The results of calculations aimed at providing a better understanding of how protein structural parameters affect 15N nuclear magnetic resonance (NMR) chemical shifts, using ab initio quantum chemical methods, are reported. The results support previous empirical observations that the two backbone dihedral angles closest to the peptide group (ψi-1 and φi) have the largest effects on 15N chemical shifts, contributing a range of about 20 ppm. The adjacent torsion angles φi-1 and ψi have a smaller contribution, up to 8 ppm, but also need to be considered when predicting protein chemical shifts. Different side chain conformations produce chemical shift variations of up to ∼4 ppm. Hydrogen bonding to peptide carbonyl groups can also contribute to 15N shielding, as can longer range electrostatic field effects, but these effects are smaller than those due to torsions. Calculations of 15N chemical shifts of nonhelical alanine residues in a Staphylococcal nuclease, dihydrofolate reductase from Lactobacillus casei, and ferrocytochrome c551 from Pseudomonas aeruginosa show a good correlation between experimental observation and ab initio prediction, but the shielding of helical residues is overestimated by ∼8 ppm, due most likely to electric field effects from the helix dipole. 15N NMR chemical shifts are very sensitive probes of protein conformation and have potential for structure validation, although at present they are less useful than are 13C shifts for prediction and refinement, because of their more complex dependence on multiple torsional, as well as electrostatic field, effects.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry