Introduction of a method for three-dimensional mapping of the charge motion in bacteriorhodopsin

Electric signals associated with the photocycle of bacteriorhodopsin carry valuable information about the proton transport process. Photocurrents measured by different experimental methods are interpreted in terms of intramolecular charge displacements. Permanent electrical asymmetry of the sample is considered to be a prerequisite for the detection of electric signals. The various photoelectric measuring techniques can be distinguished by the way of achievement of this asymmetry. A common feature of the available methods, however, is that the samples are cylindrically symmetric. Consequently, intramembraneous charge displacements can normally be monitored only along the axis of the membrane normal. We developed a novel method that allows also the detection of the in-plane components of the charge displacements. Samples containing oriented purple membrane fragments were used in the experiments, and the rotational symmetry was transiently broken via anisotropic excitation of the bR molecules by linearly polarized light. Kinetics of the normal and in-plane components were measured and interpreted as a result of spatial charge displacements associated with the proton transport process in bacteriorhodopsin.