SELECTIVE-INHIBITION OF CRYSTAL-GROWTH ON OCTACALCIUM PHOSPHATE AND NONSTOICHIOMETRIC HYDROXYAPATITE BY PYROPHOSPHATE AT PHYSIOLOGICAL CONCENTRATION

Octacalcium phosphate, Ca8H2(PO4)6.5H2O (OCP), appears to be a precursor in biomineral formation. The formation of OCP as the precursor is supported by the observation that stoichiometric hydroxyapatite, Ca5(PO4)3OH (OHAp), cannot form directly because of the presence of its growth inhibitors in serum. Therefore, the effects of the physiological concentration of pyrophosphate (P2O74-), one of the most important calcium phosphate growth inhibitors in blood, on calcium phosphate growth rates on OCP and nonstoichiometric OHAp) (apatite) seeds were measured. The amounts of seed crystals used to initiate the growth were adjusted by trial and error so that the control growth rates (in the absence of P2O74-) were the same on both OCP and apatite seeds at a given supersaturation. The crystal growth on both kinds of seed crystals from supersaturated solutions in the presence of 1-mu-M P2O74- added once ("one-time" addition) at constant pH (7.4) and 25-degrees-C was determined by KOH titration and decreases in Ca and PO4 concentrations in the solutions. Crystal growth on OCP seed crystals in the presence of a constant concentration of 1-mu-M P2O74- was also measured. The growing phases were characterized by DELTA-Ca/DELTA-PO4 ratios, chemical potential plots, X-ray diffraction (XRD) and Fourier transform infrared (FTIR). The results of this study show that: (1) P2O74- ions inhibited the growth on the apatite seeds more than on the OCP seeds; (2) apparently OCP precipitated on both types of seeds, followed by its hydrolysis to a more apatite-like phase; (3) slower crystal growth was observed on OCP seeds in the presence of a constant physiological concentration of P2O74- (1-mu-M) than in the "one-time" addition of P2O74-.