Observations of 3 nm Silk Nanofibrils Exfoliated from Natural Silkworm Silk Fibers

Silk fibers are one of the most attractive natural materials that exhibit enchanting luster and superior mechanical properties. Exploring the unique hierarchical architecture structures of natural silk fibers is the basis to understanding these unique properties. Here, we report observations of 3.1 +/- 0.8 nm silk nanofibrils and 3.7 +/- 0.9 A silk molecule chains exfoliated from natural silkworm silk fibers. Interestingly, the individual nanofibrils and protein chains show periodic diameters fluctuating along their axes. We further find that the thicker regions are relatively softer and the thinner regions are stiffer, and these can be assigned to alternatively distributed alpha-helix and beta-sheet domains, respectively. Based on these observations, we proposed a refined structure model of natural silk, which ranges from the molecular level to the fiber scale. These findings provide new opportunities to understand and exploit the unique structure-property relationships found in natural silk fibers, including inspiring the design of new artificial materials.