15N chemical shift tensors and conformation of solid polypeptides containing 15N-labeled glycine residue by 15N NMR

Abstract

The correlation between the isotropic 15N chemical shift ( δ iso) and 15N chemical shift tensor components ( δ 11, δ 22 and δ 33) and the main-chain conformation such as the polyglycine I (PGI: β-sheet), II (PGII: 3 1-helix), α-helix and β-sheet forms of solid polypeptides [Gly∗,X] n consisting of 15N-labeled glycine (Gly∗) and other amino acids (X: natural abundance of 15N) has been studied by solid-state 15N NMR method. A series of polypeptides [Gly∗,X] n (X = glycine, l-alanine, l-leucine, l-valine, l-isoleucine, β-benzyl l-aspartate, γ-benzyl l-glutamate, ϵ-carbobenzoxy l-lysine, and sarcosine) were synthesized by the α-amino acid N-carboxy anhydride (NCA) method. Conformations of these polypeptides in the solid state were characterized on the basis of conformation-dependent 13C chemical shifts in the 13C cross-polarization-magic angle spinning (CP-MAS) NMR spectra and by the characteristic bands in the IR and far-IR spectra. The δ iso, δ 11, δ 22 and δ 33 of the polypetides were determined from the 15N CP-MAS and 15N CP-static (powder pattern) spectra. It was found that the δ iso, δ 11, δ 22 and δ 33 in the PGI form (δ 83.5, 185, 40.7 and 25 ppm, resp.) are upfield from those in the PGII form (88.5, 194, 42.1 and 29 ppm, resp.), which were reproduced by the calculated 15N shielding constants using the finite perturbation theory (FPT)-INDO method. It was also found that the δ 22 of the Gly∗ of [Gly∗,X] n is closely related to the main-chain conformation and the neighboring amino acid sequence, although the δ iso is almost independent of the glycine content and conformation. Consequently, the δ 22 value of Gly∗ containing copolypeptides is useful for the structural (main-chain conformation and neighboring amino acid sequence) analysis in the solid state by 15N NMR, if the 15N-labeled copolypeptide or natural protein can be provided. In addition, it is shown that the δ iso of the glycine residue is useful for the conformational study of some fibrous proteins such as silk fibroins and collagen fibrils in the solid state.