Direct Observation of Native Silk Fibroin Conformation in Silk Gland of Silkworm.

To understand the natural silk spinning mechanism, synchrotron Fourier transform infrared (S-FTIR) microspectroscopy was employed in this study to monitor the conformation changes of silk protein in the silk gland of silkworm. The ultrahigh brightness of S-FTIR microspectroscopy allowed the imaging of the silk gland with micrometer-scale spatial resolution. Herein, tissue sections of a silk gland, including cross-section slices and longitudinal-section slices, were characterized. The results obtained clearly confirm that the conformation of the silk fibroin changes gradually along the silk gland from the tail to the spinneret. In the middle silk gland, silk fibroin mainly contains random coil/helix conformation. When it comes to the spinneret through the anterior silk gland, the content of β-sheet increases, but the content of random coil/helix instead reduces gradually. Further, the β-sheet distribution in the cross-section of the anterior silk gland was imaged using S-FTIR mapping technique. The results show that the structural distribution of the silk fibroin in cross-section is uniform without significant shell-core structure, which implies that the primary driving force to induce the conformation transition of silk fibroin from random coil/helix to β-sheet during the spinning process is elongational flow of silk fibroin in the silk gland and not the shear force between the silk fibroin and the lumen wall of silk gland. These direct pieces of evidence of silk fibroin structure in the silk gland would definitely promote a deeper understanding of the natural spinning process.