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.

Atomic force microscopy study of human tooth enamel surfaces.

Human enamel features from individual crystals up to prisms were observed by atomic force microscopy (AFM). Low magnification images of vestibular tooth surfaces show the existence of enamel prisms appearing as deep holes. Individual, parallel enamel crystals show lateral faces elongated and formed by the (100) planes of hydroxyapatite (HA). Height differences between (001) faces create the roughness of enamel surface. Individual (001) crystal faces can be observed clearly at higher magnification and show the characteristic hexagonal shape with 60 degree angles between (100) faces. This study confirms the applicability of AFM for studying biological hydroxyapatite crystals.