Biomacromolecules within bivalve shells: Is chitin abundant?

Area of Science:

• Biomineralization
• Materials Science
• Nacre Science

Background:

• Bivalve shells are advanced inorganic-organic nanocomposites.
• Their superior material properties are attributed to the organic matrix, often presumed to be chitin and proteins.
• Understanding the organic phase is key to biomimetic design.

Purpose of the Study:

• To identify the specific biopolymers in the organic matrix of various bivalve shells.
• To challenge the prevalent assumption of abundant chitin in bivalve shell organic fractions.
• To elucidate the role of identified biomolecules in shell material properties.

Main Methods:

• Solid-state CP-MAS NMR, FTIR, and SEM analyses were performed.
• Organic extracts were obtained after decalcification of shell samples.
• Intact shells, organic extracts, and periostracum were studied across seven bivalve species.

Main Results:

• Chitin was not found in significant amounts in the organic fraction of any examined bivalve shells.
• Proteinaceous material, rich in Ala and Gly and organized as β-sheets, was identified in nacreous shell extracts.
• A conserved protein was found in the periostracum of nacreous shells, while homogeneous and crossed lamellar shells showed different or insufficient organic matter for characterization.

Conclusions:

• The organic matrix of bivalve shells primarily consists of proteins, not chitin.
• Proteinaceous matter, including silk-like β-sheets, contributes significantly to the enhanced mechanical properties of bivalve shells.
• This finding necessitates a revision of biominerization models and informs the design of novel bioinspired materials.