The results of calculations aimed at providing a better understanding of how protein structural parameters affect N-15 nuclear magnetic resonance (NMR) chemical shifts, using nb initio quantum chemical methods, are reported. The results support previous empirical observations that the two backbone dihedral angles closest to the peptide group (psi(i-1) and phi(i)) have the largest effects on N-15 chemical shifts, contributing a range of about 20 ppm. The adjacent torsion angles phi(i-1) and psi(i) 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 similar to 4 ppm. Hydrogen bonding to peptide carbonyl groups can also contribute to N-15 shielding, as can longer range electrostatic field effects, but these effects are smaller than those due to torsions. Calculations of N-15 chemical shifts of nonhelical alanine residues in a Staphylococcal nuclease, dihydrofolate reductase from Lactobacillus casei, and ferrocytochrome c(551) from Pseudomonas aeruginosa show a good correlation between experimental observation and nb initio prediction, but the shielding of helical residues is overestimated by similar to 8 ppm, due most likely co electric field effects from the helix dipole. N-15 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 C-13 shifts for prediction and refinement, because of their more complex dependence on multiple torsional, as well as electrostatic field, effects.