Organized Arrangement of Calcium Carbonate Crystals, Directed by a Rationally Designed Protein

The integration of a "brick and mortar" structure and superior fracture toughness of nacre has attracted intensive attention recently. Elucidating the interactions between biomacromolecules and inorganic phases is beneficial to understand the structure-forming process of nacre. Herein, a recombinant protein, ChiSifiCa, mimicking features of the organic matrix in nacre is rationally designed. This protein contains three functional domains, chitin binding (Chi), silk fibroin (Sifi), and calcium ion binding (Ca). The domain order in ChiSifiCa is in the same manner as the distribution of organic matrix in nacre. When ChiSifiCa binds to the surface of chitin, it provides a confined microenvironment facilitating CaCO3 mineralization. The formation of spherical vaterite minerals with a hollow structure was observed under the function of ChiSifiCa. These minerals are assembled by well aligned nanoplatelets and nanoparticles, distributed in the outer and inner region of the sphere, respectively. The nanoparticles were oriented radially from the center to the edge. These nanoplatelets with stacked multilayers display almost the same crystallographic orientation. During the biomineralization process, there is change of secondary structure of ChiSifiCa from random coil to alpha-helix.