OCTACALCIUM PHOSPHATE CARBOXYLATES .1. PREPARATION AND IDENTIFICATION

The formation of octacalcium phosphate carboxylates, Ca8(HPO4)m(carbox ylate)n(PO4)4.yH2O, by conversion of alpha-Ca3(PO4)2 in solutions of 1 9 ammonium carboxylates (monocarboxylates; saturated, unsaturated, hyd roxy, keto, and amino dicarboxylates; and tricarboxylates) was investi gated. The various solid phases formed, depending on initial pH's and conversion times, were determined by X-ray diffraction and infrared an d Raman spectroscopy. Octacalcium phosphate carboxylates containing st ructurally incorporated malonate, succinate, adipate, suberate, sebaca te, fumarate, malate, and citrate ions were formed and identified. Oct acalcium phosphate carboxylates were also formed from pyruvate and alp ha-ketoglutarate solutions but with uncertain carboxylate ion structur es. All of these identified compounds are structurally similar to octa calcium phosphate, Ca8(HPO4)2(PO4)4-5H2O, but have expanded a-axis uni t-cell dimensions that generally increased with increasing number of c arbon atoms in the carboxylate ion. Among these compounds, of special importance are those containing carboxylates that are present as inter mediates in the Krebs cycle. The possible precipitation of these octac alcium phosphate carboxylates in mitochondria and their possible role as precursors in calcified tissue formation are discussed.