Dissolution of synthetic hydroxyapatite in the presence of acidic polypeptides

This article deals with the effect of two acidic polypeptides [polyaspartic acid (PA) and polyglutamic acid (PG)] onto hydroxyapatite (HAP) dissolution by separately considering their influence when they are present only at the HAP interface and when they are both adsorbed and present in the bulk solution. We first determined the amount of adsorbed PA and PG at pH 7.0 and 5.0 onto 10 mg of HAP. Dissolution experiments were performed at pH 5.0 under pH stat conditions by continuously following the consumed protons and released calcium versus time with the aid of specific electrodes. The released phosphate ions were determined by spectrophotometric analysis. The data show that, because of their calcium chelating properties, the polypeptides act as a driving force for HAP dissolution when PA and PG remain present in solution and the interfacial beneficial effect of the adsorbed peptides is erased by the chelating properties of PA and PG present in the solution. When the polypeptides are only adsorbed at the interface, even if a partial PA or PG desorption occurs, HAP dissolution inhibition is still observed.