Ab Initio IGLO Study of the φ- and ψ-Angle Dependence of the ¹³C 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 ¹³C chemical shifts in the model peptide N-acetyl-N′-methylglycinamide. A surface plot of the calculated ¹³C isotropic chemical shifts for the Cα carbon was constructed at 30° grid intervals of the φ and ψ angles. These data are used to examine the relationship between chemical shifts and protein secondary structure. The Ca carbons in α-helix and β-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 β-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 Cα 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 Cα carbon are important to the total shift but each of these has a different (φ, ψ) angle dependence. The LMOC from the Cα-C′ bond provides the largest contribution to the chemical shift difference between the a-helix and the β-sheet conformations.