Energetics of polar side-chain interactions in helical peptides: salt effects on ion pairs and hydrogen bonds

The energetics of the interaction between the polar side chains of glutamate or aspartate with lysine and glutamate with histidine have been determined using a model alanine-based peptide helix. An evaluation of the effects of NaCl and pH on the interactions between these acidic and basic residues in several different orientations and spacings in an alpha-helical peptide has been made. For many of the peptides, we find a considerable interaction between the polar side chains. In general, the shorter side chains show stronger interactions, but there are more restrictions on the precise geometry of the interactions as dictated by the spacing and orientation of the polar residues in the alpha-helical peptide. The energetics of the interaction between the fully-charged ion pairs can be diminished by added salt, but the interaction is not completely screened even at 2.5 M NaCl. The strength of the interaction between a charged and neutral side chain is not as sensitive to the ionic strength of the solution, suggesting that solvent-exposed hydrogen bonds are forming. All the interactions between the polar residues employed here stabilize helix formation, suggesting that solvent-exposed ion pairs and hydrogen bonds can contribute to the conformational stability of proteins and peptides.