Salivary protein adsorption onto hydroxyapatite and SDS-mediated elution studied by in situ ellipsometry

Whole unstimulated saliva from two donors was investigated both with respect to adsorption characteristics and SDS-induced elutability Salivary protein adsorption onto hydroxyapatite (HA) discs was studied by means of in situ ellipsometry in the concentration range 0.1-20% saliva. The adsorbed amounts on HA were found to be similar to those on silica, but the rates of adsorption were lower. Protein adsorption was virtually unaffected by the presence of Na+, whereas Ca2+ induced nucleation of calcium phosphate at the surface, the deposition rate being influenced by the pellicle age but not by the presence of saliva in bulk solution. The SDS elutability of adsorbed pellicles was determined on HA as well as on silica surfaces. Desorption from both surfaces was found to occur in the same SDS concentration range, although a residual layer was observed on HA. The slight net positive charge and lower charge density of HA as compared to the strongly negatively charged silica, may, at least partly, account for this observation by causing a reduction in the repulsive force between protein-surfactant complexes and the surface. Interindividual differences, observed in the adsorption as well as elution experiments, are thought to relate to the compositional differences observed by SDS-PAGE.