Oriented Assembly of Purple Membrane on Solid Support, Mediated by Molecular Recognition

Fusion of purple membranes triggered by immobilization on carbon nanomembranes

A freestanding ultrathin hybrid membrane was synthesized comprising two functional layers, that is, first, a carbon nanomembrane (CNM) produced by electron irradiation-induced cross-linking of a self-assembled monolayer (SAM) of 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) and second, purple membrane (PM) containing genetically modified bacteriorhodopsin (BR) carrying a C-terminal His-tag. The NBPT-CNM was further modified to carry nitrilotriacetic acid (NTA) terminal groups for the interaction with the His-tagged PMs forming a quasi-monolayer of His-tagged PM on top of the CNM-NTA. The formation of the Ni-NTA/His-tag complex leads to the unidirectional orientation of PM on the CNM substrate. Electrophoretic sedimentation was employed to optimize the surface coverage and to close gaps between the PM patches. This procedure for the immobilization of oriented dense PM facilitates the spontaneous fusion of individual PM patches, forming larger membrane areas. This is, to our knowledge, the very first procedure described to induce the oriented fusion of PM on a solid support. The resulting hybrid membrane has a potential application as a light-driven two-dimensional proton-pumping membrane, for instance, for light-driven seawater desalination as envisioned soon after the discovery of PM.

A Review on Bacteriorhodopsin-Based Bioelectronic Devices

Bacteriorhodopsin protein extracted from Halobacterium salinarum is widely used in many biohybrid electronic devices and forms a research subject known as bioelectronics, which merges biology with electronic technique. The specific molecule structure and components of bR lead to its unique photocycle characteristic, which consists of several intermediates (bR, K, L, M, N, and O) and results in proton pump function. In this review, working principles and properties of bacteriorhodopsin are briefly introduced, as well as bR layer preparation method. After that, different bR-based devices divided into photochemical and photoelectric applications are shown. Finally, outlook and conclusions are drawn to inspire new design of high-performance bR-based biohybrid electronic devices.

Detergent-resistant oligomeric Leptosphaeria rhodopsin is a promising bio-nanomaterial and an alternative to bacteriorhodopsin

Bacteriorhodopsin has attracted remarkable attention as a photoactive bio-nanomaterial in the last decades. However, its instability in the presence of detergents has restricted the extent to which bacteriorhodopsin may be applied. In this study, we investigated the oligomerization of a eukaryotic light-driven H⁺-pump, Leptosphaeria rhodopsin, using circular dichroism spectroscopy and other biophysical and biochemical methods. Our findings revealed that Leptosphaeria rhodopsin assembled into oligomers in the cell membrane and also in 0.05% DDM detergent micelles. Moreover, unlike bacteriorhodopsin in purple membrane, Leptosphaeria rhodopsin retained its oligomeric structure in 1% Triton X-100 and demonstrated strong resistance to other common detergents. A maximal photocurrent density of ∼85 nA/cm² was consistently generated, which was substantially larger than that of solubilized bacteriorhodopsin (∼10 nA/cm²). Therefore, oligomeric Leptosphaeria rhodopsin may be a promising bio-nanomaterial, and an alternative to bacteriorhodopsin, especially with the use of detergents.

Selective electroless deposition of metal clusters on solid-supported bacteriorhodopsin: applications to orientation labeling and electrical contacts

Controlling the orientation of bacteriorhodopsin (bR) monolayers is an important step in studying and utilizing such membranes in a solid-state configuration in, for example, photoelectric applications. Macroscopic monolayers of bR have been fabricated in a variety of ways, but characterization of the distribution of the two possible orientations in which the membrane fragments can adsorb has not yet been addressed experimentally. Here, an approach is presented that labels only one of the membrane surfaces by electroless growth of metal nanoparticles on top of the solid-supported membranes. In this way, it is possible to observe which surface of the membranes is actually adsorbed to the substrate. How this technique serves to interface the membranes with a top metal contact for further electrical measurements is also demonstrated.

