Large Conductance Changes in Peptide Single Molecule Junctions

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Large Conductance Changes in Peptide Single Molecule Junctions Controlled by pH Lisa Scullion,† Thomas Doneux,†,§ Laurent Bouffier,†,|| David G. Fernig,‡ Simon J. Higgins,† Donald Bethell,† and Richard J. Nichols*,† † ‡

Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom Department of Structural and Chemical Biology, University of Liverpool, Liverpool L69 7ZB, United Kingdom ABSTRACT: Because electron transfer is often highly sensitive to bridge length and molecular conformation, changes in these can have a large impact on the conductance of single molecule electrical junctions. In this study, pH is used to control the conformation and effective bridge length of single molecule junctions containing the peptide sequence H(EL)5C (where H stands for histidine, E for glutamic acid, L for leucine, and C for cysteine). The ionizable glutamic acid residues in this peptide result in an oligo-peptide structure highly sensitive to pH. At low pH, the H(EL)5C bridge exists in its more compact R-helical state, while at high pH, deprotonation leads to electrostatic repulsion between the charged carboxylate groups of the glutamic acid residues, promoting more extended conformations. An scanning tunneling microscopy-based method is used to measure the single molecule conductance of Au|H(EL)5C|Au two-terminal junctions in buffered electrolyte solution at low pH (2) and higher pH (6.9). In its more compact R-helical state at low pH, a single molecule conductance of 1.7 nS has been recorded, with the conductance then dropping to below 0.10 nS when the pH is raised to 6.9. This large pH-controlled drop in conductance shows that oligo-peptides can provide particularly sensitive motifs for controlling long-range electron transfer.

’ INTRODUCTION The study of charge transfer across molecular junctions has been given great impetus in the past decade by the development of new experimental methods for forming single molecular junctions and advanced theoretical routines for computing the conductance of such junctions.1 Prior to this, direct electrical measurements could only be made on bulk materials or on thin films. Single molecule techniques have enabled the determination of the intrinsic electrical properties of molecules in the absence of ensemble effects. There are now many instances of phenomena distinguished in single molecule junctions, which would have not been apparent in bulk films or large ensembles, for example, stochastic switching,2,3 gated single molecule junctions and single molecular transistor behavior,4
14 anomalous temperature behavior,6,15 or single molecule solvation effects.6,16 Such studies now form part of the burgeoning contemporary field of single molecule electronics, which can be conceptually traced back to 1974, when Aviram and Ratner suggested that an organic molecule could operate as a rectifier,17 a concept that has only much more recently come to experimental fruition.18,19 The switching of molecular conductance has been a key theme in molecular electronics. There are now diverse demonstrations of how switching from a low to higher conductance state can be achieved in ensemble and even single molecule molecular junctions. Because switching provides one of the most basic electronic functions, this is of general interest in molecular electronics, and it may eventually have technological ramifications. Changes in junction conductance have been r 2011 American Chemical Society

achieved by a variety of stimuli, for instance, application of voltage pulses, light, electrochemical potential, external gate voltages, and chemical change (see ref 20 and references therein). The application of voltage pulses to switch catenane molecular junctions between two bistable states represents an early demonstration of memory device behavior in molecular electronics.21 In another highly cited demonstration, sharp on
off conductance switching (negative differential resistance, NDR) as the bias voltage is swept has been achieved within twoterminal molecular devices,22 although nonmolecular transport mechanisms such as gold filament formation have also come into discussion.23 Electrochemistry is very widely used to achieve conductance changes of redox-active conjugated molecular materials, thin films, or nanomaterials.24 Likewise, irradiation of bulk and monolayer materials has also been widely deployed to bring about conductance changes by photochemistry.20 A large proportion of the studies of conductance switching have been performed on ensemble molecular materials, in either bulk form or molecular monolayers. On the other hand, the single molecule approach enables a more direct link to be established between conductance changes and chemical, electronic, redox, or conformation changes in the junction. Mechanically formed break junctions (BJs) and scanning probe Received: February 7, 2011 Revised: March 17, 2011 Published: March 31, 2011 8361

