Interplay of the hydroxyapatite structure and morphology with the thermodynamic parameters at hydroxyapatite/protein interface

Hydroxyapatite (HAP) is a major inorganic component of human body’s hard tissues. For bioengineering applications involving tissues regeneration, the understanding of the interaction of HAP materials with proteins is essential in determining the extent of interaction with cells and tissues in vivo and thus, can be critical for understanding the subsequent physiological responses. Here we report the results obtained from a study of the interaction between various types of HAP materials and a model protein, i.e. bovine serum albumin (BSA). Three types of HAP powders were prepared by bovine bone extraction (HAPnatural) and by hydrothermal synthesis, in acidic (HAPacid) and in basic (HAPbasic) medium, respectively. The HAP powders were characterized by XRD, FT-IR, BET, DLS and SEM, and they show different characteristics in terms of morphology, particle size and surface properties. Further, according to our approach, the following issues have been addressed: (i) The BSA loading capacity onto HAP by spectrophotometric (UV–Vis) measurements, (ii) The analysis of the thermodynamic binding characteristics represented by the binding constant K, binding stoichiometry n, enthalpy ΔH, Gibb’s energy ΔG and entropy ΔS changes of the binding interaction obtained by means of Isothermal Titration Calorimetry. The change of the binding energy is negative for all systems, showing the spontaneity of the binding process. The thermodynamic profile of the protein−HAP interactions appears to be strongly dependent on the physicochemical properties of the HAP materials, the enthalpic character of the interaction decreasing in the order HAPbasic > HAPacid > HAPnatural.