Hydrogen bond length and 15N NMR chemical shift of the glycine residue of some oligopeptides in the solid state

Abstract

CP-MAS and CP-static 15N NMR spectra were measured for a variety of solid oligopeptides containing the glycine residue, the crystal structures of which had already been determined by X-ray diffraction. From the results of the observed 15N chemical shifts, it was found that the isotropic 15N chemical shifts (σ iso) of the glycine residues move downfield with a decrease of hydrogen bond lengths ( R N⋯O) between the nitrogen and oxygen atoms in the amide groups, and that the pricipal value of σ 33 moves linearly downfield with a decrease of R N⋯O. There is no relationship between the principal value of σ 11 or σ 22 and R N⋯O. This indicates that such a linear downfield shift of σ 33 contributes predominantly to the downfield shift σ iso. Quantum chemical calculations of the 15N shielding constant for the model compounds were carried out by the FPT-INDO method, and the relationship between 15N chemical shift and R N⋯O discussed.