Juming Yao

13C CP/MAS NMR study on structural heterogeneity in Bombyx mori silk fiber and their generation by stretching

It is important to resolve the structure of Bombyx mori silk fibroin before spinning (silk I) and after spinning (silk II), and the mechanism of the structural transition during fiber formation in developing new silk‐like fiber. The silk I structure has been recently resolved by 13C solid‐state NMR as a “repeated β‐turn type II structure.” Here, we used 13C solid‐state NMR to clarify the heterogeneous structure of the natural fiber from Bombyx mori silk fibroin in the silk II form. Interestingly, the 13C CP/MAS NMR revealed a broad and asymmetric peak for the Ala Cβ carbon. The relative proportions of the various heterogeneous components were determined from their relative peak intensities after line shape deconvolution. Namely, for 56% crystalline fraction (mainly repeated Ala‐Gly‐Ser‐Gly‐Ala‐Gly sequences), 18% distorted β‐turn, 13% β‐sheet (parallel Ala residues), and 25% β‐sheet (alternating Ala residues). The remaining fraction of 44% amorphous Tyr‐rich region, 22% in both distorted β‐turn and distorted β‐sheet. Such a heterogeneous structure including distorted β‐turn can be observed for the peptides (AG)n (n > 9 ). The structural change from silk I to silk II occurs exclusively for the sequence (Ala‐Gly‐Ser‐Gly‐Ala‐Gly)n in B. mori silk fibroin. The generation of the heterogeneous structure can be studied by change in the Ala Cβ peak of 13C CP/MAS NMR spectra of the silk fibroin samples with different stretching ratios.

Production and characterization of a silk-like hybrid protein, based on the polyalanine region of Samia cynthia ricini silk fibroin and a cell adhesive region derived from fibronectin

There are a variety of silkworms and silk fibroins produced by them. Silks have many inherent suitable properties for biomaterials. In this paper, a novel silk-like hybrid protein, [DGG(A)(12)GGAASTGRGDSPAAS](5), which consists of polyalanine region of silk fibroin from a wild silkworm, Samia cynthia ricini, and cell adhesive region including Arg-Gly-Asp (RGD) sequence, derived from fibronectin, was designed and produced. The genes encoding the hybrid protein were constructed and expressed in Escherichia coli. The main conformation of the polyalanine region, that is, either alpha-helix or beta-sheet, could be easily controlled by treatment with different acidic solvents, trifluoroacetic acid or formic acid, respectively. This structural change was monitored with 13C CP/MAS NMR. Higher cell adhesive and growth activities of the hybrid protein compared with those of collagen were obtained.

Change in the structure of poly(tetramethylene succinate) under tensile stress monitored with solid state 13C NMR

Abstract A new solid-state NMR experimental equipment was developed to monitor the structural change of polymer and protein fibers under active uniaxial deformation. The equipment was applied to monitoring change in the structure of a biodegradable aliphatic polyester fiber, poly(tetramethylene succinate) (PTMS; [–O(CH2)4OCO–(CH2)2CO–]n), as a function of tensile stress. The crystal transition from α to β forms for PTMS was induced by stress and monitored by the change of the methylene region in the 13C NMR spectra. The 13C chemical shielding constant for the model compound of poly(tetramethylene succinate) was calculated using the GIAO–CHF (gauge invariant atomic orbitals–coupled Hartree–Fock) with ab initio 6-311G∗∗ basis set. This calculation acceptably explains the experimental results.