Methanol-induced conformation transition of gland fibroin monitored by FTIR spectroscopy and terahertz spectroscopy.

Silk fibroin extracted from the gland of Bombyx mori silkworm is employed as an ideal system to investigate its conformation transition in methanol/D2O solution. The transition process was monitored by Fourier transform infrared (FTIR) spectroscopy coupled with terahertz time domain spectroscopy (THz-TDS). Analysis of FTIR spectra suggests that, with increasing time of treatment, an increasing band at 1634 cm(-1) is observed indicating the formation of β-pleated sheets coincident with the loss of intensity of a band at 1673 cm(-1) indicating decrease of the random coil structure. In addition, there is a burst phase of 33% occurring during the first 2 minutes when the gland fibroin membranes are immersed into methanol/D2O solution. THz spectra present distinct features for conformations of silk fibroin, in combination with the results obtained from FTIR; the peaks observed at 1.54 THz (51 cm(-1)), 1.67 THz (55 cm(-1)), and 1.84 THz (61 cm(-1)) can be attributed to a β-pleated sheet, α-helix, and random coil, respectively. Intensity change of bands centered at 1.54 THz and 1.84 THz confirms the formation of the β-pleated sheet and the disappearance of the random coil. Kinetic curves obtained from THz spectra indicate that the methanol-induced conformation transition from the random coil to the β-pleated sheet is fitted with an exponential function. The results suggest that THz-TDS presents great potential as a complementary approach in studying the secondary structure of a protein, providing significant insight into the silk-spinning process in vivo.