Formation of calcium-phosphate-based supra-ceramics in solutions containing glutaric acid derivatives

Octacalcium phosphate (OCP) can incorporate specific carboxylic acids into its crystal lattice. Such substances accomodating a molecular component in their crystal lattice have been recently categorised as supra-ceramics. However, the physicochemical properties that govern the ability of carboxylic acids to be incorporated into the OCP crystal are unknown. In this study, OCP was synthesised in aqueous solutions of glutaric acid and its derivatives, each with different dissociation constants. Synthesis experiments were conducted to determine whether these carboxylic acids can be incorporated into OCP. Glutaric acid, iminodiacetic acid, and diglycolic acid could be incorporated into OCP, whereas 2,2'-thiodiglycolic acid was not incorporated into OCP, revealing for the first time that iminodiacetic acid and diglycolic acid can be incorporated into OCP. In addition, the dissociation constant of 2,2'-thiodiglycolic acid, which could not be incorporated, had an intermediate value among the studied carboxylic acids, suggesting that the dissociation constant of carboxylic acids is unlikely to be a factor governing their ability to be incorporated into OCP. These findings are expected to contribute to a deeper understanding of the incorporation of carboxylate ions into the OCP crystal.