Ion-pair and charged hydrogen-bond interactions between histidine and aspartate in a peptide helix

The effect on helix stability of placing a single pair of His-Asp or Asp-His residues, spaced (i, i + 3), (i, i + 4), or (i, i + 5), in an alanine-based peptide has been determined. The peptides have identical amino acid compositions, intrinsic helix propensities, and closely similar charge-helix dipole interactions, but they have different side chain interactions. Their helix contents are measured by circular dichroism over the pH range of 2-9, and the strength of a particular side chain interaction is determined from the increase in helix content over the reference peptide with the (i, i + 5) spacing. Side chain interactions are found for both the (i, i + 3) and (i, i + 4) spacings but only in the His-Asp orientation. Charged hydrogen-bond interactions occur at extreme pH values, and they are almost as strong as the ion-pair interactions at pH 5.5; but only the (i, i + 4) His-Asp peptide forms a strong hydrogen bond at pH 2, and only the (i, i + 3) peptide forms a strong hydrogen bond at pH 8.5. The ion-pair interactions are not screened effectively by 1 M NaCl, and hydrogen bonds probably account for most of their strength.