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

Abstract

AbstractThe 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 Cβ 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).