30—the Chemistry of Silk Fibroin I. The Action of Some Proteolytic Enzymes on Fibroin in Aqueous Solution, and Its Bearing on the Molecular Structure of Silk

The precipitate formed from an aqueous solution of fibroin by the action of pancreatic enzymes has been analysed, and shown to consist almost entirely of the three simplest amino acid residues, of glycine (G), alanine (A), and serine (S). The corresponding liquor contains all the amino acids of silk that have bulky side-chains. The insoluble fraction has a composition 30 G. 20 A. 10 S., and an experimentally-determined molecular weight of about 4,000. It has a high degree of crystalline order, as shown by X-ray analysis. The action of chymotrypsin, and of trypsin, on an aqueous solution of fibroin has also been investigated and the precipitates formed have been examined. The results are discussed in relation to Bergmann's rules for the specificity of proteolytic enzymes, and in relation to the molecular architecture of the fibroin molecule.