Application of the turbidity measurements to study in situ crystallization of calcium phosphates

Kinetics of the initial crystallization stages of calcium phosphates from supersaturated aqueous solutions containing NaCl, NaN3, and Tris–buffer was followed by turbidity measurements during 10 h. The crystallization was studied inside a silica cell at 25 °C and solution pH 7.4. The crystals obtained were found to consist of a mixture of poor crystallized hydroxyapatite (HA) and octacalcium phosphate (OCP). The threshold concentration (when crystallization started) measured by the turbidity technique appeared to be 5.5 mM. Presence of poly-l-glutamic acid previously adsorbed onto the silica seemed to promote the crystallization, while similar layer of poly-l-aspartic acid was found to have no influence. When dissolved, both polypeptides were found to inhibit the crystallization.