Marked changes in electron transport through the blue copper protein azurin in the solid state upon deuteration (original) (raw)
Related papers
Solid-State Electron Transport via the Protein Azurin is Temperature-Independent Down to 4 K
The Journal of Physical Chemistry Letters, 2019
Solid-state electronic transport, ETp, via the electron transfer copper protein, Azurin (Az), was measured in Au/Az/Au junction configurations down to 4 K, the lowest temperature for solid-state protein-based junctions. Not only does lowering the temperature help observing fine features of electronic transport, but it also limits possible electron transport mechanism(s). Practically, wire-bonded devices-on-chip, carrying Az-based microscopic junctions, were measured in liquid He, minimizing temperature gradients across the samples. Much smaller junctions, in a conductingprobe AFM, served, between room temperature and the protein's denaturation temperature (~323K), to check that conductance behaviour is independent of device configuration or contact nature, and, thus, is a property of the protein itself. Temperature-independent currents were observed from ~320 to 4K. The experimental results were fitted to a single-level Landauer model to extract effective energy barrier and electrode-molecule coupling strength values, and compare data sets. Our results strongly support that quantum tunneling, rather than hopping dominates ETp via Az.
Advanced Science, 2015
Electron transfer (ET) reactions are central to a wide range of biological processes, notably those of energy conversion such as the respiratory and photosynthetic chains. These employ mostly proteins, between which electrons are shuttled from one electron mediator to another by a redox process, driven by a free energy difference (driving force) between or within the mediators. The separation distance over which an electron is transferred within proteins can reach in some cases around 2.5 nm. One of the most intensively studied group of ET proteins is the blue copper proteins (e.g., plastocyanin, azurin, and the family of multi-copper oxidases). [ 1 ] The copper binding sites were shown to confer unique spectroscopic and thermodynamic properties on the copper ions. [ 2 ] These properties attracted studies aiming at understanding their role in the ET via proteins. Here we focus on the bacterial blue copper protein azurin (Az), functioning as an electron mediator in certain bacterial respiratory chains. [ 3 ] Most of the studies of the ET process in Az were done in solution, using spectroscopy, mainly fl ash-quench [ 4 ] or pulse-radiolysis [ 5 ] techniques. A common denominator of these studies is monitoring the ET process between the Cu ion of Az and an intramolecular donor/acceptor. In another experimental approach for measuring intramolecular ET process in Az, the protein is bound to a conductive electrode and ET can be measured between the Cu ion and the electrode as a rate-determining step of an electrochemical redox process in an electrochemical cell. [ 6 ] In contrast, electronic transport (ETp) via the protein has more recently been studied by measuring conductance across Az, trapped between two electrodes in a solid-state confi guration. [ 7 ] One pronounced difference between measuring ET in solution by spectroscopy or electrochemistry and measuring the ETp in a solid-state confi guration is that while the former requires a redox process to occur and, thus, the presence of a redox-active center (such as the Cu ion, a disulphide bond or a bound external Ru complex), the latter does not. Electron transfer (ET) proteins are biomolecules with specifi c functions, selected by evolution. As such they are attractive candidates for use in potential bioelectronic devices. The blue copper protein azurin (Az) is one of the most-studied ET proteins. Traditional spectroscopic, electrochemical, and kinetic methods employed for studying ET to/from the protein's Cu ion have been complemented more recently by studies of electrical conduction through a monolayer of Az in the solid-state, sandwiched between electrodes. As the latter type of measurement does not require involvement of a redox process, it also allows monitoring electronic transport (ETp) via redox-inactive Az-derivatives. Here, results of macroscopic ETp via redox-active and-inactive Az derivatives, i.e., Cu(II) and Cu(I)-Az, apo-Az, Co(II)-Az, Ni(II)-Az, and Zn(II)-Az are reported and compared. It is found that earlier reported temperature independence of ETp via Cu(II)-Az (from 20 K until denaturation) is unique, as ETp via all other derivatives is thermally activated at temperatures >≈200 K. Conduction via Cu(I)-Az shows unexpected temperature dependence >≈200 K, with currents decreasing at positive and increasing at negative bias. Taking all the data together we fi nd a clear compensation effect of Az conduction around the Az denaturation temperature. This compensation can be understood by viewing the Az binding site as an electron trap, unless occupied by Cu(II), as in the native protein, with conduction of the native protein setting the upper transport effi ciency limit. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
2012
We report conducting probe atomic force microscopy (CP-AFM) measurements of electron transport (ETp), as a function of temperature and force, through monolayers of holo-azurin (holo-Az) and Cudepleted Az (apo-Az) that retain only their tightly bound water, immobilized on gold surfaces. The changes in CP-AFM current-voltage (I-V) curves for holo-Az and apo-Az, measured between 250-370K, are strikingly different. While ETp across holo-Az at low force (6 nN) is temperatureindependent over the whole examined range, ETp across apo-Az is thermally activated, with calculated activation energy of 600±100 meV. These results confirm our results of macroscopic contact area ETp measurements via holo-and apo-Az, as a function of temperature, where the crucial role of the Cu redox centre has been observed. While increasing the applied tip force from 6 to 12 nN did not significantly change the temperature dependence of ETp via apo-Az, ETp via holo-Az changed qualitatively, namely from temperature-independent at 6 nN to thermally activated at forces ≥ 9 nN, suggesting changes in the protein structure upon increasing the applied force. The capability of exploring ETp by CP-AFM over a significant range of temperatures, with varying tip force to detect possible pressure-induced changes in the sample, significantly adds to the ability to study ETp through proteins and of using ETp to study proteins, with this approach.
