Silk with a difference
IN THESE days of new materials and corn - posites, it is interesting to note that sci- entists are increasingly turning towards nature in order to understand the origin of mechanical properties of material~. Spider dragline silk is one fibre whose unique properties have, 10l1g beep coveted by material scientists.
A combination of elasticity and ten- sile strength gives this silk the highest energy among natural or synthetic fibres- about 100,000 joules/kg com- pared to 80,000 joules/kg for rubber . These properties are essential for the material to support a falling spider, The molecular origins of such won- derful properties have been a matter of debate. It has been known for some time that silk is a semi-crystalline polymer, though the amount or orientation of its conipol1ents has not been very clear. The primary constituents are the two amino acids, alanine and glycine, which together make up about 67 per cent of the structure. The remaining is made up of other amino acids. Alanine is organised into short polyalanine sequences.
Recently, Lynn Jelinski and hisco- workers at the Center of Advanced Technology in Biotechnology, Cornell University, us, have r:eported a detailed analysis of the molecular orientation of the silk. Using a solution of 10 percent perdeutromethyl L-alanine (replacing ordinary hydrogen with heavy hydro- gen, 01 deuterium) which was fe4 to adult spiders, the researchers were able to get,silk fibres with about four per cent deuteration level in the residues. This enabled them to obtain high qual- ity deuterium nuclear magnetic reso- nance spectra, which offered insights into the structure of 'the material.
The researchers have found that the crystalline fraction of the silk consists of two types ofalanine-rich regions. One of them is highly oriented, while the other is less densely packed and poorly oriented. They have proposed a new model for 1he structure of the molecules of the material, as well as their arrange- ment in fibres.
Material scientists have a lot to learn f:i:omnitureyet. With Jelinski's work, a major insight has been provided into the working of nature's high perfor- man,ce fibre: A deeper understanding of how,nature and evolution has given us such rugged structures, could, in fact, revolutionise the whole field of synthetic materials.