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 C-13 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 C(beta)HABC(alpha)HCD, and we have computed the effects of X(1) ((HCCH)-C-beta-H-alpha) on C-alpha, C-beta shielding in ethane, propane, 2-methylpropane, aminoethane, propanal, and 2-aminopropanal, as well as the effects of phi, psi, and X(1) torsion angles on C-alpha, C-beta 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 C-alpha substitution causes a much larger effect on C-beta shielding (as a function of X(1)) than on C-alpha shielding. For the two peptide model compounds, phi, psi torsions strongly affect C-alpha, C-beta shielding, with the largest X(1) effect being seen with valine C-beta. These dependencies are discussed in relation to some of the chemical shift nonequivalencies due to folding observed in the C-13 NMR spectra of Drosophila melanogaster calmodulin and Staphylococcal nuclease.