Structural Determination of the Tandem Repeat Motif in Samia cynthia ricini Liquid Silk by Solution NMR

The wild silkworm Samia cynthia ricini produces silk fibroin containing polyalanine sequences. The structure of a typical tandem repeat sequence YGGDG­G­(A)12GGAG within S. c. ricini silk fibroin in liquid silk was determined by solution NMR. 13C, 15N, and 1H shifts were assigned from solution NMR spectra for the tandem repeat. TALOS-N was then used to predict the backbone dihedral angles from the chemical shifts. Dihedral angles revealed a well-structured α-helix for the polyalanine region and less stable capping motifs for the N- and C-terminal regions. Consistent with this, the amide proton temperature coefficients of the last glycine in the N-terminal region and the first two glycines in the C-terminal region show more positive values than that of a random coil structure. Thus, the α-helical structure of the polyalanine region of S. c. ricini liquid silk is stabilized by capping motifs.