HYDROGEN-BONDING EFFECT ON C-13 NMR CHEMICAL-SHIFTS OF AMINO-ACID RESIDUE CARBONYL CARBONS OF SOME PEPTIDES IN THE CRYSTALLINE STATE

C-13 cross polarization-magic angle spinning NMR spectra were measured for a series of peptides containing L-valine, L-leucine and L-aspartic acid residues, for which the crystal structures were already determined by X-ray diffraction, in order to investigate the relationship between hydrogen-bond lengths (R(N ... O)) and C-13 chemical shifts of amide carbonyl carbons in the peptides. From these experimental results, it was found that the isotropic C-13 chemical shifts (delta(iso)) of the amino acid residues move linearly downfield with a decrease in RN(...)O within the hydrogen-bonded length range considered here and also shown in our previous work on glycine and L-alanine residues as expressed by delta(150)(ppm) = a-bR(N ... O)(Angstrom) where a and b are 215.4 (ppm) and 14.2 (ppm)Angstrom(-1) for the L-valine residue, 202.2 (ppm) and 10.0 (ppm Angstrom(-1)) for the L-leucine residue, and 199.0 (ppm) and 9.6 (ppm Angstrom(-1)) for the L-aspartic acid residue, respectively. Using these relations, the R(N ...)O values of some polypeptides in the crystalline state were determined through the observation of the amide carbonyl carbon chemical shifts. These values were compared with those determined by the X-ray diffraction method. Furthermore, quantum-chemical calculation of the C-13 Shielding constant for a model compound was carried out by the finite perturbation theory INDO method in order to ascertain the C-13 shielding behavior in the formation of hydrogen bonds.