A New Class of Purple Membrane Variants for the Construction of Highly Oriented Membrane Assemblies on the Basis of Noncovalent Interactions

Purple membranes (PM) from Halobacterium salinarum have been discussed for several technical applications. These ideas started just several years after its discovery. The biological function of bacteriorhodopsin (BR), the only protein in PM, is the light-driven proton translocation across the membrane thereby converting light energy into chemical energy. The astonishing physicochemical robustness of this molecular assembly and the ease of its isolation triggered ideas for technical uses. All basic molecular functions of BR, that is, photochromism, photoelectrism, and proton pumping, are key elements for technical applications like optical data processing and data storage, ultrafast light detection and processing, and direct utilization of sunlight in adenosine 5'-triphospate (ATP) generation or seawater desalination. In spite of the efforts of several research groups worldwide, which confirmed the proof-of-principle for all these potential applications, only the photochromism-based applications have reached a technical level. The physical reason for this is that no fixation or orientation of the PMs is required. The situation is quite different for photoelectrism and proton pumping where the macroscopic orientation of PMs is a prerequisite. For proton pumping, in addition, the formation of artificial membranes which prevent passive proton leakage is necessary. In this manuscript, we describe a new class of PM variants with oppositely charged membrane sides which enable an almost 100% orientation on a surface, which is the key element for photoelectric applications of BR. As an example, the mutated BR, BR-E234R7, was prepared and analyzed. A nearly 100% self-orientation on mica was obtained.