Conformation and Orientation of Proteins in Various Types of Silk Fibers Produced by Nephila clavipes Spiders

Silk fibers harvested from the web, cocoon, and prey wrapping of the spider Nephila clavipes have been studied by polarized Raman spectromicroscopy. The technique is efficient to differentiate the various types of silk by probing monofilaments produced by the major ampullate (MA), minor ampullate (MI), cylindriform, flagelliform, and aciniform glands. The spectra show that the MA, MI, and cylindriform silks belong to the same structural class and are composed of highly oriented β-sheets (35−37%) with other slightly oriented secondary structures. Spectral markers of particular motifs involved in the β-sheets have been identified. The flagelliform silk represents a second, very peculiar structural class. It displays a heterogeneous disordered conformation without any preferential orientation. Such characteristics certainly play a role in the large extensibility of this silk. The aciniform silk represents a third class of silk dominated by moderately oriented β-sheets (∼30%) and α-helices (∼24%). Such a structure seems important in explaining the high toughness of this silk.