Octacalcium phosphate with incorporated meso-2,3-dimercaptosuccinate ions exhibiting continuous interplanar spacing expansion

Abstract

Octacalcium phosphate (OCP) has a layered structure and can accommodate various carboxylate ions, resulting in different interplanar spacings of its (100) plane. An example of the continuous expansion of the interplanar spacing of OCP has been demonstrated using mercaptosuccinic acid (MSA). The mercapto groups of carboxylic acid likely exhibit important roles in the continuous interplanar spacing expansion of the OCP with incorporated carboxylate ions. However, the origin of the continuous interplanar spacing expansion of the OCP incorporating carboxylate ions with mercapto groups is still unclear. In this study, we investigated the incorporation of meso-2,3-dimercaptosuccinic acid (MDSA) with two mercapto groups into the OCP interlayers and investigated whether continuous expansion of the interplanar spacing occurred. We synthesized OCP in aqueous solutions of various MDSA concentrations and characterized it by X-ray diffraction. OCP was formed at low MDSA concentrations, whereas at higher MDSA concentrations, MDSA inhibited the formation of OCP. The interplanar spacing of the OCP formed at low MDSA concentrations increased with increasing MDSA concentration. In other words, MDSA was incorporated into the OCP and caused its continuous interplanar spacing expansion. Hence, we successfully obtained compelling evidence indicating the importance of mercapto groups of carboxylic acid on the continuous interplanar spacing expansion of OCP. Our findings will contribute to the advancements in the chemistry of OCPs with incorporated carboxylate ions.