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

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.