Influence of pH Value on the Conformation Transition of Silk Fibroin

Both C-13 CP-MAS solid state NMR and laser Raman scattering spectroscopy were used to investigate the influence of pH values on the conformation transition of silk fibroin of Bambyx mori silkworm. The regenerated silk fibroins in the phosphate buffer solution were used to imitate the silk fibroins of the gland. A series of membranes formed in buffer solutions with different pH values were obtained by the evaporation of water at room temperature(for mimicking the gradual loss of water in spinning process). The composes and conformation in the fibroin membranes were determined qualitatively by Raman and Generalized Two-Dimensional Correlation Raman Spectroscopy for amide I region(1 600-1 700 cm(-1)) of the silk fibroins. There are four types of conformations at least existing in the fibroin membranes, including conformations of typical silk I (a-form and/or random coil), typical silk I (beta-sheet), silk I transition and silk I transition, etc.. Based on the qualitative results from 2D Raman spectrum 1 the relationship between pH value and conformation contents in the studied systems was determined quantitatively by simulation of C-beta peak (delta 14. 5-22) in C-13 CP-MAS solid state NMR spectrum. The results show that acidic solution (pH = 5.2) is more favorable than neutral (pH = 6.9) and basic (pH = 8.0) ones for the conformation transition from Silk I to Silk I because the silk fibroin molecular backbones are easier to extend and then to form beta-sheets in acidic solution.