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The Journal of Physical Chemistry C microscopes have been key techniques for the formation and study of single molecule electrical junctions. In the case of scanning tunneling microscopy (STM) studies, two broad classes of junctions can be formed; in the first, the target molecule does not chemically bridge the tip
substrate gap, while in the second case, chemical contact is achieved between molecule and both the STM tip and the substrate. Both approaches have been widely applied to study conductance switching at the single molecule level. A classic example of the first approach is the “xenon atom switch” reported by Eigler25 in which the junction conductance in an STM is switched between bistable states by transferring a Xe atom between a tungsten tip and a nickel substrate. Another impressive demonstration of the first approach is the STM observation of switching of the conductance state of substrate bound molecules by changes in the charge state of neighboring silicon surface atoms.26 In the second approach, chemical contact to the typically metal electrodes is achieved at both ends of the molecule, to create metal|molecule|metal junctions. Reed et al. demonstrated in 1997 that such molecular junctions can be formed and electrically characterized within mechanically controlled BJs.27 In another approach, Cui et al.28 formed single molecule bridges of dithiol molecular wires between a gold substrate and a gold nanoparticle sitting on top of an alkanethiol self-assembled monolayer (SAM). Electrical probing of the junction was achieved by “touching” the nanoparticle with a conducting atomic force microscopy (AFM) tip. Importantly, by repeating this measurement many times on different nanoparticle contacts and statistically analyzing the data, they could pinpoint the conductance of a single metal|molecule|metal junction.28 Such an objective statistical analysis has also played a key role in STM4,29,30 and other BJ methods31
33 for forming molecular electrical junctions. These techniques provide a strong experimental basis for both investigating charge transfer in single molecule junctions and investigating conductance-switching mechanisms. Single molecule examples of the latter include control of junction conductance by electrochemical switching of redox-active molecular bridges,4
6,34
36 photoisomerization,37 cis
trans isomerization,38 or thermal activation.15 In most of these cases, single molecule junction conductance is typically analyzed through conductance histograms recorded before and after imposition of the stimulus or in the case of electrochemical gating by recording histograms as the electrode potential is progressively stepped through an electrode potential range. One potentially effective method for imposing large conductance changes, which could have applications in conductance switching of single molecules, is through control of longrange electron transfer (ET). In recent years, there has been renewed interest in long-range transfer through macromolecules such as DNA, peptides, and proteins.39
41 This has a clear context in molecular biology, where long-range ET plays a central role in the function of redox metallo-enzymes, photosynthetic cycles, and DNA damage.42
44 Potential applications of long-range ET in molecular electronics and organic photovoltaics are also apparent. Long-range intramolecular charge transfer through macromolecules is generally expected to be extremely sensitive to the bridge structure. Both DNA and oligopeptides provide good platforms for investigating and eventually controlling long-range ET, and “switchable” oligopeptides provide the motivation for our current study. There are a number of studies in the literature of electrical junctions formed from peptide monolayers. Much of the work

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done in recent years to investigate the ET characteristics of short peptide chains within SAMs has been undertaken by Sek et al.45
50 and Kimura et al.39,40,51
62 Such investigations have usually employed electrochemical methods or STM to probe charge transfer properties. Generally, in these studies, the peptides have been self-assembled to form a monolayer film or alternatively embedded within a SAM of alkanethiols. In the latter case, the target peptides can be considered to be diluted within an alkanethiol “monolayer matrix”. Many of these studies have employed sequences that favor helix formation, such as alanine-based peptides, in the expectation that helical peptide structures will be better conduits for longer range charge transfer. In particular, Kimura et al. have used synthetic peptides comprising modified alanine amino acids with two methyl groups instead of one; this strategy promotes a very strong R-helix conformation even with short length peptides. Issues addressed with electrical characterization of peptides include rectifying the behavior of oriented helical peptide monolayers,57,59,62 molecular dipole effects,52,57,58 the influence of electric field,63 the distance dependence of ET rates and conductance,46,56 the influence of the metal surface to peptide chemical linking group,45,58 the quantitative evaluation of single molecule conductance,45,46,64 and changes to single molecule conductance upon metal ion binding.64 The diversity of such studies shows that peptides are particularly interesting targets in single molecular electronics. The ET properties of peptide monolayers can be controlled by pH, as changes in the charge states of ionizable residues in the peptide chain can influence the peptide conformation. For example, Zimmerman et al. have demonstrated that surface films of polyglutamic acid undergo a helix
coil transition upon variation of the pH.65,66 Because charge transfer is typically very sensitive to distance, this would lead to significant changes in ET rate constants. As changes in the pH lead to changes in the charge states of ionizable residues, this alters the charge distribution of the molecule and hence its dipolar properties. Yasutomi et al. have formed peptide SAMs on a gold surface with a carboxylic acid group at the outer surface of the peptide monolayer.67 By switching between the protonated carboxylic acid terminus and its deprotonated form, they were able to control the photocurrent direction through the SAM helical peptides to an electron acceptor or donor in solution. They attributed this to changes in the dipole moment of the SAM, which would in turn enhance ET in the same direction as the electric field generated by the dipole.67 pH has also been used to control the conductance of other types of molecular wires; for example, Guo et al. have shown that the conductance of oligoaniline covalently bridging gaps in single-walled carbon nanotubes responds to changes in pH.68 In a recent study, Doneux et al. have shown that pH can be used to control charge transfer across peptide monolayers on gold surfaces.69 The secondary structure of the peptides was chosen so that they would respond to pH through the deployment of ionizable glutamic acid residues in the peptide sequence. At low pH, the peptides within these monolayers existed as Rhelices, while at higher pH, deprotonation of the carboxylic acid groups of the glutamic acid residues led to a more extended conformation and substantially suppressed electrochemically determined ET across the molecular film.69 In the present study, we explore whether these ideas can be extended to the single molecule level by investigating single molecule conductance of a pH responsive peptide sequence at different solution pH. 8362

