Electrical-to-mechanical coupling in purple membranes: Membrane as electrostrictive medium

In this paper, we present acousto-electrical measurements performed on dry films of purple membranes (PM) of Halobacterium salinarium. The purpose of these measurements is to determine the relation between mechanical and elec trical phenomena in bacteriorhodopsin and to define the role of the protein in the proton transfer process. Electrical-to-mechanical coupling in PMs m anifests itself as direct and inverse piezoelectric effects. Measurements p erformed on the samples with different degrees of PM orientation and at var ious values of the externally applied cross-membrane electric field indicat e that piezoelectric phenomena in PMs arise from the electric asymmetry of the membranes, i.e., they originate from electrostriction. Experiments with samples made of oriented PMs allow estimation of the value of the intrinsi c cross-membrane electric field, which is approximate to 10(8) V/m. A hypot hetical model of PM is presented where the electrical-to-mechanical couplin g is suggested to be the main driving force for the proton translocation ag ainst the Coulomb forces acting in the membrane.