Structure of Silk using Solid-State NMR

Abstract

Silks are natural protein fibers produced by insects and spiders. The mechanical properties of silks span a wide range, and include combinations, for example, strength and extensibility, that are difficult to achieve in synthetic polymers. A variety of solid-state NMR techniques have played a crucial role in the understanding of the structure and microscopic basis of the properties of silks. Isotropic chemical shifts measured with CP/MAS reflect the secondary structure of the protein backbones. Two-dimensional correlation spectroscopy experiments have allowed more direct measurement of backbone conformations, and measurements of anisotropic interactions such as chemical shift anisotropies, and dipolar and quadrupolar couplings have allowed measurement of molecular orientation in oriented fiber samples. The silks that have been most thoroughly studied with NMR, domesticated silkworm (Bombyx mori) cocoon silk and spider dragline silks, are semicrystalline fibers containing oriented β-sheet crystallites. The noncrystalline regions in these silks are much less well ordered, but in at least some cases do appear to have preferred structures. To date, only a handful of different silks from the enormous variety of silk-producing organisms in nature have been studied with NMR in any detail. Keywords: fibers; biopolymers; protein secondary structure; CSA; DECODER; DOQSY; spin diffusion; beta-sheet; REDOR; DARR