beta-turn formation during the conformation transition in silk fibroin

The changes of the secondary structure of silk proteins, including beta-sheet, helical and random coil content during the conformation transition process have been widely studied with various methods by many researchers, however few reports relate to beta-turn formation. In this paper, beta-turn formation during the conformation transition in both regenerated silk fibroin (RSF) solutions and films was monitored by time-resolved FTIR spectroscopy. The results show that the kinetics of beta-turn formation varies not only in RSF solution or film, but also depends on the concentration of ethanol used to induce the conformation transition. Our observations confirm that the absorption band around 1690 cm(-1) is not the high frequency band of anti-parallel beta-sheet, but represents beta-turns possibly together with a component derived directly from beta-sheet as proposed in our previous work. Our findings also suggest that the free adjustment of silk fibroin macromolecular chains is very important for the formation of regular beta-sheet structure, essential for the strength of silk fibers, with important implications for the natural spinning mechanism of animal silks and attempts to copy this industrially.