Identification of mineral modulation sequences within the nacre-associated oyster shell protein, n16

Nature's use of proteins to direct and control the synthesis of inorganic solids represents an important paradigm for bioinspired materials synthesis. However, the mechanism by which polypeptides direct inorganic synthesis, particularly with regard to selective formation of crystals polymorphs, remains unknown. An important step in understanding polypeptide-directed inorganic synthesis is the identification of sequence regions in biomineralization proteins that can affect crystal growth. In this report, we identify that the 30 AA N- and C-terminal sequence regions (n16-N and n16-C, respectively) of the oyster shell aragonite-associated protein, n16, exhibit control over the morphology of calcium carbonate crystals grown in geologic calcite overgrowth assays and polyimide (Kevlar)-based assays. Here, we find that calcium carbonate crystals, which grow in the presence of model peptides representing the n16-N and n16-C sequences, adopt dendritic or circular overgrowth in geological calcite overgrowth assays and "staircase" structures in Kevlar-based assays, as compared to negative controls and to parallel assays conducted in the presence of AP24-1, the 30 AA N-terminal sequence region of the nacre-associated protein, AP24, which interrupts step edge growth. Synchrotron X-ray diffraction studies reveal that the crystals grown in the presence of n16-N are calcite. Circular dichroism spectrometry studies of n16-N and n16-C model peptides reveal qualitatively similar solution state conformations that consist of either random coil in equilibrium with other secondary structures (e.g., β-strand, turn, loop, polyproline type II) or, at higher concentrations, a β-strand secondary structure in equilibrium with random coil. We conclude that the N- and C-terminal sequence regions of the nacre-associated n16 protein most likely play a role in n16-mediated effects on calcium carbonate crystal growth in the nacre layer.