Nucleation and Growth of CaCO3 Mediated by the Egg-White Protein Ovalbumin : A Time-Resolved in situ Study Using Small-Angle Neutron Scattering

Mineralization of calcium carbonate in aqueous solutions starting from its initiation was studied by time-resolved small-angle neutron scattering (SANS). SANS revealed that homogeneous crystallization of CaCO3 involves an initial formation of thin plate-shaped nuclei which subsequently reassemble to 3-dimensional particles, first of fractal and finally of compact structure. The presence of the egg-white protein ovalbumin leads to a different progression of mineralization through several stages; the first step represents amorphous CaCO3, whereas the other phases are crystalline. The formation and dissolution of the amorphous phase is accompanied by Ca2+-mediated unfolding and cross-linking of about 50 protein monomers showing the characteristic scattering of linear chains with a large statistical segment length. The protein complexes act as nucleation centers for the amorphous phase because of their enrichment by Ca2+ ions. SANS revealed the sequential formation of CaCO3 starting from the amorphous phase a...