This study focuses on the conformational characterization of differently processedBombyx 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 −1 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 a-helical conformation in Bombyx mori silk but, from the results reported, they should rather be attributed to the Silk I form.

This study was focused on the conformational characterization of Bombyx mori silk Ðbroin in Ðlm, Ðber and powder form by means of Fourier transform Raman spectroscopy. Native and regenerated silk Ðbroin Ðlms prepared by casting dilute silk Ðbroin solutions (AE1% , 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 a-helix. Liquid silk, prepared by casting the silk gland content (Ðbroin concentration 20-25% , w/v), shows almost the same wavenumbers in the amide I and III ranges, while di †erences appear below 1000 cm-1, where three bands at 952, 930 and 867 cm-1 increase in intensity. The spectral di †erences between Ðlms and liquid silk are discussed with a view to identifying possible markers for silk I structure, a crystalline modiÐ-cation of silk Ðbroin. The treatment of both native and regenerated Ðlms with 50% (v/v) methanol solution induces the conformational transition to a b-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 ÄC, the prevailing conformation taken by silk Ðbroin is b-sheet, whatever the rate of drying. By comparing the Raman spectra of silk Ðbroin Ðber and powder, both having a b-sheet structure, a di †erence in the tyrosine doublet bands and in the amide I band can be observed. The value of the intensity ratio increases in I 853 /I 830 (R tyr ) 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 Ðber.

The structural organization of Bombyx mori silk fibroin was investigated by infrared (IR) spectroscopy. To this aim, (AG)15 and other model peptides of varying chain length, containing tyrosine (Y), valine (V), and serine (S) in the basic (AG)n sequence were synthesized by the solid phase method and their spectroscopic properties were determined. Both the position and the relative content of Y, V, and S residues in the (AG)n model system appeared critical in determining the preferred conformation, i.e., silk I, silk II, and unordered structures. Curve fitting analysis in the amide I range showed that the model peptides with prevailing silk II structure displayed different beta-sheet content, which was dependent on the degree of interruption of the (AG)n sequence. In this regard, the bands at about 1000 and 980 cm(-1), specifically assigned to the AG sequence of the B. mori silk fibroin chain, were identified as marker of the degree of interruption of the (AG)n sequence.A stable silk I structure was observed only when the Y residue was located near the chain terminus, while a silk I --> silk II conformational transition occurred when it was positioned in the central region of the peptide. Analysis of the second-derivative spectra in the amide I range allowed us to identify a band at 1639 cm(-1) (4 --> 1 hydrogen-bonded type II beta-turns), which is characteristic of the silk I conformation.

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Structural changes of tussah (Antheraea pernyi) silk fibroin films treated with different water‐methanol solutions at 20°C were studied as a function of methanol concentration and immersion time. X‐ray diffraction measurements showed that the α‐helix structure, typical of untreated tussah films, did not change for short immersion times (2 min), regardless of methanol concentration. However, crystallization to β‐sheet structure was observed following immersion of tussah films for 30 min in methanol solutions ranging from 20 to 60% (v/v). IR spectra of tussah films untreated and methanol treated for 2 min exhibited strong absorption bands at 1265, 892, and 622 cm−1, typical of the α‐helix conformation. The intensity of the bands assigned to the β‐sheet conformation (1245, 965, and 698 cm−1) increased for the sample treated with 40% methanol for 30 min. Raman spectra of tussah films with α‐helix molecular conformation exhibited strong bands at 1657 (amide I), 1263 (amide III), 1106, 908, 530, and 376 cm−1. Following α → β conformational transition, amide I and III bands shifted to 1668, and to 1241, 1230 cm−1, respectively. The band at 1106 cm−1 disappeared and new bands appeared at 1095 and 1073 cm−1, whereas the intensity of the bands at 530 and 376 cm−1 decreased significantly. ©1995 John Wiley & Sons, Inc.