Analytical Sciences

Structural Transition of Bombyx mori Liquid Silk Studied with Vibrational Circular Dichroism Spectroscopy

Toshinori MORISAKU,* Sho ARAI,* Kohzo KONNO,* Yu SUZUKI,** Tetsuo ASAKURA,*** and Hiroharu YUI*

*Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
**Tenure-Track Program for Innovative Research, University of Fukui, 3-9-1 Bunkyo, Fukui, Fukui 910-8507, Japan
***Department of Biotechnology, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan

We investigated the structural transition from liquid silk to silk fibers with vibrational circular dichroism spectroscopy. Liquid silk showed a major right-handed optically active band at around 1650 cm⁻¹ and a minor one at around 1680 cm⁻¹. The former disappeared over time, while the intensity in the latter increased. With the former wavenumber, liquid silk mainly adopted a random-coil structure. In contrast, the latter may reflect an intermediate structure in the transition. Furthermore, two right-handed bands at around 1630 and 1660 cm⁻¹ appeared with the disappearance of the major band, and then the wavenumber of the former shifted to around 1620 cm⁻¹. The shift results from the decrease in the frequency of the CO stretching mode due to the stacking of the β-sheet that comprises fibers. The band at 1660 cm⁻¹ may reflect another intermediate structure due to its strong correlation with that at 1620 cm⁻¹ in terms of their temporal change in intensity.

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