Differential adsorption and chemical affinities of proteins for apatitic surfaces

Studies are reviewed which identify the interacting groups involved in protein adsorption onto hydroxyapatite. Thus, carboxyl and phosphoester groups bind to calcium sites in the adsorbent, and basic groups bind to phosphate sites. Detailed adsorption studies have been performed to quantitate some of these interactions. An adsorption model, based on the Langmuir adsorption isotherm, adequately fitted the data from experiments using selected amino acids, bovine albumin and two human salivary proteins. Adsorption parameters (affinities and maximum number of sites) were obtained for several apatitic adsorbents, with affinities increasing considerably in the series hydroxy- (HA), fluorhydroxy- (FHA) and fluorapatite (FA). A modest increase in the number of sites was also noted. The change in adsorption behavior, with increasing fluoride content, was attributed to a reduction in the surface free energy of the adsorbent, with a concomitant decrease in the interaction of the adsorbent with water, and a consequent enhancement of the adsorption bond. It is suggested that this effect may play a role in the cariostatic effect of fluoride. Unusual structural aspects of the salivary proteins are discussed in relation to their adsorption behavior, and the molecular segments responsible for binding to the adsorbent tentatively identified.