The role of the air-liquid interface in protein-mediated biomineralization of calcium carbonate

Crystals formed by natural biomineralization processes often display exquisite morphologies not normally seen in synthetic laboratory preparations. This may be attributable to the influence of biomolecules that act to modify or direct crystal growth and assembly. Many attempts have been made to mimic these processes in synthetic systems to render novel crystal shapes and to form otherwise unstable crystal polymorphs. To determine protein influenced crystallization mechanisms, polymers, synthetic peptides, and polypeptides are often used. An established biomineralization model is the use of carbon dioxide gas diffusion into a calcium ion solution since it yields a slow precipitation and so lends itself to observation. However, reported observations are often assumed to be purely solution-phase phenomena,(1)despite it being well-known that macromolecules adopt particular conformations at the air-liquid interface.(2)In this study, we demonstrate that novel nanostructured cones of calcium carbonate (calcite) formed in the presence of osteopontin are created at the air-liquid interface. On one hand, this suggests that caution should be exercised in interpreting data from this clearly nonbiomimetic model. On the other, we demonstrate that we could physically manipulate air-liquid interface crystallization to align anisotropic nanostructures using 1D microscale guidance.