Preparation and characterization of new hybrid organic/inorganic systems derived from calcium (alpha-aminoalkyl)-phosphonates and -phosphonocarboxylates.

We have studied the phenomenon of calcium complexation by lab synthesized amphiphilic (alpha-aminoalkyl)-phosphonocarboxylic or -phosphonic acids. The electrical conductivity of aqueous solutions of sodium salts of all these acids was measured versus the volume of a calcium salt solution added. It appeared that calcium complexes are formed in a Ca/P atomic ratio close to 1. Calcium phosphonocarboxylates and calcium phosphonates were also precipitated by mixing aqueous solutions of disodium salts of phosphorus amphiphiles and calcium nitrate solutions. Before chemical analysis, these complexes were calcined to remove the organic part. In the mineralized products, calcium and phosphate were assayed: the Ca/P atomic ratio was equal to 1. X-ray diffraction and IR spectroscopy showed that they are made entirely of beta pyrophosphate (Ca2P2O7), a result in agreement with previous chemical analysis. The chemical formula of the starting calcium complexes could be written as CaL2H2O (L=ligand). The SEM micrographs of these complexes show plate-like structures. XRD patterns are characteristic of layered structures. These facts suggest that calcium complexes are composed of alternating bimolecular layers of calcium alkylphosphonocarboxylates or calcium alkylphosphonates, the chains being tilted and partially interdigitated.

Calcium phosphate interactions with titanium oxide and alumina substrates: an XPS study.

Besides the excellent mechanical properties of titanium and alumina (Al(2)O(3)) in the case of load bearing applications, their bone-bonding properties are very different. In osseous environment, Al(2)O(3) ceramic is encapsulated by fibrous tissues, whereas bone can bind directly to titanium, via its natural titanium dioxide (TiO(2)) passivation layer. So far, this calcification dissimilarity between TiO(2) and Al(2)O(3) was attributed to respectively their negative and positive surface charge under physiological conditions. The present study aims at studying the chemical interactions between TiO(2) and Al(2)O(3) (phase alpha) with the diverse ions contained in simulated body fluids (SBFs) buffered with trishydroxymethyl aminomethane (TRIS) at pH=6.0 and pH=7.4. After 1 h of immersion, TiO(2) and alpha-Al(2)O(3) powders were analyzed by X-ray photoelectron spectroscopy (XPS). The results indicated that Ca and HPO(4) groups were present on TiO(2) surface. In addition, HPO(4) groups were found to be in a higher amount than Ca on TiO(2), which does not comply with the surface charge theory. With regard to Al(2)O(3), little HPO(4) but no Ca was detected on its surface, and TRIS bound to Al(2)O(3) substrate in all of the immersion experiments. The fact that both Ca and HPO(4) were present at the vicinity of TiO(2) might be at the origin of its calcification ability. On the other hand, Al(2)O(3) did not show any affinity towards Ca and HPO(4) ions. This might explain the inability of Al(2)O(3) substrate to calcify.

Adsorption of O-Phospho-L-Serine and L-Serine onto Poorly Crystalline Apatite.

The adsorption of phosphoserine and serine was studied to determine the effect of amino acid functional groups on the surface reactivity of synthetic poorly crystalline apatite similar to bone mineral. The experimental results for phosphoserine and serine uptake agree respectively with the Langmuir and Freundlich models. Phosphoserine exhibits stronger adsorption capacity and a higher affinity constant for the surface crystals compared to serine molecules. The enhanced adsorption capacity noted for phosphoserine might be related to the presence of phosphate groups in the molecule, which are specific attachment sites. This observation suggests that the strength of phosphate bonds to the solid surface, especially to calcium ions, is higher than that of carboxyl and hydroxyl ones. Spectroscopic observations provide evidence of an adsorption mechanism involving the anionic species of the amino acids and the surface of the crystals. Thus, a change in the position of the band of carboxyl groups occurred for the adsorbed molecules compared to the native amino acids. This revealed that the molecular residues do interact with apatite surface calcium. The shift noted in the frequencies of the bands associated with carboxylate vibrations is more pronounced for phosphoserine, confirming the stronger interaction noted for this molecule. Based on these results, one can conclude that the sorbent and sorbate charged species play an important role in the mechanism of uptake of the amino acids onto crystal surfaces. This may contribute to a better understanding of the mechanism by which phosphoproteins could influence mineralization processes and caries. Copyright 2001 Academic Press.