Nanoscale Electron Transport and Photodynamics Enhancement in Lipid-Depleted Bacteriorhodopsin Monomers

Abstract

Potential future use of bacteriorhodopsin (bR) as a solid-state electron transport (ETp) material requires the highest possible active protein concentration. To that end we prepared stable monolayers of protein-enriched bR on a conducting HOPG substrate by lipid depletion of the native bR. The ETp properties of this construct were then investigated using conducting probe atomic force microscopy at low bias, both in the ground dark state and in the M-like intermediate configuration, formed upon excitation by green light. Photoconductance modulation was observed upon green and blue light excitation, demonstrating the potential of these monolayers as optoelectronic building blocks. To correlate protein structural changes with the observed behavior, measurements were made as a function of pressure under the AFM tip, as well as humidity. The junction conductance is reversible under pressure changes up to ∼300 MPa, but above this pressure the conductance drops irreversibly. ETp efficiency is enhanced significantly at >60% relative humidity, without changing the relative photoactivity significantly. These observations are ascribed to changes in protein conformation and flexibility and suggest that improved electron transport pathways can be generated through formation of a hydrogen-bonding network.

Structure dependent spin selectivity in electron transport through oligopeptides

“…The same concept was applied in the past both with CP-AFM and STM for investigating length dependent [28][29][30][31][32] and force dependent conduction. 4,[33][34][35][36] The present study demonstrates the use of CP-AFM to study the spin dependent electron transfer through oligopeptide monolayers. A magnetic tip is used in the present case, unlike conventional CP-AFM, to study the spin specific electron conduction by tuning the magnetic polarity of the tip.…”

Protein bioelectronics: a review of what we do and do not know

“…Under higher applied forces ( 10 nN), the protein exhibits structural deformations and the change in conductance with force is no longer reversible, i.e., plastic deformation occurs ( Figure 18). 365 Force-dependent ETp investigations of Az, bR, CYP2C9, and other proteins after immobilizing onto gold substrates, have demonstrated added protein stability in the presence of cofactors or small molecule binders. 146 The response to an applied force of the conductance of an Az monolayer under cyclic mechanical loading (with CP-AFM) resembled memristive behavior, which could be modeled empirically using the viscoelastic property of the protein.…”

“…473 To enhance photoconduction across bR-containing monolayers, ETp was probed across higher protein concentrations in partially delipidated WT-bR, where ∼75% of the PM lipids were removed. 365 bR was immobilized as a monolayer, taking advantage of interactions of the WT surfaces, which are normally in contact with lipids, with the surface of highly ordered pyrolytic graphite (HOPG) used as a conducting substrate (Figure 33a).…”

Mechanisms responsible for the photocurrent in bacteriorhodopsin

“…Patches of PM have been used for several purposes: to produce metal-protein-metal junctions [10][11][12], to perform c-AFM investigations [13,14], to develop solar cells of new generation [15], etc. As a matter of fact, films of bR resist to thermal, electrical and also mechanical stress [10][11][12][13][14] and show a substantial photocurrent when irradiated by a visible (green) light [10,12,16].…”

“…Patches of PM have been used for several purposes: to produce metal-protein-metal junctions [10][11][12], to perform c-AFM investigations [13,14], to develop solar cells of new generation [15], etc. As a matter of fact, films of bR resist to thermal, electrical and also mechanical stress [10][11][12][13][14] and show a substantial photocurrent when irradiated by a visible (green) light [10,12,16]. Therefore, bR can be used as an optoelectrical switch, to convert radiant energy into electrical energy [15], in pollutants remediation systems [17], to produce optical memories [18], to control neuronal and tissue activity [19,20], etc.…”