MOLECULAR CONFOMATIONS OF SILK FIBROIN IN SILK GLAND

Molecular conformation of the coagulated fibroin in silk gland was studied by means of X-ray diffractometry and infrared spectrometry. It was ascertained by the infrared dichroism the coagulated silk fibroin in posterior gland has the parallel β form. However most of the fibroin in posterior gland appears in random coil. The α-β transition occurs at the anterior part of posterior division. Neither α-helical structure nor cross-β- structure was seen in the fibroin molecules of this middle division. The structure of α-form of silk fibroin is similar to polyglycine II with its hydrogen bonds projected out perpendicularly to the polypeptide chain. From these structures, it may be expected that α--β transformation in silk fibroin molecules occurs easily without serious rearrangement of the hydrogen bond. The X-ray powder diagrams of silk fibroin in anterior division indicate β-form structure. It may be considered that the α-form structure of silk fibroin molecules are broken to form partly β-form while passing through the anterior silkgland.