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Figure 1. (A) Selection of seven overlaid I(s) curves recorded for Au(111) samples at pH 2, which show monotonic exponential behavior and are used to determine d ln I/ds values without the presence of molecules bridging between the tip and the substrate. Utip =
200 mV, and I0 = 10 nA. The gold surface was functionalized with H(EL)5C although no molecular junction was formed for these cases where purely exponential decay curves are observed. (B) A representative d ln I/ds curve obtained from the data in panel a, showing a slope of
9.1 nm
1. Such curves are used to estimate the tip-to-sample distance and the distance at which molecular bridges break (see the text). The slight nonlinearity at the beginning of the ln(I) versus distance curve is attributed to an initial inertia in the retraction process, for example, caused by an initial piezo delay or an attractive interaction between tip and sample.

’ EXPERIMENTAL SECTION The peptide sequence H(EL)5C was purchased from Anaspec >90%. Purity was determined using high-performance liquid chromatography and mass spectroscopy, and it was used without further purification. HClO4 (70%) was purchased from Merck, and K2HPO4 and KH2PO4 were purchased from Fisons; all chemicals were used as received. All pH measurements were performed on a Hanna 2211 bench pH meter, which was calibrated with standard buffer solutions. The pH of the electrolyte solutions was adjusted just prior to experiments to either pH 2 or pH 6.9. The ruthenium-tagged peptide, Ru
H(EL)5C, was synthesized as described in ref 69. Gold films were employed as substrates in the I(s) measurements, and these were purchased from Arrandee Germany. These were flame-annealed at approximately 800
1000 °C with a Bunsen burner immediately prior to use. This procedure is known to result in atomically flat Au(111) terraces.70 For these measurements, a pH 2 solution was made using Milli-Q water and perchloric acid. This was used to make a 1  10
4 M solution of the peptide. Following immersion of the gold substrate for 2 min, the sample was rinsed with Milli-Q water and dried in a stream of nitrogen. It was then placed on the STM sample plate,

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and the Teflon cell was mounted. This cell was then filled with the pH 2 solution and a cut and Apiezon wax-coated, gold STM tip was used for I(s) measurements. Samples for high pH measurements were prepared in the same way using pH 6.9 solutions of freshly prepared K2HPO4/KH2PO4. The I(s) technique is a STM method for forming single molecular electrical junctions and measuring their electrical conductance. I refers to current, while s refers to the distance between the tip and the substrate. In this method, molecular bridges are formed between the STM tip and the substrate by approaching the tip to a given set-point current (I0). The tip is then rapidly retracted as the current flowing between the STM tip and the substrate is monitored as a function of distance. The resulting current
distance [I(s)] curves differ very substantially, depending on whether a molecule bridges between the tip and the substrate. A monotonic exponential decay of the current with distance results if no molecules bridge the tip
substrate gap. On the other hand, if a molecule is captured in the junction, then the current falls off much less steeply and a plateau is typically formed. As the molecular junction is further extended, it eventually cleaves, and the end of the plateau is marked by a sudden drop in the current. Many of these I(s) retraction curves are collected and then statistically analyzed in histogram plots, from which conductance values are extracted. The I(s) technique is related to the in situ BJ method,29,71,72 with both techniques using an STM to form single molecule electrical junctions. However, the I(s) technique avoids contact between STM tip and substrate, while the BJ techniques rely on first achieving metallic contact between STM and substrate and then cleaving these metallic junctions, which are then bridged by molecules. A comparison of the two methods for alkanedithiol molecular junctions has been described in a recent publication.1 The I(s) method also allows for the determination of the tip
sample separation at which the Au|molecule|Au junction cleaves. We call this distance the break-off distance (stotal). To obtain stotal, the initial distance away from the surface (s0) at the beginning of an I(s) scan is added to the distance displacement (Δs) of the STM tip from the surface during the I(s) scan. The distance s0 is obtained through calibration of the distance dependence of I0 and through determination of a distance reference point. We take this reference point by extrapolating the ln(I) versus s curves to the metallic point contact conductance (G0). This is achieved by recording several I(s) scans during the conductance measurements, which show no evidence of molecular bridge formation. Such curves for “bare” junctions display a monotonic exponential decrease of the tunneling current as the tip is retracted and are suitable for the calibration process; examples are shown in Figure 1 for the pH 2 solution. These exponential decay curves are then plotted as ln(I) versus s, and the slopes of these plots are then taken to give d ln(I)/ds (Figure 1b). Note the initial part of the ln(I) versus s curves is not used since in this region the curves are nonlinear due to inertia as the tip picks up speed during the retraction. These ln(I) versus s plots are then extrapolated back to the current, which corresponds to the metal-to-metal point contact conductance (77.4 μS). The following calculation is used to determine the distance between tip and surface (s0) at a given I0 value: s0 ¼

