P ratio whereas those for the LSZ system decreased from −37.9 kJ/mol to −30.8 kJ/mol. A diminished dependence of ΔGads on the Ca/P ratio was observed in the MGB system. The adsorptions of LSZ and MGB onto Hap were exothermic which again supports an electrostatic attraction mechanism. In contrast, the adsorption of BSA onto Hap exhibited a large positive value for ΔHads, indicating that the process was endothermic. The ΔSads value of the LSZ system was negative while those for the BSA and MGB systems were positive. Positive values for ΔSads, especially for the BSA system, indicate that the adsorption processes for BSA and MGB (both with a lower structural stability) were driven by an entropy gain, i.e. ΔSads > 0. Adsorption of these proteins onto the Hap surface was accompanied by a structural rearrangement.