CHEMICAL-SHIFTS IN PROTEINS - AN AB-INITIO STUDY OF C-13 NUCLEAR-MAGNETIC-RESONANCE CHEMICAL SHIELDING IN GLYCINE, ALANINE, AND VALINE RESIDUES

Using gauge-including atomic orbital self-consistent field ab initio quantum chemical methods, we have computed the effects of bond lengths, bond angles, and torsion angles on the carbon-13 chemical shielding of C(alpha) (and C(beta)) sites in model fragments for glycine, alanine, and valine residues in proteins. Predicted chemical shieldings are highly sensitive to bond length variations, and we show that it is essential to relax or energy minimize protein structures (to remove large errors associated with bond length uncertainties) in order to successfully predict experimental C-13 NMR spectra.