Conformational Transformation Exhibited by the Peptide Extracted from Crystalline Region of Bombyx mori Silk Fibroin in Solid and Solution States

The conformational transformation of a 30-residue peptide H(Ala-Gly-Ser-Gly-Ala-Gly)₅OH, i.e., (AGSGAG)₅, extracted from highly crystalline region of Bombyx mori (B. mori) silk fibroin was described by using the high resolution solid state ¹³C NMR, and CD spectroscopies. Based on the conformation-dependent ¹³C NMR chemical shifts of the Ala, Gly and Ser residues and the line-shape analysis of the conformation sensitive Ala C_β resonance, the peptide revealed a strong preference for silk II structural form, i.e., an antiparallel β-sheet structure (ø=−140±20° and ψ=135±20°) in solid state. On the contrary, the CD spectra of this peptide in the two non-native hexafluorinated fibre spinning solvents, hexafluoroisopropanol (HFIP) and hexafluoroacetone (HFA), exhibited the existence of an unusual tightly-folded conformation resembling 3₁₀-helix (ø=−60±20° and ψ=−30±20°), as judged from the R ratio of [θ]₂₂₂/[θ]₂₀₃ in HFIP solution, whereas a dynamically averaged unordered structure in HFA. Taken together, the information inclined to hypothesis that the primary structure of the highly crystalline regions of B. mori silk fibroin may be easily accessible to the large conformational changes, which in turn may be critical for facilitating the structural transformation from unprocessed silk fibroin (silk I form) to processed silk fiber (silk II form).