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An X-ray investigation of a precipitate obtained by the action of pancreatic enzymes on an aqueous solution of silk fibroin suggests that the precipitate has essentially the same crystal structure as the silk fibre. Since the precipitate consists almost entirely of glycine, alanine and serine residues, it is concluded that only one crystalline phase, composed of these three aminoacids, is present in silk fibroin.The precipitate, which is more highly crystalline than silk fibroin, has a density of 1.40 g.cm. -3. This value is taken as an approximation to the density of the crystalline regions of the silk fibre.From photographs obtained when silk fibres are oscillated in the X-ray beam, it is concluded that the fibroin structure possesses a twofold screw axis.Consideration of all the available evidence suggests an orthorhombic unit cell and the space group P22121 for silk fibroin. A detailed structure is proposed, and it is shown that intensities of X-ray reflexions calculated from it are in reasonable agreement with those observed.

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synopsisA study of the swelling of Acala 4-42 cotton in aqueous alkalis and acids has been carried out. The degree of swelling of the cotton in the reagents has been indicated by measurement of the variations in width of the cotton at its widest point. The nature of the swelling process whether interfibrillar or intrafibrillar has been deduced from microscopical and x-ray evidence. It has been shown that a similarity of shape of the swelling curves, i.e., an equal degree of swelling in the same or other reagents, does not imply the same mechanism of swelling. The results are more readily explained in terms of the crystalline-fibril theory than by the fringe-micellar theory, especially when high degrees of swelling have been achieved.