HYDROGEN-BONDING EFFECT ON C-13 NMR CHEMICAL-SHIFTS OF L-ALANINE RESIDUE CARBONYL CARBONS OF PEPTIDES IN THE SOLID-STATE

In order to investigate the relationship between hydrogen-bond length and C-13 NMR chemical shifts of L-alanine carbonyl carbons in peptides in the solid state, C-13 CP-MAS NMR spectra were measured for a series of peptides containing L-alanine residues, for which the crystal structures were already determined by X-ray diffractions. From the results of the observed C-13 chemical shifts, it was found that the isotropic C-13 chemical shifts (delta(iso)) of L-alanine residues move linearly downfield with a decrease of hydrogen-bond length (R(N...O)) as expressed by delta(iso) = 237.5-21.7 R(N...0) ppm. Such a downfield shift of delta(iso) predominantly arises from the large downfield shift of delta-22, one of the chemical shift tensor components (delta-11, delta-22, and delta-33) with decreasing R(N...0). Delta-11 moves somewhat upfield with R(N...0) and delta-33 is not sensitive to a change of R(N...0). The quantum-chemical calculation of the C-13 shielding constant for the model compound was carried out by the FPT-INDO method, and the hydrogen-bonding effect on the C-13 chemical shift in the ]C = O...H-N[ type hydrogen bond and the nature of the hydrogen bond are discussed.