N-15 CHEMICAL-SHIFT TENSORS AND CONFORMATION OF SOLID POLYPEPTIDES CONTAINING N-15-LABELED L-ALANINE RESIDUE BY N-15 NMR .2. SECONDARY STRUCTURE REFLECTED IN SIGMA-22

A series of polypeptides [Ala⋆,X]ncontaining 15N-labeled L-alanine (Ala⋆) and other amino acids (X; natural abundance of 15N) such as glycine, L-alanine, D-alanine, L-leucine, β-benzyl L-aspartate, γ-benzyl L-glutamate, γ-methyl L-glutamate, L-valine, L-isoleucine, 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 of the characteristic bands in the infrared (IR) and far-IR spectra. Further, isotropic 15N chemical shift (σiso) and chemical shift tensors (σ11, σ22and σ33) of the polypeptides were measured by the 15N CP-MAS and the 15N CP-static (powder pattern) methods, respectively. It was found that σisois useful for the conformational study of homopolypeptides and copolypeptides with identical primary structures (amino acid sequences). In addition, it was demonstrated that the σ22value of the Ala⋆residue in copolypeptide is closely related to the main-chain conformations (such as the right-handed and left-handed α-helices, and the β-sheet forms) rather than the amino acid sequence. Consequently, the σ22value is very useful for conformational analysis of solid copolypeptides. © 1990, American Chemical Society. All rights reserved.