RAMAN-SPECTROSCOPIC STUDIES OF SILK FIBROIN FROM BOMBYX-MORI

This study was focused on the conformational characterization of Bomby x mori silk fibroin in film, fiber and powder form by means of Fourier transform Raman spectroscopy. Native and regenerated silk fibroin fil ms prepared by casting dilute silk fibroin solutions (<1%, w/v) displa y characteristic conformationally sensitive bands at 1660 cm(-1) (amid e I), in the range 1276-1244 cm(-1) (a complex amide III region with m ultiple detectable maxima) and at 1107 and 938 cm(-1). This spectral p attern can be related to a prevalently random coil conformation, with traces of alpha-helix. Liquid silk, prepared by casting the silk gland content (fibroin concentration 20-25%, w/v), shows almost the same wa venumbers in the amide I and III ranges, while differences appear belo w 1000 cm(-1), where three bands at 952, 930 and 867 cm(-1) increase i n intensity. The spectral differences between films and liquid silk ar e discussed with a view to identifying possible markers for silk I str ucture, a crystalline modification of silk fibroin. The treatment of b oth native and regenerated films with 50% (v/v) methanol solution indu ces the conformational transition to a beta-sheet structure, as demons trated by the shift of amide I to 1665 cm(-1) and the appearance of ne w maxima at 1262 and 1236 cm(-1) (amide III) and at 1084 cm(-1). When liquid silk is cast at above 50 degrees C, the prevailing conformation taken by silk fibroin is beta-sheet, whatever the rate of drying. By comparing the Raman spectra of silk fibroin fiber and powder, both hav ing a beta-sheet structure, a difference in the tyrosine doublet bands and in the amide I band can be observed. The value of the I-853/I-830 (R-tyr) intensity ratio increases in the powder while amide I shifts to lower wavenumbers, suggesting that the hydrogen bonds involving the tyrosil residues are weaker in the powder than in the fiber. (C) 1998 John Wiley & Sons, Ltd.