A new class of Co9 S8 @MoS2 core-shell structures formed on carbon nanofibers composed of cubic Co9 S8 as cores and layered MoS2 as shells is described. The core-shell design of these nanostructures allows the advantages of MoS2 and Co9 S8 to be combined, serving as a bifunctional electrocatalyst for H2 and O2 evolution.

The molecular conformation of silk fibrion is characterized by solid-state 13C NMR before spinning (silk I structure) and after spinning (silk II structure). We compare native silk fibers with the quasi-crystalline Cp-fraction and a synthetic model peptide (Ala-Gly)15, both of which can be converted either into silk I by dialysis from 9 M LiBr or into silk II by treatment with formic acid. Our results demonstrate that silk II fibers are intrinsically heterogeneous, consisting of beta-sheets, distorted beta-turns, and distorted beta-sheets. This higher-order heterogeneity is revealed by the 13C-NMR Cbeta-peak of Ala, indicating that the Ala side chains are stacked partially in parallel and partially face-to-face, at a ratio of 1:2.

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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 13 C solid-state NMR as a "repeated ␤-turn type II structure." Here, we used 13 C solid-state NMR to clarify the heterogeneous structure of the natural fiber from Bombyx mori silk fibroin in the silk II form. Interestingly, the 13 C CP/MAS NMR revealed a broad and asymmetric peak for the Ala C␤ 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 ␤-turn, 13% ␤-sheet (parallel Ala residues), and 25% ␤-sheet (alternating Ala residues). The remaining fraction of 44% amorphous Tyr-rich region, 22% in both distorted ␤-turn and distorted ␤-sheet. Such a heterogeneous structure including distorted ␤-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 C␤ peak of 13 C CP/MAS NMR spectra of the silk fibroin samples with different stretching ratios. Keywords:Bombyx mori silk fiber; antiparallel ␤-sheet structure; poly(Ala-Gly); 13 C CP/MAS NMR; silk I and silk II; heterogeneous structure of silk fiber Because of the exceptional strength and high toughness of Bombyx mori silk fiber, much attention has been focused on the structure determination of silk fibroin before and after spinning and the factors that contribute to the conformational change (Asakura and Kaplan 1994;Gosline et al. 1999). The remarkable properties of silk fibers are attributed to the distribution of microcrystalline and amorphous domains, which are formed in the process of spinning by protein-protein interactions. The overall composition of silk fibroin in mol% consists of glycine (42.9%), alanine (30.0%), serine (12.2%), tyrosine (4.8%), and valine (2.5%) (Shimura 1980). The complete primary structure of B. mori fibroin has been determined by Mita et al. (1994) and more recently by Zhou et al. (2000). In addition to the wellknown sequence motif AGSGAG, which is the main repetitive unit in the crystalline regions of the silk fiber (Fraser et al. 1966;Strydom et al. 1977;Saito et al. 1984), the protein also contains semicrystalline repeating motifs such as AGYGAG and AGVGYGAG (Shimura 1980;Asakura et al. 1984 Asakura et al. , 2001b Asakura et al. , 2002bMita et al. 1994;Zhou et al. 2000) as well as the irregular unit, GAAS (Asakura et al. 2002a) and the amorphous motifs.Fibroin can assume two distinct structures in the solid state, namely silk I before spinning, and silk II after spinning, that is, the silk fiber. The corresponding structures have...

The solid-state (13)C CP-MAS NMR spectra of biosynthetically labeled [(13)C(alpha)]Tyr, [(13)C(beta)]Tyr, and [(13)C(alpha)]Val silk fibroin samples of Bombyx mori, in silk I (the solid-state structure before spinning) and silk II (the solid-state structure after spinning) forms, have been examined to gain insight into the conformational preferences of the semicrystalline regions. To establish the relationship between the primary structure of B. mori silk fibroin and the "local" structure, the conformation-dependent (13)C chemical shift contour plots for Tyr C(alpha), Tyr C(beta), and Val C(alpha) carbons were generated from the atomic coordinates of high-resolution crystal structures of 40 proteins and their characteristic (13)C isotropic NMR chemical shifts. From comparison of the observed Tyr C(alpha) and Tyr C(beta) chemical shifts with those predicted by the contour plots, there is strong evidence in favor of an antiparallel beta-sheet structure of the Tyr residues in the silk fibroin fibers. On the other hand, Tyr residues take a random coil conformation in the fibroin film with a silk I form. The Val residues are likely to assume a structure similar to those of Tyr residues in silk fiber and film. Solid-state (2)H NMR measurements of [3,3-(2)H(2)]Tyr-labeled B. mori silk fibroin indicate that the local mobility of the backbone and the C(alpha)-C(beta) bond is essentially "static" in both silk I and silk II forms. The orientation-dependent (i.e., parallel and perpendicular to the magnetic field) solid-state (15)N NMR spectra of biosynthetically labeled [(15)N]Tyr and [(15)N]Val silk fibers reveal the presence of highly oriented semicrystalline regions.