Raman Spectroscopic Investigation of the Denaturation Process of Silk Fibroin

Abstract

The mechanical denaturation process of silk fibroin is examined by Raman spectroscopy. The fresh silk fibroins from the middle gland of mature silkworms are drawn to various ratios on a tensile tester ( R = ldrawn/ linitial, where l is length) and their conformations are measured with Raman spectroscopy. Undrawn silk fibroin is mainly in the random coil structure with some α-helical conformation, the characteristic bands appearing at 1252 and 1660 (random coil) and at 942, 1106, and 1270 cm−1 (α-helix). When the samples are drawn up to R = 4 at an extension rate of 500 mm/min, two peaks at 1233 cm−1 (the amide III band) and 1085 cm−1 appear; it is shown that the β-sheet conformation is then formed. With an increase in drawing ratios, the intensities of these β-sheet bands increase and those of the random coil and α-helical bands decrease gradually. These changes indicate that, under the action of stress, the conformation of fibroin is altered from random coil and α-helix to β-sheet structures. This result is quite similar to the results achieved by the spinning of the silkworm. The effect of the water content in liquid silk on this conformational transition process is revealed and discussed.

Structure of silk by raman spectromicroscopy: From the spinning glands to the fibers

“…Still, the first Raman spectrum of silk thread was published as early as 1985 for the degummed cocoon silk fiber of B. mori. 13 The position of amide and other skeletal bands have clearly shown the predominance of b-sheets in agreement with results obtained previously from other techniques. The first polarized Raman spectra of a single silk fiber were obtained in 1994 for B. mori and Nephila clavipes (N. clavipes) silk using a conventional Raman spectrometer.…”

“…13,62 Experiments made in vitro (in solution) can provide several relevant FIGURE 6 Polarized spectra of the flagelliform silk fiber of N. clavipes harvested from the spider web and washed with water. The spectra presented are the average of 12 experimental spectra.…”

“…13,62 The manual drawing of the B. mori dope solution induced the appearance of bands at 1676 (amide I) and 1233 (amide III) cm 21 attributed to the formation of b-sheets. A more recent study in which regenerated fibers have been produced with a hypodermic syringe has later shown that, beside drawing, shearing was also important for the development of birefringence and for the formation of b-sheets.…”

Vibrational 13C‐cross‐polarization/magic angle spinning NMR spectroscopic and thermal characterization of poly(alanine‐glycine) as model for silk I Bombyx mori fibroin

“…Magoshi et al 37 and Zheng et al 38 attributed this band to the presence of the ␣-helix conformation in silk fibroin, in striking contrast with the results reported by Saito et al 24 and Asakura et al, 25 who excluded the possibility that silk fibroin chains in the solid state may assume this conformation on the basis of X-ray and 13 C-CP/MAS NMR results. More recently, we interpreted the 1276 cm Ϫ1 band as an amide III component of the silk I form of silk fibroin.…”

Nanomechanics of single silkworm and spider fibres: a Raman and micro‐mechanical in situ study of the conformation change with stress

“…31 Vibrational signatures of˛-helix and of the alternativeˇ-sheet conformations have been proposed in the literature. Monti et al 31 along with Magoshi et al 41 and Zheng et al 42 claim that the 1275 and 1107 cm 1 bands are characteristic of Silk I and that 1230 cm 1 is characteristic of theˇ-sheet form. The same conclusion has been made by Preghenella et al 43,44 The assignment of the 1085 cm 1 peak remains disputed.…”