Imaging the surface potential of active purple membrane

Bacteriorhodopsin, the light-driven proton pumping protein in the purple membrane of Halobacterium salinarium, is known to be fully functional in various environments. Because of this, the protein's potential for technological application has been recognized. To investigate bacteriorhodopsin's light-induced surface potential changes in humid air, we modified our commercial atomic force microscope to allow imaging at controlled humidities and toggling between defined lighting conditions. The purple membrane patches are prepared from solution onto freshly cleaved highly oriented pyrolytic graphite and imaged in the surface potential detection mode, also called Kelvin probe force microscopy, to identify their topography and surface potential in parallel. Precise tuning of the detection mechanism is crucial to allow imaging of the relatively small potential differences on the purple membrane (in the millivolt range). Our results show that the two orientations of the purple membrane can be identified by their surface potential. This is verified by high-resolution topography images of the patches. Further experiments also show that, for the first time, it was possible to toggle the surface potential of the purple membrane patches by 3-5 mV when imaging the sample in dark or illuminated surrounding in humid air. Copyright (C) 2002 John Wiley Sons, Ltd.