Amino Acidic Control of Calcium Phosphate Precipitation by Using the Vapor Diffusion Method in Microdroplets

Calcium phosphate precipitation was carried out in the Presence of L-aspartic acid (L-asp, iep = 2.77), L-alanine (L-ala, iep = 6.00), and L-arginine (L-arg, iep = 10.76) at different concentration's by using a vapor diffusion Sitting drop method (VDSD) in microdroplets. Irrespective of the nature and the concentration of the amino acid used, the early stage in the precipitation consisted in the formation of a white viscous suspension composed of amorphous calcium phosphate (ACP) spherulites. After 1 week, different calcium phosphate phases were found depending on the amino acid nature and concentration. At higher concentrations of L-aspartic acid, brushite (dicalcium phosphate dihydrate, DCDP) platelets and a few needle-like carbonate-hydroxyapatite (HA) crystals were found. In the presence of higher concentrations of L-alanine, the precipitate was composed of both needle-like HA and octacalcium phosphate (OCP) platelets. Finally, at higher concentrations of L-arginine, we obtained carbonate-HA nanocrystals with length of 20-40 nm and a few OCP crystals, as in the blank experiment (without amino acids). The results are explained on the basis of the influence of these amino acids on the pH evolution of the solution and on the-nature and strength of the interactions of the major charged species of the amino acids with the surface lattice ions of the apatite precursor Phases (DCPD or OCP). A thermodynamic model, based on the temporal existence of OCP in the solution, is proposed to explain the formation of the HA nanocrystals.