Salt Bridges Do Not Stabilize Polyproline II Helices^†

Interactions between side chains, and in particular salt bridges, have been shown to be important in the stabilization of secondary structure. Here we investigate the contribution of a salt bridge formed between a lysine and a glutamate to the polyproline II (P_II) helical content of proline-rich peptides. Since this structure has precisely three residues per turn, charged residues spaced three residues apart are on the same side of the helix and are best situated to interact. By contrast, computer simulations show that charged residues spaced four residues apart are both too far apart to interact strongly and are oriented such that interactions are unlikely. We have measured the P_II content of peptides containing a lysine and glutamate pair spaced three or four residues apart using circular dichroism spectroscopy. Somewhat surprisingly we find that the P_II content is insensitive to both the spacing and the pH. These findings indicate that i → i + 3 salt bridges do not stabilize the P_II helical conformation. The implications of these observations for both P_II helix formation and denatured protein conformations are discussed.