The authors are grateful to Dr. S. Amiya and Mr. K. Enomoto at Kuraray Corp. for their helpful discussion in artificial spinning. T.A. acknowledges support from the Program for Promotion of Basic Research Activities for Innovative Biosciences, Japan.

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-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 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 13C CP/MAS NMR spectra of the silk fibroin samples with different stretching ratios.

There are many kinds of silks spun by silkworms and spiders, which are suitable to study the structureproperty relationship for molecular design of fibers with high strength and high elasticity. In this review, we mainly focus on the structural determination of two well-known silk fibroin proteins that are from the domesticated silkworm, Bombyx mori, and the wild silkworm, Samia cynthia ricini, respectively. The structures of B. mori silk fibroin before and after spinning were determined by using an appropriate model peptide, (AG)15, with several solid-state NMR methods; 13C two-dimensional spin-diffusion solid-state NMR and rotational echo double resonance (REDOR) NMR techniques along with the quantitative use of the conformation-dependent 13C CP/MAS chemical shifts. The structure of S. c. ricini silk fibroin before spinning was also determined by using a model peptide, GGAGGGYGGDGG(A)12GGAGDGYGAG, which is a typical repeated sequence of the silk fibroin, with the solid-state NMR methods. The transition from the structure of B. mori silk fibroin before spinning to the structure after spinning was studied with molecular dynamics calculation by taking into account several external forces applied to the silk fibroin in the silkworm.

The 13C CP/MAS NMR, X-ray diffraction, and Raman spectroscopies were used for monitoring the structural transition of Samia cynthia ricini (S. c. ricini) silk fibroin induced by stretching. Here the silk fibroin was obtained from the aqueous solution stored in the silk gland. All of these spectroscopic data indicate that the structural transition from R-helix to â-sheet occurs with increasing the stretching ratio, especially between the stretching ratios, 4 and 6. The 13C chemical shifts of Ala Câ peak in the 13C CP/MAS NMR spectrum change significantly depending on R-helix, random coil, and two kinds of â-sheet structure, which make it possible to clarify the local structure and structural transition. Actually, the fraction of individual structure in the silk fibroin samples was determined from decomposition of the Ala Câ peak by assuming a Gaussian line shape. To examine the conformational change of Ala residues of the polyalanine region in S. c. ricini silk fibroin observed with these spectroscopic methods, MD simulations for four peptide molecules, AGGAGG(A)12GGAGAG, with R-helix conformation were performed in the presence of water molecules under different tensile strengths, and then additional MM calculations were performed after removal of water molecules. The change in the conformational character of Ala residues induced by stretching is explicable by these calculations.

Bombyx mori silk fibroin fiber is a fibrous protein produced by the silkworm at room temperature and from an aqueous solution whose primary structure is highly repetitive. In this study we analyzed the structural characteristics of native peptides, derived from B. mori silk fibroin, with formic acid treatment using highresolution solid-state 13C NMR. We establish that the Ser residue bearing a shortpolar side chain has the ability to stabilize the conformation formed in the model peptides due to its ability to form intermolecular hydrogen bonds involving its hydroxyl group as a donor and the carbonyl groups of other residues as acceptors. On the other hand, insertion of Tyr residue in the basic (AG)n and (AGSGAG)n sequence motifs usually exhibited disruptive effects on the preferred conformations. Moreover, the environmental effect was investigated by mixing the native Cp fraction with the model peptides, showing that there is no significant structural difference on the Ser-containing peptides, while structural transformation was observed on the peptides containing the GAAS unit. This may be attributed to the fact that the Cp fraction promotes the formation of an antiparallel â-sheet in the Ala-Ala unit. Such periodically disrupted ordered structures in the semicrystalline region of B. mori silk fibroin may be critical not only for facilitating the conformational transformation from silk I to silk II structural form but also for having some correlation with the unique properties of the silk materials.

利用基因工程方法把含有短肽RGD的氨基酸序列连接到桑蚕丝素蛋白的结晶序列GAGAGS上，通过调节DNA的聚合度，合成了具有[TGRGDSPA（GVPGV）2GG（GAGAGS）3AS]n一级结构、不同分子量大小的桑蚕丝素rRGD融合蛋白，并且通过在M9培养基中添加[3一13C]Ala的方法进行融合蛋白的稳定同位素标记。13c CP/MAS NMR结果显示，融合蛋白中的GAGAGS部分具有与天然桑蚕丝素结晶部分相同的分子结构，即Silk I处理后为均一的分子结构，而SilkⅡ处理后为不均一的分子结构，它包含了三种不同的结构成分。另一方面，通过对小鼠成纤维细胞BALB/3T3在不同蛋白材料载体上的粘附和增殖性能的测定结果显示，融合蛋白对细胞的增殖性能与天然胶原蛋白相近，但表现出了比胶原蛋白更好的细胞粘附性能。该研究结果显示，如果对该桑蚕丝素一：RGD融合蛋白进行适当加工，可能适合于组织工程。

以碱金属离子诱导桑蚕丝素蛋白溶液发生构象转变，研究了蛋白质初始结构对其矿化作用的影响。FT-IR，XRD和SEM等测试结果显示，未经任何处理的桑蚕丝素蛋白溶液矿化后形成片状复合物，其无机相以二水磷酸氢钙（DCPD）为主；而经过K＋和Na＋金属离子处理后，桑蚕丝素溶液的结构由无规线团/螺旋构象向β-折叠发生转变，矿化后成纤维状，并相互结合呈现纳米级的三维多孔结构，其无机相以热力学稳定的羟基磷灰石（HA）为主。可以认为，丝素蛋白结构转化为较伸展的β-折叠后，使得更多的亲水基团暴露在外面，在丝素蛋白分子不断凝聚成纤过程中，HA结晶快速生长并附着在这些微纤上，最终形成纤维状的丝素蛋白/HA复合物。该结果为阐明蛋白质的生物矿化过程及其调控机理提供了理论依据，同时可以从矿化复合物的形成来反映这些微量元素可能对骨组织形成的影响，为临床骨组织的修复提供一定的参考。

Silkworms have been developed over thousands years to optimize folding and crystallization of fibroin under highly controlled conditions which have resulted in their efficient fiber formation. In this paper, we reconstructed the three-dimensional architecture of the spinneret of a wild silkworm Samia cynthia ricini from approximately 1000 optical micrographs of the semi-thin cross sections. The chitin plates and muscles were observed in the silk press part together with large change in the diameter of the spinneret lumen at the press part by large shear stress. This is similar to the case of the spinneret of Bombyx mori silkworm, indicating that the structural change in the silk fibroin of S.c. ricini silkworm occurs exclusively at the silk press part due to large shear stresses. Molecular dynamics (MD) calculations were then performed to study the structural change that occurs in the crystalline region of S.c. ricini silk fibroin under shear stress. Namely, using the peptide AGGAGG(A)12-GGAGAG as a model of the crystalline part of the silk fibroin under different shear stresses in the presence of water molecules and followed by molecular mechanics (MM) calculation after removal of water molecules. The simulation indicates that the Ala residues in the model eptides adopt a predominantly b-sheet structure under shear stresses of above 1.0 GPa.

以体积分数为60%的乙醇处理制得不溶性再生丝素膜，通过碳二亚胺法将Cecropin B抗菌肽共价接枝到丝素膜表面，利用红外光谱、扫描电镜、X射线能谱和抗菌性测试等手段对制得的丝素膜结构和表面特性及其抗菌性能进行了测试分析。结果表明，Cecropin B抗菌肽成功地接枝到了丝素膜的表面，接枝后的丝素膜水溶性低，并具有良好、持久的抗菌能力。