The variable effective dielectric constant and the importance of the direction of the peptide dipole moment: An investigation of dipole—dipole interactions in αα- and βαβ-units of proteins

Electrostatic interactions between the peptide dipoles in α-helices and in parallel β-strands have been investigated. The “dielectric screening factor”, or “effective dielectric constant”, has been evaluated for two α-helix dipoles, surrounded by water, using the method of image charges. The effective dielectric “constant” appears to vary considerably. One of the most important factors determining its value is the distance of the charges to the solvent surrounding the helices. The parallel β-sheet has a small, but significant, dipole moment if the direction of the individual peptide dipole moment runs parallel to the NH and CO bonds. Then, a stabilization of βαβ-units as observed in actual proteins occurs by favourable electrostatic interactions between the dipole moments of the α-helix and the β-strands. This interaction energy appears, however, to be very sensitive to the charge distribution selected to represent the peptide dipole moment. Therefore, further investigations of the backbone dipole moment of β-strands and β-sheets are essential in order to obtain correct values for the electrostatic energies of globular protein molecules