Inhibitory Activity of Oligo- and Poly-L-glutamic Acids against Calcium Phosphate Insolubilization and Calcium Binding with Special Relevance to Their Molecular Weight Dependence

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The inhibition by oligo- and poly-L-glutamic acids of calcium phosphate insolubilization was investigated with special attention to their molecular weights. The inhibitory activity was evaluated by the induction time from base titration experiments. The inhibitory activity of oligo-L-glutamic acids increased as their molecular weights (residue numbers) increased, and that of oligo-L-glutamic acid (20 mer) was almost the same as that of poly-L-glutamic acids (70 mer or 340 mer). Calcium binding properties of oligo-L-glutamic acids were measured by using a calcium-ion-selective electrode. The apparent amount of calcium bound to oligo-L-glutamic acids tended to increase as their molecular weights increased. The calcium-binding isotherm of oligo-L-glutamic acid (20 mer) was almost identical with that of poly-L-glutamic acid (70 mer). Moreover, the effect of poly-L-glutamic acid on the crystalline forms of insolubilized calcium phosphates was investigated by X-ray diffraction analysis, with the result that CaHPO₄·2H₂O (DCPD), a precursor of hydroxyapatite, was not observed.