Solid-state NMR structure determination of whole anchoring threads from the blue mussel Mytilus edulis

The molecular structure of the blue mussel Mytilus edulis whole anchoring threads was studied by two-dimensional (13)C solid-state NMR on fully labeled fibers. This unique material proves to be well ordered at a molecular level despite its heterogeneous composition as evidenced by the narrow measured linewidths below 1.5 ppm. The spectra are dominated by residues in collagen environments, as determined from chemical shift analysis, and a complete two-dimensional assignment (including minor amino acids) was possible. The best agreement between predicted and experimental backbone chemical shifts was obtained for collagen helices with torsion angles (-75°, +150°). The abundant glycine and alanine residues can be resolved in up to five different structural environments. Alanine peaks could be assigned to collagen triple-helices, β-sheets (parallel and antiparallel), β-turns, and unordered structures. The use of ATR-FTIR microscopy confirmed the presence of these structural environments and enabled their location in the core of the thread (collagen helices and antiparallel β-sheets) or its cuticle (unordered structures). The approach should enable characterization at the molecular level of a wide range of byssus macroscopic properties.