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 f unctional group has been undertaken to determine the magnitude of thes e 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 t he middle of the peptide. These peptides have no reactive residues oth er than the Asp side chain carboxylate group. Circular dichroism confi rmed that these peptides adopt helical conformations in aqueous soluti on over a broad pH range. The pK(a) of compound 12, where the Asp resi due 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 sig nificant perturbation was observed at the C-terminus of a helix (compo und 23), despite this being the negative pole of the dipole. We believ e that this carboxylate is drawn toward the N-terminus by electrostati c attraction to the positive pole of the dipole, resulting in position ing of the carboxylate over the middle of the helix lather than over t he C-terminus.