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Silkworms of Bombyx mori in their fifth instar were fed mulberry leaves coated with 35 S cysteine and the fibroin extracted from their glands. The 35 S fibroin was oxidized and a sample, analysed, when the 35 S was found exclusively in 35 S cysteic acid. The oxidized 35 S fibroin yielded a soluble fraction on digestion with trypsin: that which remained insoluble was then treated with chymotrypsin which solubilized part of it. Peptides containing 35 S cysteic acid were separated from these two soluble fractions by a combination of the techniques of resin chromatography, Sephadex chromatography, and paper electrophoresis. The sequences of four peptides were examined and were shown to form part of a ‘parent sequence’, Gly‐Ala‐Gly‐Ala‐Gly‐Cya‐Asx‐Ser‐Ala‐Val‐Cya(Pro, Leu), which accounted for approximately three‐quarters of the cysteic acid in silk fibroin. It is suggested that the fibroin molecule is a single chain of molecular weight 103,000 with an intrachain disulphide bond between the two half‐cystine residues indicated in the above sequence.
Robson et al. (Tue,) studied this question.