Protein Secondary Structure in Spider Silk Nanofibrils

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