Matrix interactions in biomineralization: Aragonite nucleation by an intrinsically disordered nacre polypeptide, n16N, associated with a β-chitin substrate

Previous literature by Falini et al. suggests that the cooperation between β-chitin, proteins, and a silk fibroin-like hydrogel determines polymorph selectivity within the nacreous layer of mollusk shells (favoring aragonite over calcite formation). Here we present an in vitro assay in which we combine functionalized organic surfaces with soluble peptides to probe the role of surface-peptide interactions in calcium carbonate polymorph selectivity. Specifically, we combined nl6N (a 30 amino acid peptide from the Japanese pearl oyster Pinctada fucata) and its sequence variants, n16Ns (randomly scrambled) and n16NN (global Asp → Asn, Glu → Gln substitution), with different forms of chitin (α and β). We found that the combination of n 16N adsorbed onto β-chitin leads to the formation of aragonite in vitro as well as demonstrated chitin binding ability. Negative controls, including sequence modified versions of nl6N (n16Ns and n16NN), exhibit variation inβ-chitin binding and the ability to nucleate aragonite. The peptide + α-chitin combination exhibits very little chitin binding and nucleates exclusively calcite with minor morphological effects. The n16N and n16Ns peptides used in this study are considered intrinsically disordered and have previously been shown to interact with calcium carbonate. We propose that the intrinsically disordered structure of n16N also allows the peptide to interact with the substrate creating a new organic matrix interface. The cooperation between the peptide and substrate may explain the polymorph specificity among these samples.