Thermodynamics of statherin adsorption onto hydroxyapatite

Statherin is a salivary protein that inhibits the nucleation and growth of hydroxyapatite crystals in the supersaturated environment of the oral cavity. The thermodynamics of adsorption of statherin onto hydroxyapatite crystals have been characterized here by isothermal titration calorimetry and equilibrium adsorption isotherm analysis. At 25 °C, statherin adsorption is characterized by an exothermic enthalpy of ∼3 kcal/mol that diminishes to zero at ∼25% surface coverage. The initial heat of statherin adsorption increases with temperature, displaying a positive heat capacity change of 194 ± 7 cal K^-1 mol^-1 at 25 °C. The heat of adsorption during this initial phase is strongly dependent on the buffer species, and from the differential heats of buffer ionization, it can be calculated that approximately one proton is taken up by the crystal or protein upon adsorption. The free energy of adsorption is dominated at all coverages by a large positive entropy (≥23 cal K^-1 mol^-1), which may be partially due to the loss of organized water that hydrates the protein and the mineral surface prior to adsorption. These results are interpreted using a two-site model for adsorption of statherin onto the hydroxyapatite crystals.