Adsorption thermodynamics of acidic proline-rich human salivary proteins onto calcium apatites.

The thermodynamics of adsorption of four acidic proline-rich salivary proteins from their aqueous solutions onto hydroxyapatite (HA) and fluorapatite (FA) have been analyzed. Enthalpies and entropies of adsorption were derived from isotherms at 37 degrees C and 4 degrees C and led to the unexpected conclusion that adsorption of the macromolecules studied is an endothermic process. Consequently, adsorption is driven by an increase in entropy. Most of this increase originates in the adsorbate molecules, probably through disruption of internal ion pairs, hydrogen bonds, and displacement of water molecules. Integral free energies of adsorption (delta G) and the partial contributions of adsorbent and adsorbate were both calculated as a function of the adsorption coverages. For equal coverages (of each adsorbate), delta G is lower with FA than with HA. Most of the contribution to delta G originates from the adsorbate, although the adsorbent contribution becomes significant as adsorption saturation is approached. The differences in the various thermodynamic quantities are qualitatively related to the known structural features of the four proteins. The results are important in understanding the role played by the proline-rich proteins in the oral environment.