¹⁵N Chemical Shifts of Backbone Amides in Bovine Pancreatic Trypsin Inhibitor and Apamin

¹⁵N chemical shifts of backbone amides were measured at natural abundance for apamin and bovine pancreatic trypsin inhibitor using heteronuclear multiquantum proton-detected correlated spectroscopy (HMP-COSY). Chemical shift differences from values expected for random-coil peptides were calculated and examined with respect to structural features, including torsion angles and intramolecular hydrogen bonds. A correlation with the torsion angle ψ_i-1 was observed for the β-sheet residues. No other dominant factor was found to account for the large (up to ±15 ppm) variation from the model peptides. The results suggest that these ¹⁵N shifts are sensitive to nonbonded interactions, though there is no systematic variation with respect to a single structural parameter.