Controlled Crystallization of Calcium Carbonate via Cooperation of Polyaspartic Acid and Polylysine Under Double-Diffusion Conditions in Agar Hydrogels

We demonstrated the crystallization of calcium carbonate in the presence of polyaspartic acid (PAsp) and polylysine (PLys) under double-diffusion conditions in agar hydrogels. The crystallization of precipitated CaCO3 was discussed based on the equivalency rule, and it was suggested that the nucleation of CaCO3 in the presence of PLys and PAsp occurred heterogeneously on the polypeptide assemblies that act as templates for nucleation. With PLys alone, the precipitates had calcite as the major component and vaterite as the minor component, where calcite and vaterite adopted dendritic and spherulitic morphologies, respectively. In the presence of PAsp alone and a mixture of PLys and PAsp, the precipitates comprised calcite spherulites only. The spherulites had a hierarchal structure with an inner part (the core) having a loose structure and an outer part composed of calcite crystals that grew radically from the core. The cores completely dissolved during crystallization, resulting in the formation of spherulites with a hollow structure. It was speculated that the core consists of polypeptide assemblies and poorly crystalline CaCO3. PAsp and PLys exerted synergistic effects on calcite spherulite formation, where the spherulite and hollow sizes could be controlled by changing the relative amount of PLys and PAsp.