Helical peptides with three pairs of Asp‐Arg and Glu‐Arg residues in different orientations and spacings

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Abstract
The helix‐stabilizing effects of repeating pairs of Asp‐Arg and Glu‐Arg residues have been characterized using a peptide system of the same design used earlier to study Glu‐Lys (Marqusee, S. & Baldwin, R.L., 1987, Proc. Natl. Acad. Sci. USA 84, 8898–8902) and Asp‐Lys ion pairs (Marqusee, S. & Baldwin, R.L., 1990, In Protein Folding [Gierasch, L.M. & King, J., Eds.], pp. 85–94, AAAS, Washington, D.C.). The consequences of breaking ion pair and charge‐helix dipole interactions by titration to pH 2 have been compared with the results of screening these interactions with NaCl at pH 7.0 and pH 2.5. The four peptides in each set contain three pairs of acidic (A) and basic (B) residues spaced either i, i + 4 or i, i + 3 apart. In one peptide of each kind the pairwise order of residues is AB, with the charges oriented favorably to the helix macrodipole, and in the other peptide the order is BA.The results are as follows: (1) Remarkably, both Asp‐Arg and Glu‐Arg peptides show the same pattern of helix stabilization at pH 7.0 found earlier for Glu‐Lys and Asp‐Lys peptides: i + 4 AB > i + 4 BA = i + 3 AB > i + 3 BA. (2) The ion pairs and charge‐helix dipole interactions cannot be cleanly separated, but the results suggest that both interactions make important contributions to helix stability. Because the four peptides of each set have nearly identical compositions, the large differences in helix content within each set cannot be caused by differences in length and sequence. (3) Significant differences in helix content remain at 5 M NaCl, where ion pair interactions are completely screened. The residual interactions may either be nonscreenable H‐bond interactions or remaining charge‐helix dipole interactions.