Electric field-induced structural rearrangements in biomembranes

Abstract

Structure and function of biological membranes are vitally dependent on the electric membrane polarization; prolonged depolarization of the plasma membrane causes cell death. The natural cross-membrane electric field affecting the protein/lipid dielectrics are on the order of 100 kVcm_ t. The inhomogeneous field originating from ionic groups and adsorbed small ions are of the same order of magnitude; these fields are restricted to the interfacial compartments adjacent to the membrane surface. Externally applied electromagnetic fields which are strong enough to compete with the intrinsic membrane fields, cause structural rearrangements in the proteins and in the lipid bilayer parts[l-61. For example, electro-optic data of aqueous suspensions of purple membranes indicate that bacteriorhodopsin exhibits conformational flexibility in electric field pulses (l-30 kV cmi, l-100 ps). The electric dichroism shows two kinetically different structural transitions within the protein molecule[l]. The electrically induced rearrangements comprise a rapid (r z 1 ps), but concerted, change in the orientation of both retinal and tyrosine and/or tryptophan side chains. These angular changes of position are accompanied by changes in the local protein environment of the chromophores. A slower relaxation mode (7 x 100 p(s) involves alterations in the microenvironment of aromatic amino acid residues and is accompanied by pK-changes of at least two types of proton binding sites, leading to a sequential uptake and release of protons. Light scattering data are consisitent with the maintenance of the random distribution of the membrane discs within the short duration of the applied electric fields. The kinetics of the electro-optic signals and the steep dependence of the relaxation amplitudes on the electric field strength suggest a saturable induceddipole mechanism and a rather large reaction dipole moment 0E