Raman spectroscopic characterization of Bombyx mori silk fibroin:: Raman spectrum of Silk I

This study focuses on the conformational characterization of differently processed Bombyx mori silk fibroin samples by Raman spectroscopy. The Raman spectra of silk fibroin film and liquid silk are discussed in comparison with those of the crystalline fractions of Bombyx mori silk fibroin (Cp, chymotryptic precipitate) with Silk I (Silk I-Cp) and Silk II (Silk II-Cp) structures. The complete 1800-200 cm(-1) Raman spectrum of Silk I-Cp is reported for the first time. The amide I and amide III modes were found to be scarcely suitable for the spectroscopic characterization of silk fibroin in the Silk I form in the presence of a random coil conformation. Raman marker bands for the Silk I form were identified in other spectral ranges at about 1415, 950, 930, 865, 260 and 230 cm(-1). On the basis of the above findings, the comparison of the Raman spectra of film, liquid silk and Silk I-Cp in the range 1000-800 cm(-1) clearly indicates that in addition to random coil, both film and liquid silk contain local domains of Silk I structure; their amount is higher in liquid silk, as indicated by the relative intensity of the bands at about 950, 930 and 865 cm-l and by the I-1415/I-1455 intensity ratio. The assignments of the bands at about 1275 and 1107 cm(-1) are also discussed. These bands were previously assigned to the presence of alpha -helical conformation in Bombyx mori silk but, from the results reported, they should rather be attributed to the Silk I form. Copyright (C) 2001 John Wiley & Sons, Ltd.