Spider silk has a unique combination of mechanical properties that make it one of the toughest materials known. Now US chemists have used solid-state NMR spectroscopy to probe the structural features responsible for the silk's strength in an approach that could be applied to a range of biopolymers.

Spider dragline silk is made from two proteins. Its strength is thought to come from sections of protein in a -sheet conformation, while the elasticity comes from helical regions. Knowing the relative proportions of these conformations, and how this correlates to the amino acid sequence, is important for understanding the silk's mechanical properties and for developing synthetic silk.

Normally, spider silk produces broad unresolved NMR signals. When the silk is treated with water, the fibres swell in diameter and shorten by approximately 50 per cent. Gregory Holland and Jeffery Yarger at Arizona State University in Tempe, and their colleagues were studying this supercontraction when they noticed that water affected the NMR response of the helical regions.

Holland explains the process: 'Water penetrates the helical domains and causes them to become mobile, resulting in a sharpening of their NMR resonances and an increase in resolution. In contrast, water does not penetrate the -sheet domains so they remain rigid on NMR timescales and the resonances resemble those of dry silk.'

The team used an incredible natural abundance double quantum transfer experiment (INADEQUATE) NMR method to analyse spider silk. By exploiting the difference between the NMR spectra of wet and dry silk they could differentiate the signals from the helical and sheet structures and quantify the amino acid levels in each. They concentrated on the amino acids glycine and alanine, which make up 60-70 per cent of spider silk, and found that 28 per cent of the silk's glycines and 82 per cent of the alanines are in the -sheet conformation.

Holland and Yarger say that fully understanding the properties of spider dragline silk requires characterising the rest of its structure and the interactions between the two proteins. They are continuing to investigate these areas.

Michael Townsend