Inhibition of the amorphous calcium phosphate phase transformation reaction by polyphosphates and metal ions.

The induction time for amorphous calcium phosphate (ACP) phase transformation was monitored at pH 7.4 and T = 25 degrees C with [Ca2+]o = [PO4]o = 4.0 X 10(-3) M, as a function of added crystal growth inhibitors Mg2+, Sr2+, Zn2+, pyrophosphate (PP), and tripolyphosphate (TPP). Metal ions increase the induction time for the initiation of the phase change reaction in the order Zn2+ less than Sr2+ less than Mg2+. For polyphosphates it was observed that both PP and TPP are potent inhibitors with TPP more effective than PP as expected. The combination of Mg2+ or Sr2+ and PP or TPP leads to a synergistic delay in the onset of the phase conversion. The greatest inhibition was observed for Mg2+ and TPP. Reaction solutions containing 2.0 X 10(-4) M Mg2+ and 4.0 X 10(-5) M TPP resulted in a 90% increase in the induction time over what would be anticipated from an additive effect from these species.