Charged histidine affects alpha-helix stability at all positions in the helix by interacting with the backbone charges.

To determine whether a charged histidine side chain affects alpha-helix stability only when histidine is close to one end of the helix or also when it is in the central region, we substitute a single histidine residue at many positions in two reference peptides and measure helix stability and histidine pKa. The position of a charged histidine residue has a major effect on helix stability in 0.01 M NaCl: the helix content of a 17-residue peptide is 24% when histidine is at position 3 compared to 76% when it is at position 17. This dependence of helix content on histidine position decreases sharply in 1 M NaCl, as expected for counterion screening of the charge-helix dipole interaction. Results at interior positions indicate that the position of a charged histidine residue affects helix stability at these positions. Unexpectedly high values of the helix content are found when either neutral or charged histidine is at one of the last three C-terminal positions, suggesting that either form can stabilize an isolated helix by hydrogen bonding to a main-chain CO group.