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 statistical mechanical calculation previously used for (AG)15, the probable lamellar structures of (AGSGAG)5 in the solid state are proposed. The models of two turns in the central part of the sequence of (AGSGAG)5 consist of approximately 8−12 amino acids. The effect of the introduction of Ser residue on the local structure of Ala-Gly copolymeric peptides is also discussed on the basis of the evidence from 13C solid state spin−lattice relaxation experiments.