AB-INITIO IGLO STUDY OF THE PHI-ANGLE AND PSI-ANGLE DEPENDENCE OF THE C-13 CHEMICAL-SHIFTS IN THE MODEL PEPTIDE N-ACETYL-N'-METHYLGLYCINAMIDE

The ab initio IGLO (individual gauge for localized orbitals) method was used to examine the conformational dependencies of the isotropic C-13 chemical shifts in the model peptide N-acetyl-N'-methylglycinamide. A surface plot of the calculated C-13 isotropic chemical shifts for the Calpha carbon was constructed at 30-degrees grid intervals of the phi and psi angles. These data are used to examine the relationship between chemical shifts and protein secondary structure. The Calpha carbons in alpha-helix and beta-sheet conformations are calculated to be shifted 2.3 ppm downfield and 2.9 ppm to high field, respectively, of the random coil value. Considering the spread in experimental values, especially for the beta-sheet conformations, these secondary shifts are in reasonable agreement with the average experimental values of 3.2 and -1.2 ppm, respectively, for glycyl residues in peptides and proteins. The smaller differences predicted for other types of secondary structures are also consistent with the experimental results. Thus, for the Calpha carbon it is not necessary to include interresidue hydrogen-bonding effects to explain the major chemical shift trends. An analysis of the localized MO contributions (LMOC) shows that all four bonds directly connected to the Calpha carbon are important to the total shift but each of these has a different (phi, psi) angle dependence. The LMOC from the Calpha-C' bond provides the largest contribution to the chemical shift difference between the alpha-helix and the beta-sheet conformations.