Mechanisms of Calcium and Phosphate Ion Association in Aqueous Solution

The initial steps of nucleation of calciumphosphates in aqueous solution are elucidated by means of quantum / classical molecular mechanics simulations. A special focus is dedicated to the role of the protonation state of the phosphate ion. The crystallization of calciumphosphates including entirely deprotonated phosphate ions is found at much lower pH values than required for finding the (PO₄)^3? species in water. In such cases the depronation of the hydrogenphosphate ion has to occur during crystal growth. According to our findings, the Ca²⁺··(PO₄)^3?··Ca²⁺] ion triple is the smallest stable aggregate, which may be expected to contain an entirely deprotonated phosphate ion.