Structural changes induced in tussah silk (Antheraea pernyi) fibroin films by immersion in methanol

The crystalline transition induced by immersion in a methanol/water mixture of tussah silk fibroin (from Antheraea pernyi) film obtained by casting from a 1% solution was studied by x-ray diffraction, differential scanning calorimetry, and infrared spectroscopy. The molecular conformation of the fibroin, consisting mostly of the α-helix and random-coil forms, was transformed into a random-coil and β-rich conformation containing only a small amount of α-helix after immersion for no more than 5 min. The intersheet packing of the β-crystal of the original tussah silk fibroin film was imperfect in the early stage of immersion. However, crystallization proceeded further when the immersion time exceeded 10 min. As a result the sheets in the β-form crystal became closely packed because of the decrease in the content of the random-coil form. The exothermic peak, which appeared at 226°C in the original fibroin and was attributed to the random-coil → β-structure transition, disappeared completely after immersion for 5 min.