Paper: Protein Secondary Structure in Spider Silk Nanofibrils

(a) False-colored scanning electron microscope (SEM) image of a strand of the ribbon silk of a Loxosceles (recluse) spider, which is entirely composed of nanofibrils. (b) A female Chilean recluse (Loxosceles laeta) spider. © Atomic force microscope (AFM) topography image of a Loxosceles silk ribbon. The white arrows indicate the “X” and “Z” directions in the plane of the ribbon, perpendicular and parallel to the ribbon axis, respectively. Inset: nanofibrils parallel to the ribbon axis Z are revealed at higher AFM magnifications. Color bars: 0–100 nm; inset: 0–12 nm. (d, e) Optical microscopy (OM) images of the FTIR (d) and Raman (e) samples. (e) Optical microscopy image of a location tested on the Raman sample.

Nanofibrils play a pivotal role in spider silk and are responsible for many of the impressive properties of this unique natural material. However, little is known about the internal structure of these protein fibrils. We carry out polarized Raman and polarized Fourier-transform infrared spectroscopies on native spider silk nanofibrils and determine the concentrations of six distinct protein secondary structures, including β-sheets, and two types of helical structures, for which we also determine orientation distributions. Our advancements in peak assignments are in full agreement with the published silk vibrational spectroscopy literature. We further corroborate our findings with X-ray diffraction and magic-angle spinning nuclear magnetic resonance experiments. Based on the latter and on polypeptide Raman spectra, we assess the role of key amino acids in different secondary structures. For the recluse spider we develop a highly detailed structural model, featuring seven levels of structural hierarchy. The approaches we develop are directly applicable to other proteinaceous materials.

Our paper has been published in the journal Nature Communications (2021 impact factor: 17.69).

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