OCTACALCIUM PHOSPHATE CARBOXYLATES .2. CHARACTERIZATION AND STRUCTURAL CONSIDERATIONS

Detailed physicochemical characterization of octacalcium phosphate car boxylates (OCPCs) with structurally incorporated succinate, adipate, s uberate, sebacate, fumarate, and citrate ions is reported. Composition al formulas of the OCPCs were derived from Ca, P, C, H, and H2O analys es. In octacalcium phosphate dicarboxylates with the general formula C a8(HPO4)2-x(dicarboxylate)x(PO4)4.yH2O, a maximum of one HPO42- was re placed by a dicarboxylate ion (x less-than-or-equal-to 1). The water c ontent increased with increasing chain length of the dicarboxylate ion (5.7 less-than-or-equal-to y less-than-or-equal-to 8.0). For octacalc ium phosphate tricarboxylate, Ca8(HPO4)2-1.5x(tricarboxylate)x(PO4)4.y H2O, with incorporated citrate ions the values of x and y were 0.4 and 7.8, respectively. X-ray diffraction patterns of OCPCs show structura l similarity with the parent compound octacalcium phosphate (OCP); the b and c axes of the unit cells were nearly the same as those of OCP b ut the a axes were progressively expanded concomitant with carbon chai n length. Infrared and Raman assignments were made for nearly all band s of these six OCPCs; an OCP-succinate containing the deuterated succi nate ion was prepared to facilitate band assignments. Spectra of the O CPCs, as compared to that of OCP, showed the presence of carboxylate g roups, changes in water bonding, only slight changes in PO4 environmen ts, and preferential reduction in HPO4(5) content. OCP has two crystal lographically nonequivalent HPO4 groups (designated 5 and 6); the pref erential replacement of HPO4(5) by the dicarboxylate ion is plausible considering lattice geometry. By utilizing combined data from the diff erent methods, the possible positions of carboxylate ions in the OCPC structures are discussed.