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 ~(13)C CP/MAS NMR, REDOR, and Statistical Mechanical Calculations

摘要

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 ~(13)C CP/MAS NMR spectra of 10 different [3-~(13)C]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 ~(13)C 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 ~(13)C?~(15)N atomic distances of five versions of the above peptide with different [1-~(13)C]Gly-Ser-[~(15)N]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 ~(13)C solid state spin?lattice relaxation experiments.