Local structure in spider dragline silk investigated by two-dimensional spin-diffusion nuclear magnetic resonance

The local structure of dragline silk from the spider Nephila madagascariensis is investigated by solid-state nuclear magnetic resonance. Two-dimensional (2D) spin-diffusion experiments show that the alanine-rich domains of the protein form beta-sheet structures in agreement with one-dimensional NMR results from a different species of the genus Nephila (Simons, A.; Ray, E.; Jelinski, L. W. Macromolecules 1994, 27, 5235) but at variance with diffraction results. The microstructure of the glycine-rich domains is found to be ordered. The simplest model that explains the experimental findings is a 3(1)-helical structure. Random coils, planar beta-sheets, and alpha-helical conformations are not found in significant amounts in the glycine-rich domains. This observation may help to explain the extraordinary mechanical properties of this silk, because 3(1)-helices can form interhelix hydrogen bonds.