The fibrosis process of silk protein from sol-like fibroin to solid silk is a conformation transition of protein, e.g. from random coil to β-sheet, which is induced by some factors, i.e., shear stress or stretching force. However, the spinning mechanism of natural silk is still an open issue. Presently, the conformation transition and β-sheet aggregation growth of silk fibroin are identified in the very dilute aqueous solution of Bombyx mori regenerated silk fibroin (0.1 mg/mL) by the circular dichroism (CD) spectroscopy. From this result, we could explain why the molecular chain in native silk has such well orientation under a very low spinning speed (around 1 cm/s), comparing to those of synthetic fibers.

      On another side, the process of silk formation is rather faster than that of cast a silk fibroin membrane with the same concentration of fibroin aqueous solution in vivo. Therefore, we investigated an extreme case, e.g. the ethanol-induced conformation transition of regenerated Bombyx mori silk fibroin membrane from a poorly defined to the well ordered state, by timeresolved Fourier transform infrared (FTIR) spectroscopy. From the analysis of FTIR difference spectra, taken on time scales as short as 6 s and up to 1 h after addition of ethanol, intepsity vs. time plots of an increasing band at 1618 cm^－1 were observed indicating formation of a β-sheet coincident with the loss of intensity of a band at 1668 cm^－1 indicating decreases of random coil and/or silk I structure. Both infrared markers were fitted with identical biphasic exponential decay functions,however, there was a clear burst phase occurring prior to the onset of the observed transitions. In this case, the conformation transition process is suggested to either proceed sequentially through at least two intermediate states that contain different levels of β-sheet structure or to have parallel pathways of initial β-sheet formation followed by a slower “perfection” phase. The first observed process forms in a burst phase a few seconds after mixing or even faster, prior to the collection of the first spectrum at 6 s. The second observed process occurs with a time constant of 0.5 rain, the intermediate presents at this stage then continues with a time constant of 5.5 rain completing the observed formation of the β-sheet.

     Thus, the conformation transition of silk protein analyzed by both of CD and FTIR provides insight into a part of the fascinating process of cocoon formation in Bombyx mori.

论文来源:Young Scientist-Forum on Macromolecular Science,April 9-12, 2002, Beijing, China Sino-German