Effects of the alpha-helix dipole upon the functioning and structure of proteins and peptides

The alpha-helix carries a considerable dipole moment, the effect of which can be approximated by placing 0.5-0.7 positive unit charge near the N-terminus and 0.5-0.7 negative unit charge near the C-terminus of the helix. The effects of the alpha-heliz dipole on the distribution of charged residues along the alpha-helix in globular proteins is discussed as well as the helix dipole assisted anion binding by helices in a variety of proteins. In about 9 enzymes the active site is situated close to the N-terminus of a helix, in such a manner that the electrical field of the dipole may increase reaction rates. In addition, the effects, or postulated effects, of the alpha-helix dipole on the following phenomena are briefly reviewed: (i) the packing of alpha-helices in proteins; (ii) electron, proton and ion conduction along the helix axis; (iii) ion transport along multiple helix channels in membranes; (iv) flow of electrons guided by helix dipole fields in photosynthetic reaction centres; (v) voltage-dependent pore-formation in lipid membranes; (vi) packing of helical peptides in water-free crystals; (vii) stabilisation and destabilisation of oligopoptides by the interaction between charged side chains and the helix dipole; and (viii) a pK shift of a histidine caused by the helix dipole field.