Bacteriorhodopsin (bR) as an electronic conduction medium: Current transport through bR-containing monolayers

Abstract

Studying electron transport (ET) through proteins is hampered by achieving reproducible experimental configurations, particularly electronic contacts to the proteins. The transmembrane protein bacteriorhodopsin (bR), a natural light-activated proton pump in purple membranes of Halobacterium salinarum, is well studied for biomolecular electronics because of its sturdiness over a wide range of conditions. To date, related studies of dry bR systems focused on photovoltage generation and photoconduction with multilayers, rather than on the ET ability of bR, which is understandable because ET across 5-nm-thick, apparently insulating membranes is not obvious. Here we show that electronic current passes through bR-containing artificial lipid bilayers in solid ''electrode-bilayer-electrode'' structures and that the current through the protein is more than four orders of magnitude higher than would be estimated for direct tunneling through 5-nm, water-free peptides. We find that ET occurs only if retinal or a close analogue is present in the protein. As long as the retinal can isomerize after light absorption, there is a photo-ET effect. The contribution of light-driven proton pumping to the steady-state photocurrents is negligible. Possible implications in view of the suggested early evolutionary origin of halobacteria are noted. molecular electronics ͉ vesicles ͉ bioelectronics B acteriorhodopsin (bR) is a protein-chromophore complex that serves as a light-driven proton pump in the purple membrane (PM) of Halobacterium salinarum (1). It has been shown that the protein is composed of seven transmembrane helices with a retinal chromophore covalently bound in the central region via a protonated Schiff base to a lysine residue (Fig. 1A). The PM is organized in a 2D hexagonal crystal lattice with a unit cell dimension of Ϸ6.2 nm. Electron crystallography has indicated that bR is organized into trimers in which lipids mediate intertrimer contact (2). Light absorption by bR initiates a multistep reaction cycle with several distinct spectroscopic intermediates: J 625 , K 590 , L 550 , M 412 , N 560 , and O 640 . More details on the molecular alterations that occur during the photocycle were recently obtained from x-ray diffraction studies (see ref. 3 for a recent review). The light-adapted form of bR contains only all-trans retinal, whereas the dark-adapted form contains a 1:1 mixture of 13-cis and all-trans (4). Because of its long-term stability against thermal, chemical, and photochemical degradation and its desirable photoelectric and photochromic properties, bR has attracted much interest as a material for biooptics and bioelectronics (5). Most of these efforts focused on multilayers and their photovoltage͞photocurrent generation (6-8) and photoconduction (9).In principle, PM patches (Ϸ5 nm thick, a few m in size; see Fig. 1B) can serve as a model protein material that is important for both planar junction fabrication and current transport measurements, because the 5-nm membrane is well beyond the thickness over which tu...

Molecular Scale Conductance Photoswitching in Engineered Bacteriorhodopsin

“…Despite this, recent analyses have concluded that the lipidated protein can sustain current magnitudes comparable to those observable with conjugated organics of comparable length. 30 The probe-independent observations we report herein not only highlight the role of the retinal moiety in mediating electronic conductance through BR 20 but also the significance of partial delipidation in facilitating a more robust coupling of this conduit to electrodes. The combined effects of partial delipidation and chemisorption on BR electrical properties, long-term stability and on the different intermediates of its photocycle are currently under study.…”

“…20−22 Attempts to engage monolayers of BR in the native PM for electronic measurements, through the use of either lithographically fabricated contacts or C-AFM probes, have led to reported junction resistances in the TΩ range 23 and low (pA range) currents per trimer. 20,23,24 The photoactivity of BR is retained in dried films of PM, 25,26 making it an attractive material for developing biomolecular devices, such as photodetectors or optical memories. 7,27,28 In all previous work, to the best of our knowledge, the protein photoresponse has been analyzed in the PM, within relatively large (micrometer sized) patches or in PM dispersions contained in wells.…”

Spin-dependent electron transport in protein-like single-helical molecules

“…All of these results seem to be inconsistent with previous experiments' conclusions that the single-stranded helical molecules, such as ssDNA, may not polarize the electrons (5). We note that the electron transport/transfer has been widely investigated in many proteins (19)(20)(21)(22)(23)(24)(25)(26). However, to our knowledge, the underlying physics is still unclear for spin-selective phenomenon observed in the α-helical protein and for the contradictory behaviors between the protein and the ssDNA.…”

Cellular and organellar membrane-associated proteins in haloarchaea: Perspectives on the physiological significance and biotechnological applications

“…BR possesses an extremely stable hexagonal lattice structure, and is resistant against chemical, thermal and photochemical degradation [68], which makes it a potentially optimal biomaterial for use in biological devices under conditions of high temperature, strong light intensity and high ionic strength. More importantly, the optical properties of BR, and its ability to form two-dimensional films are being investigated for use in optical nanodevices, including exploitation of the photochromic properties of BR, which mainly involves interchange of the initial B and M states.…”

“…The light-adapted form contains only all-trans retinal whereas the dark-adapted form contains equal amounts of 13-cis and all-trans retinal. Both forms go through a cyclic photoreaction, but only the all-trans retinal-containing BR translocates protons across the membrane [67,68]. Briefly, proton translocation by BR can be described as follows.…”