Hydrogen-bonding effect on 13C NMR chemical shifts of amino acid residue carbonyl carbons of some peptides in the crystalline state

Abstract

13C 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 13C chemical shifts of amide carbonyl carbons in the peptides. From these experimental results, it was found that the isotropic 13C chemical shifts ( δ iso) of the amino acid residues move linearly downfield with a decrease in R N…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 δ iso ( ppm) = a − bR N…O ( A ̊ ) where a and b are 215.4 (ppm) and 14.2 (ppm Å −1) for the l-valine residue, 202.2 (ppm) and 10.0 (ppm Å −1) for the l-leucine residue, and 199.0 (ppm) and 9.6 (ppm Å −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 13C shielding constant for a model compound was carried out by the finite perturbation theory INDO method in order to ascertain the 13C shielding behavior in the formation of hydrogen bonds.