The effect of a peptide helix macrodipole on the pK(a) of an Asp side chain carboxylate

A study of the effect of a helix dipole on the pK(a) of a side chain functional group has been undertaken to determine the magnitude of these electrostatic effects in the absence of interfering influences from a protein matrix. Three helical peptides were prepared: two containing Asp residues at the N- or C-terminus and one with an Asp residue in the middle of the peptide. These peptides have no reactive residues other than the Asp side chain carboxylate group. Circular dichroism confirmed that these peptides adopt helical conformations in aqueous solution over a broad pH range. The pK(a) of compound 12, where the Asp residue is at the N-terminus of the helix, is 3.81 ± 0.31. This is lower than the pK(a) of an Asp residue in a short nonhelical model compound (4.09 ± 0.21) and lower than the pK(a) values of 23, where the Asp residue is at the C-terminus of a helix (4.17 ± 0.24), and 19, where the Asp residue is in the middle of the helix (4.17 ± 0.29). No significant perturbation was observed at the C-terminus of a helix (compound 23), despite this being the negative pole of the dipole. We believe that this carboxylate is drawn toward the N-terminus by electrostatic attraction to the positive pole of the dipole, resulting in positioning of the carboxylate over the middle of the helix rather than over the C-terminus.