An Assessment of the Effect of the Helix Dipole in Protein Structures

The locations of the cations bound to the peptide group at the C-termini and the anions attached to the main-chain NH group at the N-termini of helices are analysed. The ions are hardly found along the helical axis, where the effect due to the helix macrodipole is likely to be the maximum. The disposition of the ions appears to be controlled more by the stereoelectronic requirements of the ligand group rather than any long distance electric field. This and other related structural observations call for some circumspection in assigning a role for the helix dipole in protein structure and function.