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[摘要]：Peptides with a combination of hydrophilic and hydrophobic sequences mimicking the primary structure of Bombyx mori silk fibroin were synthesized and studied in the solid state by NMR using C-13 selective labeling coupled with C-13 conformation-dependent chemical shifts and 2D solid-state spin-diffusion NMR. The hydrophilic sequence was poly(L-glutamic acid) (E)(n), and the hydrophobic one was the consensus sequence of file crystalline fraction of B. mori silk fibroin, (AGSGAG)(4). The balance of hydrophilic and hydrophobic characters of the peptide was controlled by changing the relative length, n, of (E)(n), from 4 to 8. When n = 4 and 5, the structure of the hydrophobic sequence is basically Silk I (the structure of B. mori silk fibroin before spinning in the solid state), and the polyglutamate sequences are random coil. On the other hand, when n = 6-8, the structure of the polyglutamate sequence changes gradually from random coil to beta-sheet, and the hydrophobic sequence adopts a mixture of beta-sheet and random coil/distorted beta-turn forms, although the fraction of the latter form decreases gradually by increasing the number it from 6 to 8. Molecular dynamics and molecular mechanics calculations were also performed to examine the stability of the aggregated domains of the peptides in the solid state. The conformational change of (E)(4)(AGSGAG)(4) was monitored in the solid state by decreasing the pH of the aqueous solution during the sample preparation. |
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