Protein Chemical Shifts Arising from α-Helices and β-Sheets Depend on Solvent Exposure

The NMR chemical shifts of certain atomic nuclei in proteins (1Hα,13Cαand13Cβ) depend sensitively on whether or not the amino acid residue is part of a secondary structure (α-helix, β-sheet), and if so, whether it is helix or sheet. The physical origin of the different chemical shifts of atomic nuclei in α-helices versus β-sheets is a problem of long standing. We report that the chemical shift contributions arising from secondary structure (secondary structure shifts) depend strongly on the extent of exposure to solvent. This behavior is observed for1Hα,13Cαand13Cβ(sheet), but not for13Cβ(helix), whose secondary structure shifts are small. When random coil values are subtracted from the chemical shifts of all1Hαnuclei (Pro residues excluded) and the residual chemical shifts are summed to plot the mean values against solvent exposure, the results give a funnel-shaped curve that approaches a small value at full-solvent exposure. When chemical shifts are plotted instead against Elocal, the electrostatic contribution to conformational energy produced by local dipole-dipole interactions, a well characterized dependence of1Hαchemical shifts on Elocalis found. The slope of this plot varies with both the type of amino acid and the extent of solvent exposure. These results indicate that secondary structure shifts are produced chiefly by the electric field of the protein, which is screened by water dipoles at residues in contact with solvent.