Ion-Pair and Charged Hydrogen-Bond Interactions between Histidine and Aspartate in a Peptide Helix

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Abstract
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 acount for most of their strength.

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