Theoretical variation of the H alpha chemical shift in acetyl-glycyl-N-methylamide and oligoglycines with molecular conformation and environment.

The sum of the magnetic anisotropy and polarization contributions to the magnetic shielding constants of the alpha protons is calculated as a function of the torsion angles about the NC alpha (phi) and C alpha C' (psi) bonds of the dipeptide. The results show that the polarization or electric field effect is several fold larger than the magnetic anisotropy contribution. The calculated variations are large enough to account for the spread of the values measured for these protons in peptides and proteins. The results obtained for polyglycine alpha helices and antiparallel beta sheets are discussed in relation with molecular conformation and environmental effects on the one hand and experimental data on the other.