Molecular Scale Conductance Photoswitching in Engineered Bacteriorhodopsin

Abstract

Bacteriorhodopsin (BR) is a robust light-driven proton pump embedded in the purple membrane of the extremophilic archae Halobacterium salinarium. Its photoactivity remains in the dry state, making BR of significant interest for nanotechnological use. Here, in a novel configuration, BR was depleted from most of its endogenous lipids and covalently and asymmetrically anchored onto a gold electrode through a strategically located and highly responsive cysteine mutation; BR has no indigenous cysteines. Chemisorption on gold was characterized by surface plasmon resonance, reductive striping voltammetry, ellipsometry, and atomic force microscopy (AFM). For the first time, the conductance of isolated protein trimers, intimately probed by conducting AFM, was reproducibly and reversibly switched under wavelength-specific conditions (mean resistance of 39 ± 12 MΩ under illumination, 137 ± 18 MΩ in the dark), demonstrating a surface stability that is relevant to potential nanodevice applications.

Directed evolution of bacteriorhodopsin for applications in bioelectronics

“…In addition to enhancing the lifetime of different photocycle intermediates of BR, mutagenesis has been employed to enhance the innate dipole moment, gold-binding capabilities and specificity of ion pumping of BR. For a number of device platforms and architectures, the ability of BR to bind to gold is critical because gold is a chemically inert and electrically conductive [112][113][114]. The native protein contains no cysteine residues and thus the strategic addition of cysteine residues in the loop regions of BR via site-directed mutagenesis allows BR to rsif.royalsocietypublishing.org J R Soc Interface 10: 20130197 covalently bind to gold.…”

Spin-dependent electron transmission through bacteriorhodopsin embedded in purple membrane

“…Electron conduction through these purple membranes was measured recently (9) in ambient and as a function of temperature (10). Electrochemical and conduction studies were also performed when the purple membranes were deposited on gold (11).…”

Chirality – Beyond the Structural Effects

“…Specifically, purple membrane, which contains a mutant of bacteriorhodopsin (bR), can serve as a light‐activated spin switch. This system has been previously suggested as a candidate for bio‐electronics, and in a former study, wild‐type bR was shown to have spin‐filtering properties . Spin‐dependent electrochemical studies were conducted on purple membrane containing a mutant of bacteriorhodopsin (bR) adsorbed on nickel substrates .…”

“…An interesting aspect of usingC ISS for spintronics is applying biosystemsa sac omponent in electronic devices like spin valves.B ecause of its stable structure and ease of combining with inorganic interfaces, the use of bacteriorhodopsin (bR)w as investigated in detail among other combinations,b ecause of the possibility to use bR for controlling spintronics with light. [14] Specifically,p urple membrane,w hich contains am utant of bacteriorhodopsin (bR), can servea salight-activateds pin switch.T his system has been previously suggested as ac andidate for bio-electronics [15,16] and in af ormers tudy,w ild-type bR was shown to have spin-filtering properties. [17] Spin-dependent electrochemicals tudies were conducted on purple membrane containingamutant of bacteriorhodopsin (bR) adsorbed on nickel substrates.…”