Heterogeneous exchange behavior of Samia cynthia ricini silk fibroin during helix–coil transition studied with ¹³C NMR

The structure and structural transition of the glycine residue adjacent to the N-terminal alanine residue of the poly(L-alanine), (Ala)(12-13), region in Samia cynthia ricini silk fibroin was studied using (13)C nuclear magnetic resonance (NMR). Most of the glycine carbonyl peaks in the (13)C solution NMR spectrum of [1-(13)C]glycine-silk fibroin could be assigned to the primary structure from the comparison of the (13)C chemical shifts of seven glycine-containing tripeptides. The slow exchange between helix and coil forms in the NMR time scale was observed with increasing temperature exclusively for the underlined glycine residue in the Gly-Gly-(Ala)(12-13) sequence during fast helix-coil transition of the (Ala)(12-13) region.