HYDROGEN-BOND LENGTH AND N-15 NMR CHEMICAL-SHIFT OF THE GLYCINE RESIDUE OF SOME OLIGOPEPTIDES IN THE SOLID-STATE

CP-MAS and CP-static N-15 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 N-15 chemical shifts, it was found that the isotropic N-15 chemical shifts (sigma-iso) of the glycine residues move downfield with a decrease of hydrogen bond length (R(N)...o) between the nitrogen and oxygen atoms in the amide groups, and the principal value of sigma-33 moves linearly downfield with a decrease of R(N)...o. There is no relationship between the principal value of sigma-11 or sigma-22 and R(N)...o. This indicates that such a linear downfield shift of sigma-33 contributes predominantly to the downfield shift of sigma-iso. Quantum chemical calculations of the N-15 shielding constant for the model compounds were carried out by the FPT-INDO method, and the relationship between N-15 chemical shift and R(N)...o discussed.