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 electrical phenomena in bacteriorhodopsin and to define the role of the protein in the proton transfer process. Electrical-to-mechanical coupling in PMs manifests itself as direct and inverse piezoelectric effects. Measurements performed on the samples with different degrees of PM orientation and at various values of the externally applied cross-membrane electric field indicate 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 intrinsic cross-membrane electric field, which is approximately 10(8) V/m. A hypothetical model of PM is presented where the electrical-to-mechanical coupling is suggested to be the main driving force for the proton translocation against the Coulomb forces acting in the membrane.

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Bacteriorhodopsin is a photoactive protein performing the transmembrane proton pumping through the purple membrane of Halobacterium salinarum. Experimental results of the electrical studies of the dried purple membrane films excited by short light pulses are presented. The time constant of the photoelectric response of the purple membrane film corresponds to the optically detectable L intermediate lifetime that is tens of micro seconds. Absence of the positive part of the photoelectric response signal in the time range of tens of microseconds under acidic conditions supports the assumption about the possibility of blockage of the proton transfer. The polarization field is a stimulating factor of the active proton transfer according to the assumption of the suggested two-state model. The mechanical-electrical properties of the dipole materials and the piezoelectric effect of the hydrogen bonds are discussed in the context of the zwitterionic state of the Schiff base and its counter ion Asp85. On the basis of the recent crystallographic data and molecular dynamics simulations it is concluded that the polarization of the Schiff base is a consequence of the mechanically strained hydrogen bonds caused by the retinal photoisomerization. The reorganized H-bond network impedes the proton way back, and the proton accomplishes work while moving in the external circuit.