Asymmetric distribution of biotin labeling on the purple membrane

This work examined the biotin modification of bacteriorhodopsin (BR) in the purple membrane (PM). The results of flash kinetic absorption measurements showed that photocycle was maintained in biotinylated BR. Biotinylated BR also maintained its photoelectric activity, as indicated by the photoelectric response of the bilayer lipid membrane (BLM). Atomic force microscopy (AFM) of stretavidiin-bound biotin revealed that biotin molecules covered both surfaces of the, but the amount of biotinylated BR on the extracellular (EC) surface was markedly higher than on the cytoplasmic (CP) surface. Further studies showed that, after reaction with fluorescamine (FL), biotin labeling occurred only on the CP surface. These results are informative for future work on bioconjugation of BR as well as work on oriented assembly and the design of BR-based photoelectric devices.

Covalent attachment of bacteriorhodopsin monolayer to bromo-terminated solid supports: preparation, characterization, and protein stability

The interfacing of functional proteins with solid supports and the study of related protein-adsorption behavior are promising and important for potential device applications. In this study, we describe the preparation of bacteriorhodopsin (bR) monolayers on Br-terminated solid supports through covalent attachment. The bonding, by chemical reaction of the exposed free amine groups of bR with the pendant Br group of the chemically modified solid surface, was confirmed both by negative AFM results obtained when acetylated bR (instead of native bR) was used as a control and by weak bands observed at around 1610 cm(-1) in the FTIR spectrum. The coverage of the resultant bR monolayer was significantly increased by changing the pH of the purple-membrane suspension from 9.2 to 6.8. Although bR, which is an exceptionally stable protein, showed a pronounced loss of its photoactivity in these bR monolayers, it retained full photoactivity after covalent binding to Br-terminated alkyls in solution. Several characterization methods, including atomic force microscopy (AFM), contact potential difference (CPD) measurements, and UV/Vis and Fourier transform infrared (FTIR) spectroscopy, verified that these bR monolayers behaved significantly different from native bR. Current-voltage (I-V) measurements (and optical absorption spectroscopy) suggest that the retinal chromophore is probably still present in the protein, whereas the UV/Vis spectrum suggests that it lacks the characteristic covalent protonated Schiff base linkage. This finding sheds light on the unique interactions of biomolecules with solid surfaces and may be significant for the design of protein-containing device structures.

A nanomechanical device based on light-driven proton pumps

In this paper, a hybrid device based on a microcantilever interfaced with bacteriorhodopsin (bR) is constructed. The microcantilever, on which the highly oriented bR film is self-assembled, undergoes controllable and reversible bending when the light-driven proton pump protein, bR, on the microcantilever surface is activated by visible light. Several control experiments are carried out to preclude the influence of heat and photothermal effects. It is shown that the nanomechanical motion is induced by the resulting gradient of protons, which are transported from the KCl solution on the cytoplasmic side of the bR film towards the extracellular side of the bR film. Along with a simple physical interpretation, the microfabricated cantilever interfaced with the organized molecular film of bR can simulate the natural machinery in converting solar energy to mechanical energy.

Chemically induced enhancement of the opto-electronic response of Halobacterium purple membrane monolayer

Acetylation of purple membranes (PM) significantly enhances the surface photovoltage that they exhibit, if adsorbed as a monolayer on a solid surface; we suggest that this increase is due to the improved orientation of the PM on the surface.

Photocurrents generated by bacteriorhodopsin adsorbed on nano-black lipid membranes

Purple membranes were adsorbed on freestanding lipid bilayers, termed nano-black lipid membranes (nano-BLMs), suspending the pores of porous alumina substrates with average pore diameters of 280 nm. Nano-BLMs were obtained by first coating the upper surface of the highly ordered porous alumina substrates with a thin gold layer followed by chemisorption of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol and subsequent addition of a droplet of 1,2-diphytanoyl-sn-glycero-3-phosphocholine and octadecylamine dissolved in n-decane onto the hydrophobic submonolayer. By means of impedance spectroscopy, the quality of the nano-BLMs was verified. The electrical parameters confirm the formation of single lipid bilayers with high membrane resistances covering the porous matrix. Adsorption of purple membranes on the nano-BLMs was followed by recording the photocurrents generated by bacteriorhodopsin upon continuous light illumination. The membrane system exhibits a very high long-term stability with the advantage that not only transient but also stationary currents are recordable. By adding the proton ionophore carbonyl cyanide-m-chlorophenylhydrazone the conductivity of the nano-BLMs increases, resulting in a higher stationary current, which proves that proton conductance occurs across the nano-BLMs.