Raman spectroscopic studies of silk fibroin fromBombyx mori

This study was focused on the conformational characterization of Bombyx mori silk fibroin in film, fiber and powder form by means of Fourier transform Raman spectroscopy. Native and regenerated silk fibroin films prepared by casting dilute silk fibroin solutions (<1%, w/v) display characteristic conformationally sensitive bands at 1660 cm-1 (amide I), in the range 1276–1244 cm-1 (a complex amide III region with multiple detectable maxima) and at 1107 and 938 cm-1. This spectral pattern can be related to a prevalently random coil conformation, with traces of α-helix. Liquid silk, prepared by casting the silk gland content (fibroin concentration 20–25%, w/v), shows almost the same wavenumbers in the amide I and III ranges, while differences appear below 1000 cm-1, where three bands at 952, 930 and 867 cm-1 increase in intensity. The spectral differences between films and liquid silk are discussed with a view to identifying possible markers for silk I structure, a crystalline modification of silk fibroin. The treatment of both native and regenerated films with 50% (v/v) methanol solution induces the conformational transition to a β-sheet structure, as demonstrated by the shift of amide I to 1665 cm-1 and the appearance of new maxima at 1262 and 1236 cm-1 (amide III) and at 1084 cm-1. When liquid silk is cast at above 50°C, the prevailing conformation taken by silk fibroin is β-sheet, whatever the rate of drying. By comparing the Raman spectra of silk fibroin fiber and powder, both having a β-sheet structure, a difference in the tyrosine doublet bands and in the amide I band can be observed. The value of the I853/I830 (Rtyr) 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. © 1998 John Wiley & Sons, Ltd.