Matrix - Mineral Interfaces In Biomineralization: Designing An In Vitro Assay For Nacre Formation

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The formation of biogenic crystals in vivo (biomineralization) is controlled in part by the interface between organic and inorganic components. These interfaces can control the nucleation and growth of crystals, leading to the desired crystal polymorph, morphology, size, and orientation. By studying these interfaces, and developing increasingly complex synthetic systems, scientists can better understand biological systems and develop new materials with novel materials properties. Mollusk nacre (mother-of-pearl) is a composite material composed of aragonite (CaCO3) and various biopolymers. Synthetically, aragonite is difficult to nucleate. Mollusks utilize an organic matrix ([beta]-chitin, silk fibroin-like hydrogel, and proteins) to control shell formation. Here, I present an in vitro assay for calcium carbonate mineralization where the assay complexity is systematically increased to understand the role of each matrix component in controlling crystallization. First, functionalized organic surfaces with soluble peptides were combined to probe the role of surface-peptide interactions in polymorph selectivity. Specifically, n16N (a 30 amino acid peptide from the Japanese pearl oyster Pinctada fucata) and its sequence variants, n16Ns (randomly scrambled) and n16NN (global Asp [RIGHTWARDS ARROW] Asn, Glu [RIGHTWARDS ARROW] Gln substitution), were combined with different forms of chitin ([alpha] and [beta]) as well as synthetic Self-Assembled Monolayers (SAMs). Only the combination of n16N adsorbed onto [beta]-chitin leads to the formation of aragonite in vitro. The n16N peptide and its variants have different binding affinities for [beta]-chitin which correlate to their ability to nucleate aragonite. The complexity of the original in vitro assay was further increased to probe the role of another matrix component: silk fibroin hydrogels. With the addition of silk to synthetic, functionalized surfaces (SAMs), the ability of the SAM to affect crystal orientation and nucleation (as compared to controls) changed due to protein adsorption and denaturation on the functionalized surfaces. With the addition of silk fibroin to the chitin-n16N system, orientation and morphological control is gained (regardless of the n16N sequence). Flat vaterite crystals and amorphous calcium carbonate deposits are oriented with the [beta]-chitin fibers. The work in this thesis provides evidence for the possible roles of the chitin-protein interface on mineralization in nacre.

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