INTERACTION OF SODIUM POLYACRYLATE WITH OCTACALCIUM PHOSPHATE

摘要

NaPA inhibits nucleation and growth of OCP, as well as its hydrolysis into HA. The first of the two phenomena is clearly due to a nonspecific interaction, as indicated by the lack of any significative adsorption of the polyelectrolyte on OCP crystals and by the isotropic reduction of the coherence lengths of the perfect crystalline domains on increasing polymer concentration in solution. On the other hand, the inhibition of OCP crystals. We verified that, in similar conditions, the adsorption of NaPA on HA crystals reaches a maximum relative amount of 2 wt%, in agreement with previous studies about NaPA adsorption on HA. The polymer preference for OCP structure is confirmed by the comparison among the different crystal habits of HA crystals obtained by hydrolysis at different NaPA concentrations. The crystals obtained in 10~(-2) mM NaPA, that is when just a partial hydrolysis occurs, exhibit a morphology quite similar to that of HA crystals obtained by hydrolysis in water, whereas those obtained in 10~(-3) mM NaPA are clearly affected by the polymer. This finding suggests that when the polyelectrolyte can choose, as in partially hydrolyzed material, it is preferentially adsorbed on OCP rather than on HA crystals. On the basis of the structure of OCP and HA, it must be supposed that the preference of the polyelectrolyte for OCP is due to the presence of the hydrated layer, where it is adsorbed on the (100) face causing an anisotropic variation of the coherence lengths. The morphology of HA crystals obtained by OCP hydrolysis is very similar to that of the plate-like crystals of bone and dentine, and supports the hypothesis of an OCP involvement in the biomineralization process of these tissues. The adsorption on the hydrated layer of the (100) face prevents OCP hydrolysis into HA, in agreement with the hypothesis that hydrolysis takes place along this face, as suggested to explain the contemporary presence of OCP and HA in the same crystal of calcifying dentine. Although these results cannot be extrapolated to in vivo interaction between acidic proteins and calcium phosphates, they point out the important role that the presence of acidic macromolecules can play on the relative stability of octacalcium phosphate and hydroxyapatite, as well as on the shape, dimensions and crystallinity of the final crystals.