Supercontraction and Backbone Dynamics in Spider Silk: 13C and 2H NMR Studies

Abstract

The high-performance mechanical properties of certain spider silks can be radically altered by the addition of water. For example, unconstrained silk fibers from the major ampullate gland of the golden orb-weaving spider, Nephila clavipes, contract to about half of their original length when immersed in water. In this paper we use solid-state 13C and 2H NMR to study N. clavipes silk fibers, so as to address the molecular origins of supercontraction in the wet silk. Using 13C NMR, we study backbone dynamics and demonstrate that, when in contact with water, a substantial fraction of the glycine, glutamine, tyrosine, serine, and leucine residues in the protein backbone show dramatic increases in the rate of large-amplitude reorientation. 2H NMR of silk samples that incorporate leucine deuterated at one terminal methyl group provides a probe for dynamics at specific side chains along the fiber. Only a subset of these leucine residues is strongly affected by water. We suggest that the highly conserved YGGLGS(N)QGAGR blocks found in the silk protein play a major role in the supercontraction process. Amino acid sequences are proposed to produce artificial spider silk with similar mechanical properties, but without the undesired phenomenon of supercontraction. A possible use of the “supercontracting sequence” is also suggested.