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

The ab initio IGLO (individual gauge for localized orbitals) method wa s used to examine the conformational dependencies of the isotropic C-1 3 chemical shifts in the model peptide N-acetyl-N'-methylglycinamide. A surface plot of the calculated C-13 isotropic chemical shifts for th e Calpha carbon was constructed at 30-degrees grid intervals of the ph i and psi angles. These data are used to examine the relationship betw een chemical shifts and protein secondary structure. The Calpha carbon s in alpha-helix and beta-sheet conformations are calculated to be shi fted 2.3 ppm downfield and 2.9 ppm to high field, respectively, of the random coil value. Considering the spread in experimental values, esp ecially for the beta-sheet conformations, these secondary shifts are i n 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 struct ures are also consistent with the experimental results. Thus, for the Calpha carbon it is not necessary to include interresidue hydrogen-bon ding effects to explain the major chemical shift trends. An analysis o f the localized MO contributions (LMOC) shows that all four bonds dire ctly connected to the Calpha carbon are important to the total shift b ut each of these has a different (phi, psi) angle dependence. The LMOC from the Calpha-C' bond provides the largest contribution to the chem ical shift difference between the alpha-helix and the beta-sheet confo rmations.