Unexpected β-sheets and molecular orientation in flagelliform spider silk as revealed by Raman spectromicroscopy

Abstract

Structural data relative to spider silk fibers such as the flagelliform (Flag) silk are fundamental to understand the origin of their outstanding mechanical properties. However, due to its small diameter and limited availability, experimental data relative to the structure of the Flag silk are almost nonexistent. Raman spectromicroscopy is one of the rare techniques that can provide structural information about this type of silk. We have thus used this technique to characterize Flag silk fibers spun by three orb-weaving spiders in their native state. The polarized spectra reveal that Flag fibers spun by Araneus diadematus and Argiope aurantia spiders unexpectedly contain β-sheets and exhibit a certain level of molecular orientation. In contrast, the Flag silk from Nephila clavipes possesses very few β-sheets and a low molecular orientation, the proteins being essentially disordered. These observations account for the higher strength and lower extensibility of Araneus diadematus and Argiope aurantia Flag fibers compared with Nephila clavipes. The analysis of the primary structure strongly suggests that the “spacer” motifs of the Flag silk are more prominent and have a higher β-sheet propensity for spider species that spin fibers containing β-sheets. These results nicely support the Raman data and provide the first hypothesis regarding the role played by these spacers in the structure of Flag silk. The structural data obtained provide a molecular basis for the tensile properties of these fibers showing that overall, considering the different types of spider silk, the β-sheet appears to be a universal structural element used by nature to provide strength to fibers that exhibit dissimilar tensile properties.