Structural Modulation of Octacalcium Phosphate Originating from Hydrogen-Bond Network Engineering in the Hydrated...

Octacalcium phosphate (OCP) consists of an apatite layer and a hydrated layer. The hydrated layer, located between the apatite layers, is composed of hydrogen phosphate ions (HPO₄^2-) and structural water molecules, and the HPO₄^2- ions can be substituted with dicarboxylate ions. In the previously reported substitution systems using linear saturated dicarboxylate ions, it has been reported that the local environments of the phosphate ion species in the hydrated layer become polarized. However, no studies have focused on controlling these local environments by incorporating organic molecules into the hydrogen-bonding network in the hydrated layer. Similarly, no examples have been reported in which heteroaromatic dicarboxylate ions with a five-membered ring structure are substituted into OCP. Therefore, in this study, pyrazole-3,5-dicarboxylic acid (PD), a five-membered aromatic molecule possessing hydrogen-bonding ability, was used to evaluate the effects of the PD molecules substituted into the hydrated layer on the OCP crystal structure and properties. As a result, it was revealed that PD was successfully substituted into the hydrated layer of OCP. The substituted PD molecules were incorporated into the relatively strong hydrogen-bonding network region in the hydrated layer, and this incorporation induced a change in the OCP crystal structure. Furthermore, the intermolecular interactions among the PD molecules adsorbed on the crystal surfaces of OCP affected the aggregation behavior, resulting in the formation of pore structures between the crystals. These results indicate that PD, a five-membered dicarboxylate ion, was substituted into OCP in a distinct manner that is clearly different from that of conventional linear saturated dicarboxylate ions, and that the substituted PD molecules became incorporated into the relatively strong hydrogen-bonding network in the hydrated layer. This study provides a design guideline for controlling the hydrogen-bonding network in the hydrated layer and the aggregation state of OCP crystals by utilizing organic molecules.