A repeated beta-turn structure in poly(Ala-Gly) as a model for silk I of Bombyx mori silk fibroin studied with two-dimensional spin-diffusion NMR under off magic angle spinning and rotational echo double resonance.

The structure of a crystalline form of Bombyx mori silk fibroin, commonly found before the spinning process (known as silk I), was proposed by combining data obtained from two-dimensional spin-diffusion nuclear magnetic resonance under off magic angle spinning, rotational-echo double-resonance (REDOR), previously reported X-ray diffraction analyses and 13C NMR chemical shifts. Instead of B. mori silk fibroin with silk I structure, we used the sequential model peptide (Ala-Gly)15. The structure of the sequential model peptide is characterized as silk I after dissolving the peptide in 9 M LiBr and then dialyzing against water. Moreover, 13C or 15N-labeled sites may be introduced easily at any position in (Ala-Gly)(15) by the solid phase synthesis method for these NMR experiments. The torsional angles of (Ala-Gly)15 with silk I structure were determined as (-60(+/-5) degrees, 130(+/-5) degrees ) and (70(+/-5) degrees, 30(+/-5) degrees ) for Ala and Gly residues, respectively. The formation of the intra-molecular hydrogen bonding along the chain was confirmed from REDOR NMR by determination of the inter-atomic distance between the nitrogen and carbon atoms comprising the intra-molecular hydrogen bonding. The structure is named a repeated beta-turn type II-like structure.