Nanoscale electrical conductivity of the purple membrane monolayer

Abstract

Nanoscale electron transport through the purple membrane monolayer, a two-dimensional crystal lattice of the transmembrane protein bacteriorhodopsin, is studied by conductive atomic force microscopy. We demonstrate that the purple membrane exhibits nonresonant tunneling transport, with two characteristic tunneling regimes depending on the applied voltage ͑direct and Fowler-Nordheim͒. Our results show that the purple membrane can carry significant current density at the nanometer scale, several orders of magnitude larger than previously estimated by macroscale measurements. DOI: 10.1103/PhysRevE.76.041919 PACS number͑s͒: 87.80.Ϫy, 73.40.Rw, 85.65.ϩh The purple membrane ͑PM͒ is a two-dimensional crystal lattice naturally present in the cell membrane of Halobacterium salinarum. It is composed of a lipid bilayer and a single-protein species, the Bacteriorhodopsin ͑bR͒, in a lipidto-protein ratio of 10 ͑mol/ mol͒. Bacteriorhodopsin acts as a light-driven proton pump, converting solar energy into an electrochemical proton gradient across the cell membrane ͓1,2͔. Its functional stability under different environmental conditions combined with easy and large production has made bR a model protein for studies of charge transport on cell membranes, as well as an excellent candidate for bioelectronic applications ͓3,4͔.Despite its enormous interest, only a few studies regarding the electron transport measurements of a single PM layer have been reported so far, leading to an incomplete and controversial picture ͓5-7͔. The main obstacle encountered in measuring the electrical conductivity of the PM monolayer ͑ϳ5 nm thick͒ resides on providing reliable electrical contact at the electrode-membrane interface. Measured currents can dramatically differ by orders of magnitude from measurement to measurement on supposedly identical conditions, being extremely sensitive to the electrode-membrane distance as well as the applied load on the membrane. To date, two methods have been reported: ͑i͒ measuring the current of a monolayer confined between two submillimeter-sized electrodes ͓5͔ and ͑ii͒ probing the nanoscale conductivity of the monolayer using a scanning tunneling microscope ͑STM͒ ͓6,7͔. The millimeter-sized electrode configuration demands a flat and hole-free monolayer covering the entire electrode surface, which is, however, difficult to fabricate. Furthermore, averaging of biological information on a macroscale level is inherent to this method. To study electron conduction at the molecular level, scanning probe techniques are by far the most appropriate approach. STM, however, has an intrinsic limit in the tunneling current feedback for insulating samples which impedes the control and quantification of the probe-membrane distance and forces applied on the biomolecules.In this article we use conductive atomic force microscopy ͑C-AFM͒ as an extremely controlled method to provide a comprehensive and unambiguous model of electron conduction in the PM monolayer. Conductive AFM has demonstrated to be well suited to studying th...

Charge transport in bacteriorhodopsin monolayers: The contribution of conformational change to current-voltage characteristics

“…In the slow transition from this state to the M 410 , a proton is released. Junctions prepared with bR monolayers contacted with Au electrodes, show non-linear I-V characteristics both in the dark (or blue light) and in the presence of green light [3][4][5]. It is generally assumed that green light produces a more intense current as a consequence of the modification of the chromophore-opsin complex.…”

“…(v) When the chromophore of the protein is completely removed, the conductivity is suppressed by three orders of magnitude lower, taking the value of a standard dielectric. Despite the presence of these experiments, the existing theoretical approaches only provide a phenomenological interpretation of some I-V characteristics in terms of metal-insulatormetal tunneling theories [5,6].…”

“…-In recent years, several papers reported on the electrical properties of single proteins inserted into hybrid systems [1][2][3][4][5]. The principal aim of these researches is to explore the possibility to design nanodevices, mainly nanobiosensors, with extreme sensibility and specificity.…”

Current-voltage characteristics of seven-helix proteins from a cubic array of amino acids

“…To this purpose, we have investigated three proteins belonging to the transmembrane family whose model of the tertiary structure (hereafter simply indicated as the 3D structure) is known [15,20]. For one of them, the bacteriorhodopsin (bR), the I-V characteristic is also available from experiments [14,21]. For these proteins the I-V characteristic has been calculated using an impedance network protein analogue (INPA) [10,18].…”

Electrical transport through ordered self-assembled protein monolayer measured by constant force conductive atomic force microscopy

“…4 Biomolecules open also the possibility to combine biological functions with nanoelectronics. 4,6,7 One of the many challenges of combining proteins with the nonbiological "nanoworld" is the lack of methods to control protein assembly and to ensure good electrical contact between molecules and electrodes. 8,9 The contacting becomes especially difficult with soft materials such as proteins because they may deform under contact forces.…”