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.

Development of an automated experimental setup for the study of ionic-exchange kinetics. Application to the ionic adsorption, equilibrium attainment and dissolution of apatite compounds.

An ion-selective electrode and microcomputer-based experimental setup for the study of ionic-exchange kinetics between a powdered solid and the solution is described. The equipment is composed of easily available commercial devices and a data acquisition and regularization computer program is presented. The system, especially developed to investigate the ionic adsorption, equilibrium attainment and dissolution of hard mineralized tissues, provides good reliable results by taking into account the volume changes of the reacting solution and the electrode behaviour under different experimental conditions, and by avoiding carbonation of the solution. A second computer program, using the regularized data and the experimental parameters, calculates the quantities of protons consumed and calcium released in the case of equilibrium attainment and dissolution of apatite-like compounds. Finally, typical examples of ion-exchange and dissolution kinetics under constant pH of enamel and synthetic hydroxyapatite are examined.

Kinetics of dissolution of calcium hydroxyapatite powder. III: pH and sample conditioning effects.

The kinetics of dissolution of synthetic hydroxyapatite powder (HAP) were studied at 37 degrees C and constant pH in the pH range 3.7-6.9 by continuously recording proton uptake and calcium release. The effect of sample conditioning was carefully investigated. The powder previously equilibrated in saturated solutions shows an initial dissolution rate higher than the one obtained when dry powder directly added to the dissolution solution is used. This effect is interpreted by considering surface state differences. As previously shown, dry powder contains important amounts of calcium and phosphate ions adsorbed onto apatite surface, ions which are desorbed during equilibration. It is assumed that the initial presence of these ions slows the dissolution rate during the first stage of the process by the formation of a permselective layer. Except for these adsorption phenomena which are less important for human enamel powder (HEP) having a lower specific surface area, it is shown that in spite of structural, morphological, and purity differences, the general dissolution behavior of HAP is quite similar to that of HEP, previously studied, and for which a quantitative model has been proposed. The dissolution rates are stirring dependent in a large range of stirring speeds and are proportional to [H+]0.64. Moreover, it is shown that in the whole range of studied pH, a calcium accumulation process occurs at the interface during the first minutes of the acidic attack. It is concluded that in our experimental conditions, the dissolution process is limited by the diffusion of calcium and/or phosphate ions in the interface.(ABSTRACT TRUNCATED AT 250 WORDS)