Further studies of the helix dipole model: Effects of a free α‐NH 3 + or α‐COO − group on helix stability

Interactions between the α‐helix peptide dipoles and charged groups close to the ends of the helix were found to be an important determinant of α‐helix stability in a previous study. 1 The charge on the N‐terminal residue of the C‐peptide from ribonuclease A was varied chiefly by changing the α‐NH 2 blocking group, and the correlation of helix stability with N‐terminal charge was demonstrated. An alternative explanation for some of those results is that the succinyl and acetyl blocking groups stabilize the helix by hydrogen bonding to an unsatisfied main‐chain NH group. The helix dipole model is tested here with peptides that contain either a free α‐NH   3 +α‐COO − groups, and no other charged groups that would titrate with similar p K a 's. This model predicts that α‐NH 3 α‐COO‐ groups are helix‐destabilizingand that the destabilizing interactions are electrostatic in origin. The hydrogen bonding model predicts that α‐NH 3 and α‐COO‐ groups are not themselves helix‐destabilizing, but that an acetyl or amide blocking group at the N‐ or C‐ terminus, respectively, stabilizes the helix by hydrogen bonding to an unsatisfied main‐chain NH or CO group. The results are as follows: (1) Removal of the charge from α‐NH 3 and α‐COO‐ groups by pH titration stabilizes an α‐helix. (2) The increase in helix stability on pH titration of these groups is close to the increase produced by adding an acetyl or amide blocking group. (3) The helix‐stabilizing effect of removing the charge from α‐NH 3 and α‐COO‐ groups by pH titration is screened by increasing the NaCl concentration, and therefore the effect is electrostatic in origin. (4) Replacing the C‐terminal amide blocking group with a methylester blocking group, which cannot donate a hydrogen bond, causes little change in helix stability.