Role of Ovalbumin in the Stabilization of Metastable Vaterite in Calcium Carbonate Biomineralization

The role of proteins in biomineralization has been examined in this work by studying the effect of ovalbumin on the stabilization of metastable CaCO3 phases. In the absence of ovalbumin, the mixing of Na2CO3 with CaCl2 in an aqueous solution led to the formation of metastable phases that swiftly transformed into stable calcite crystals within 4 h under the experimental conditions. However, ovalbumin was found to favor the formation and stabilization of spherical vaterites, and the effect was concentration dependent. In the presence of 2 g/L ovalbumin, for example, vaterite microspheres with diameters ranging from 0.9 to 3.0 mu m, composed of much smaller nanosized particles, were produced and stabilized even after 24 h following the initial mixing. In additon, the influence of ovalbumin on the CaCO3 mineralization process from the very beginning was carefully examined. Both amorphous calcium carbonate (ACC) and vaterite were favored with ovalbumin present, but the ACC phase formed predominantly at the initial stage of mixing followed by the vaterite formation. Vaterite could then be embedded further in the mineralization process and become stabilized many hours afterward. The stabilizing effect of ovalbumin could arise from the strong binding between carboxylate groups of ovalbumin and the calcium ions on the CaCO3 surface, preventing the metastable CaCO3 from transformation via dissolution-recrystallization processes. The strong ovalbumin adsorption on vaterite microspheres was revealed from transmission electron microscopy imaging and thermogravimetric analysis, thereby providing useful evidence to support the proposed stabilizing mechanism.