Kinetic studies involving octacalcium phosphate and hydroxyapatite: Model crystals for biomineralization

Kinetics investigations of octacalcium phosphate (OCP) and hydroxyapatite (HAP), under conditions of constant driving force were carried out. The effect of biological molecules on the mineralization of OCP and HAP and the demineralization of OCP are discussed. Also, the solution composition influence on OCP dissolution rates and the transformation kinetics of OCP to a more apatitic phase are presented. HAP was grown in the presence of synthetic peptides. The inhibitory effect of cystatin SN fragments relative to the effect exhibited by other biological molecules (Johnsson et al.) suggest that properties of biological molecules may have a differing inhibitory effectiveness depending on the rate controlling crystal growth mechanism. The effect of solution composition on OCP surface characteristics and dissolution kinetics was investigated. The relationship between surface characteristics and dissolution rate are discussed in terms of kink density and a kinetic ionic ratio theory developed by Zhang. Proteins possessing different functional groups may have dissimilar effects on crystallization kinetics due to different matching between the crystal surface and the protein structure. The kinetic effects of phosphophoryn (PPn) and polyaspartic acid (PAA) on OCP growth are compared. Although PPn covers much less surface area than PAA, PPn is a more effective growth inhibitor. The kinetics of OCP transformation to an apatitic phase was investigated. Dual constant composition kinetic experiments in combination with physicochemical characterization techniques (FTIR and X-ray powder diffraction) may suggest that an OCP dissolution/HAP growth mechanism does not sufficiently describe the transformation process.