Morphological and Structural Modifications of Octacalcium Phosphate Induced by Poly-l-Aspartate

Abstract

The results of this paper indicate that the presence of poly-l-aspartic acid (PASP) strongly inhibits the synthesis of octacalcium phosphate (OCP) in aqueous solution and provokes remarkable morphological and structural modifications. Under the experimental conditions employed, in the absence of the polyelectrolyte, OCP precipitates as individual platelets, elongated along the c-axis direction, of mean dimensions 22.0 × 1.3 × 0.2 μm3. Low PASP concentrations (up to 22 μM) induce a reduction of the mean dimensions of the platelets and the formation of aggregates, which in the TEM-ED images appear mostly constituted of fibers aggregated parallel to their length, and exhibit unidirectional disorder in the direction orthogonal to the c-axis. Relatively high PASP concentrations (up to 110 μM) greatly reduce the total amount of product, which precipitates as almost spherical aggregates with increasing structural disorder. The modification of the X-ray diffraction pattern on increasing PASP concentration indicates an increasing disorder and an increasing contribution of the apatitic layer of the OCP structure, which could be related to the observed reduction of thermal stability, and suggests a possible interaction of PASP with the hydrated layer of the OCP structure.