PROTEINS: Structure, Function, and Genetics 5:1-7(1989) Further Studies of the Helix Dipole Model: Effects of a Free a-NHQ+ or a-COO- Group on Helix Stability

ABSTRACT Interactions between the a-helix peptide dipoles and charged groups close to the ends of the helix were found to be an important determinant of a-helix stability in a previous study. ’ The charge on the N-terminal residue of the C-peptide from ribonuclease A was varied chiefly by changing the a-NH2 block-ing 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 di-pole model is tested here with peptides that con-tain either a free a-NH3+ or a-COO- group, and no other charged groups that would titrate with similar pK,’s. This model predicts that a-NH3+ and a-COO- groups are helix-destabilizing and that the destabilizing interactions are electro-static in origin. The hydrogen bonding model predicts that a-NH3+ and a-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 hy-drogen bonding to an unsatisfied main-chain NH or CO group. The results are as follows: (1) Removal of the charge from a-NH3+ and a-COO- groups by pH titration stabilizes an a-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 a-NH3+ and a-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 methyl-ester blocking group, which cannot donate a hydrogen bond, causes little change in helix stability