Biomacromolecules within mollusk shells: gateway to biomimetic complex superstructures
Department of Earth and Planetary Sciences, Macquarie University, NSW 2109 Australia.
Organisms precipitate a large variety of biominerals which are biogenic hierarchical nanocomposite materials consisting of inorganic-organic hybrid components, each with its structural motifs, inorganic crystal formation - shape, and micro- or macroscopic properties. Shells of mollusk, for instance, are formed with different ratios of inorganic-organic materials and different structural motifs resulting in a large variety of calcareous biocomposites with material properties outperforming those of their synthetic counterparts. Amongst all different shell microstructures, nacre is the most studied to date, but comparable knowledge is lacking for non-nacre shell structures such as homogeneous and crossed-lamellar structures. While the organic matrix occluded in shells is a minor component, ca 5 wt% of the total, significant fractions of the shell macromolecules have been presumed to be chitin and/or proteins. Identifying the biopolymer phase is, therefore, a crucial step in improving our understanding of design principles relevant to biominerals. We have used different bioanalytical and biochemical techniques including solid state-NMR, FTIR, Raman spectroscopy, scanning electron microscopy, ACQUITY ultraperformance liquid chromatography and SDS-electrophoresis to explore the composition of the organic part of mollusk shells and examine its interface with the inorganic part. The biopolymer content consists primarily of proteinaceous matter with structural motifs as silk-like β-sheets in nacre and collagenous gel-like matrix in non-nacre shells. Though other motifs persist in non-nacre, the composition in nacre is strikingly similar to the constituents of spider dragline silk but does not conform to the prevalent model that the biopolymer component in shells consists of well-arranged polysaccharide-chitin1. 1 Y. Levi-Kalisman, G. Falini, L. Addadi, S. Weiner, J. Struct. Biol., 2001, 135, 8-17.