Influence of concentration of calcium ion on controlled precipitation of calcium phosphate within unilamellar lipid vesicles

The controlled precipitation of calcium phosphate within large unilamellar phosphatidylcholine (PC) vesicles with diameters of 80-120 nm has been studied by transmission electron microscopy, energy dispersive X-ray analysis and selected-area electron diffraction. The vesicles were filled with the aqueous solution, whose Ca2+ concentration was 4 and 0.03 mM, Ca/P was 1.67 and pH was 7.0. The vesicles were prepared by ether vaporization technique. The precipitation from the 4 mM solution within the vesicles was amorphous calcium phosphate (ACp) and it remained stable at room temperature. The ACp was transformed into HAp crystals after treatment in hot water bath (60 degrees C) for 24 h. As for the 0.03 mM solution, which is nearly the critical saturation concentration of HAp, the precipitated crystals were small HAp. The role of the vesicle membrane in the controlled precipitation of calcium phosphate is discussed in terms of kinetic and spatial restrictions inherent in the reaction environment. The influence of concentration of calcium ion is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.