The molecular structure of spider dragline silk: Folding and orientation of the protein backbone

Abstract

The design principles of spider dragline silk, nature's highperformance fiber, are still largely unknown, in particular for the noncrystalline glycine-rich domains, which form the bulk of the material. Here we apply two-dimensional solid-state NMR to determine the distribution of the backbone torsion angles ( , ) as well as the orientation of the polypeptide backbone toward the fiber at both the glycine and alanine residues. Instead of an ''amorphous matrix,'' suggested earlier for the glycine-rich domains, these new data indicate that all domains in dragline silk have a preferred secondary structure and are strongly oriented, with the chains predominantly parallel to the fiber. As proposed previously, the alanine residues are predominantly found in a ␤ sheet conformation. The glycine residues are partly incorporated into the ␤ sheets and otherwise form helical structures with an approximate 3-fold symmetry.

Molecular and Nanostructural Mechanisms of Deformation, Strength and Toughness of Spider Silk Fibrils

“…4b,d]). The initial regime is characterized by a relatively high tangent modulus (1.07 GPa) owing to the homogeneous stretching of semiamorphous regions that are rich in hydrogen bonding in the form of 3 1 -helices and beta-turns [1,6,22].…”

“…1a) [6][7][8]. Thereby, the anti-parallel beta-sheet nanocrystals [9][10][11][12] play a key role in defining the mechanical properties as they provide stiff orderly cross-linking domains embedded in a semiamorphous matrix that consists of less orderly beta-structures, 3 1 helices and beta-turns [6,13,14].…”

A Materiomics Approach to Spider Silk: Protein Molecules to Webs

“…2c, d), showing that the computational predictions yield excellent results when compared with empirical analyses. 26,27 Further details can be found in the original paper. 20 Further analysis of the resulting structures showed that the poly-alanine region and adjacent poly-glycine-alanine segments formed a stiff orderly cross-linking Fig.…”

“…The resulting configurations were validated against experimental data of structure and composition. 26 The resultant structures then allow for mechanical characterization on multiple scales as described in the subsequent figures. (b) Detailed view of the hydrogen bonding structure within different domains is identified with the model.…”

“…In fact, this model first identified the secondary structure composition of the amorphous regions, showing a lack of alpha-helix conformations, but rather a disorderly mixture of structures resembling 3 1 helices and beta-turns, supporting a series of earlier experimental investigations. 26,[28][29][30] Silk's extraordinary properties on the macroscopic scale ultimately stem from the balance of strength and extensibility at the molecular scale. To assess the function of different protein domains, molecular-level deformation mechanisms of the protein composite were examined.…”