Structural Analysis of the Synthetic Peptide (Ala-Gly-Ser-Gly-Ala-Gly)5, a Model for the Crystalline Domain of Bombyx mori Silk Fibroin, Studied with 13C CP/MAS NMR, REDOR, and Statistical Mechanical Calculations

In our previous study, we have proposed a lamellar structure for Ala-Gly repeated copolymeric peptide, a model for crystalline region of Bombyx mori silk fibroin. Here, we propose the structure of (AGSGAG)5 with silk II form which is a more mimic of the crystalline region of B. mori silk fibroin than (AG)15. The local structure for each Ala residue was determined from 13C CP/MAS NMR spectra of 10 different [3-13C]Ala-(AGSGAG)5 peptides differing in their 13C labeling positions. The highest field peak for the Ala Cβ carbon (16.7 ppm) assigned to a distorted β-turn structure and/or random coil changes significantly depending on the 13C labeling position. In addition, the fractions of the random coil and/or distorted β-turn component of each Ser residue were determined by REDOR experiments from the 13C−15N atomic distances of five versions of the above peptide with different [1-13C]Gly-Ser-[15N]Gly positions. By combining the structural information of Ala and Ser residues from solid state NMR, with statistic...