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 q uantum chemical methods, we have computed the effects of bond lengths, bond angles, and torsion angles on the carbon-13 chemical shielding o f C(alpha) (and C(beta)) sites in model fragments for glycine, alanine , and valine residues in proteins. Predicted chemical shieldings are h ighly sensitive to bond length variations, and we show that it is esse ntial to relax or energy minimize protein structures (to remove large errors associated with bond length uncertainties) in order to successf ully predict experimental C-13 NMR spectra.