Determination of the torsion angles of alanine and glycine residues of Bombyx mori silk fibroin and the model peptides in the silk I and silk II forms using 2D spin diffusion solid-state NMR under off magic angle spinning

The structures of two crystalline forms of Bombyx mori silk fibroin, before and after the spinning process from silkworm, have been known as silk I and silk II, respectively. Most recently, we proposed a silk I structure of alternating copolypeptide (Ala-Gly)(15) as a "repeated beta-turn type II structure". In this paper, two-dimensional spin diffusion solid-state NMR under off magic angle spinning was applied to determine the torsion angles of the alanine and glycine residues of the same peptide with silk II structure. The angles were determined to be phi, psi = -150degrees, 150degrees within experimental error of +/-10 for both residues, which supports an antiparallel beta-sheet structure of silk II. Next, [1,2-C-13]glycine was incorporated into B. mori silk fibroin by feeding [1,2-C-13]glycine to silkworms to evaluate the torsion angle psi, of glycine residues in the silk fibroin directly. The angles were evaluated as psi = 30degrees and psi = 150degrees for silk I and silk II forms, respectively. The evaluation was also performed for the torsion angle of the glycine residue in (Ala-Gly)(15) in the silk I and silk 11 forms. The similar angles were obtained between the silk fibroin and (Ala-Gly)(15), which indicates that the structural models for the silk fibroin proposed from the structural study of (Ala-Gly)(15) are valid.