In order to understand better the origins of the chemical shift nonequivalencies observed in proteins due to folding, we have investigated the effects of torsion angles on 13C nuclear magnetic resonance shielding in a series of compounds, using a gauge-including atomic orbital (GIAO) method. We regard the naturally occurring L-amino acids as ethane derivatives CβHABCαHCD, and we have computed the effects of χ1 (HCβCαH) on Cα, Cβ shielding in ethane, propane, 2-methylpropane, aminoethane, propanal, and 2-aminopropanal, as well as the effects of φ, ψ, and χ1 torsion angles on Cα, Cβ shielding in the peptide models N-formyl-L-alanine amide and N-formyl-L-valine amide. Our results show for the simpler model compounds that Cα substitution causes a much larger effect on Cβ shielding (as a function of χ1) than on Cα shielding. For the two peptide model compounds, φ, ψ torsions strongly affect Cα, Cβ shielding, with the largest χ1 effect being seen with valine Cβ. These dependencies are discussed in relation to some of the chemical shift nonequivalencies due to folding observed in the 13C NMR spectra of Drosophila melanogaster calmodulin and Staphylococcal nuclease.
ASJC Scopus subject areas
- Colloid and Surface Chemistry