Temperature and Force Dependence of Nanoscale Electron Transport via the Cu Protein Azurin
ACS Nano, 2012
The mechanisms of solid-state electron transport (ETp) via a monolayer of immobilized Azurin (Az) was examined by conducting probe atomic force microscopy (CP-AFM), both as function of temperature (248 -373K) and of applied tip force (6-12 nN). By varying both temperature and force in CP-AFM, we find that the ETp mechanism can alter with a change in the force applied via the tip to the proteins. As the applied force increases, ETp via Az changes from temperature-independent to thermally activated at high temperatures. This is in contrast to the Cu-depleted form of Az (apo-Az),
ACS Nano, 2015
Surprisingly efficient solid-state Electron Transport has recently been demonstrated through "dry" proteins (with only structural, tightly bound H 2 O left), suggesting proteins as promising candidates for molecular (bio)electronics. Using inelastic electron tunneling spectroscopy (IETS), we explored electron-phonon interaction in metal/protein/metal junctions, to help understanding solid-state electronic transport across the redox protein Azurin. To that end an oriented Azurin monolayer on Au is contacted by soft Au electrodes. Characteristic vibrational modes of amide and amino-acid side groups as well as of Az-electrode contact were observed, revealing the Az native conformation in the junction and the critical role of side groups in the charge transport. The lack of abrupt changes in the conductance and the line shape of IETS point to far off-resonance tunneling as the dominant transport mechanism across Azurin, in line with previously reported (and herein confirmed) Azurin junctions. The inelastic current and hence electron-phonon interaction appears to be rather weak and comparable in magnitude with the inelastic fraction of tunneling current via alkyl chains, which may reflect the known structural rigidity of Azurin.
European Journal of Biochemistry, 1992
An intramolecular electron-transfer process has previously been shown to take place between the Cys3 -Cys26 radical-ion (RSSR-) produced pulse radiolytically and the Cu(I1) ion in the blue singlecopper protein, azurin [Farver, 0. & Pecht, I. (1 989) Proc. Nut1 Acad. Sci. USA 86, further investigate the nature of this long-range electron transfcr (LRET) proceeding within the protein matnx, we have now investigated it in two azurins where amino acids have been substituted by single-site mutation of the wild-type Pseudomonas aeruginosa azurin. In one mutated protein, a methionine residue (Met44) that is proximal to the copper coordination sphere has been replaced by a positively charged lysyl residue ([M44K]azurin), while in the second mutant, another residue neighbouring the Cu-coordination site (His35) has been replaced by a glutamine ([H35Q]azurin). Though both these substitutions are not in the microenvironment separating the electron donor and acceptor, they were expected to affect the LRET rate because of their effect on the redox potential of thc copper sitc and thus on the driving force of the reaction, as well as on the reorganization energies of the copper site.
Proteins as Electronic Materials: Electron Transport through Solid-State Protein Monolayer Junctions
Journal of the American Chemical Society, 2010
Electron transfer (ET) through proteins, a fundamental element of many biochemical reactions, has been studied intensively in solution. We report the results of electron transport (ETp) measurements across proteins, sandwiched between two solid electrodes with a long-range goal of understanding in how far protein properties are expressed (and can be utilized) in such a configuration. While most such studies to date were conducted with one or just a few molecules in the junction, we present the high yield, reproducible preparation of large area monolayer junctions of proteins from three different families: Azurin (Az), a blue-copper ET protein, Bacteriorhodopsin (bR), a membrane protein-chromophore complex with a proton pumping function, and Bovine Serum Albumin (BSA). Surprisingly, the currentvoltage (I-V) measurements on such junctions, which are highly reproducible, show relatively minor differences between Az and bR, even though the latter lacks a known ET function. ETp across both Az and bR is much more efficient than across BSA, but also for the latter the currents are still high, and the decay coefficients too low to be consistent with coherent tunneling. Rather, inelastic hopping is proposed to dominate ETp in these junctions. Other features such as asymmetrical I-V curves and distinct behavior of different proteins can be viewed as molecular signatures in the solid-state conductance.
Tunneling explains efficient electron transport via protein junctions
Proceedings of the National Academy of Sciences of the United States of America, 2018
Metalloproteins, proteins containing a transition metal ion cofactor, are electron transfer agents that perform key functions in cells. Inspired by this fact, electron transport across these proteins has been widely studied in solid-state settings, triggering the interest in examining potential use of proteins as building blocks in bioelectronic devices. Here, we report results of low-temperature (10 K) electron transport measurements via monolayer junctions based on the blue copper protein azurin (Az), which strongly suggest quantum tunneling of electrons as the dominant charge transport mechanism. Specifically, we show that, weakening the protein-electrode coupling by introducing a spacer, one can switch the electron transport from off-resonant to resonant tunneling. This is a consequence of reducing the electrode's perturbation of the Cu(II)-localized electronic state, a pattern that has not been observed before in protein-based junctions. Moreover, we identify vibronic featu...
Direct electron transfer between copper-containing proteins and electrodes
Biosensors and Bioelectronics, 2005
The electrochemistry of some copper-containing proteins and enzymes, viz. azurin, galactose oxidase, tyrosinase (catechol oxidase), and the "blue" multicopper oxidases (ascorbate oxidase, bilirubin oxidase, ceruloplasmin, laccase) is reviewed and discussed in conjunction with their basic biochemical and structural characteristics. It is shown that long-range electron transfer between these enzymes and electrodes can be established, and the mechanistic schemes of the DET processes are proposed.