lnðG0 3 Utip =I0 Þ dlnl=ds

This s0 value at a given set-point current (I0) is then added to the 8363

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’ RESULTS AND DISCUSSION The peptide, HELELELELELC, used in this study is composed from glutamic acid and leucine along its backbone. The glutamic acid in the peptide is responsible for the relationship between structure and pH. The structure of glutamic acid is shown in Scheme 1a. With its acidic side chain, glutamic acid is a hydrophilic amino acid, and leucine (Scheme 1b) with its aliphatic side chain (isobutyl) is hydrophobic. Scheme 2 shows the structure of HELELELELELC, without (a) and with redox tagging (b). For brevity, we refer to this sequence as H(EL)5C, where H stands for histidine, E for glutamic acid, L for leucine, and C for cysteine. The sensitivity of the secondary structure of H(EL)5C to pH has been demonstrated by Doneux et al.69 Molecular dynamics simulations and circular dichroism results showed that the peptide conformation switches from an R-helical state at low pH to a random coil at neutral pH.69 The peptide H(EL)5C (Scheme 2a) has histidine and cysteine end groups. For the single molecule conductance experiments with an STM, both of these groups are left unfunctionalized and act as terminal groups for attachment of the peptide between Scheme 1. Structure of (A) L-Glutamic Acid and (B) L-Leucine

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gold contacts. For the electrochemical characterization of ET across SAMs, H(EL)5C is selectively tagged at the histidine end with a ruthenium ion (Scheme 2b), as described by Doneux et al.69 In this case, attachment is through the cysteine group to the gold substrate. We refer to the ruthenium ion-tagged sequence, which is assembled into monolayers on the gold surface as Ru
H(EL)5C. Figure 2 shows cyclic voltammograms of Ru
H(EL)5C SAM in a pH 6.9 phosphate buffer solution and a pH 2.0 electrolyte, respectively. At pH 6.9, the glutamic acid residues in the peptide sequence are deprotonated, while at pH 2.0, they are protonated. At the lower pH, the peptide is in its more compact R-helical state, while deprotonation leads to electrostatic repulsion between the charged carboxylate groups of the glutamic acid residues, promoting more extended conformations.69 This extension is reflected in the ET characteristics, as probed by cyclic voltammetry. At pH 2, a clear voltammetric wave is seen, while at pH 6.9, ET kinetics are so slow that no voltammetric wave can be discerned. Doneux et al. have characterized the pH-dependent conformation of H(EL)5C with molecular dynamics simulations.69 At pH 2, they showed that the Rhelix is stable over a 200 ns simulation window, while at pH 6.9, a random coil evolves during the time frame of the simulation. They have also quantified the kinetics of charge transfer to the Ru3þ/ Ru2þ couple in Ru
H(EL)5C SAMs. This was achieved by cyclic voltammetry and a Laviron analysis to give rate constants of 78, 110, and 230 s
1, respectively, at pH 3.0, 2.0, and 1.0. The clear implication of this study was that rate of ET across these SAMs was controlled by the pH-dependent conformation of the peptide in the monolayer. Helix to random coil transitions have also been observed by others for polyglutamic acid,65,66,73 both in solution and grafted onto surfaces. Koga et al.73 reported a structural transition at a pH of around 5, driven by the deprotonation of glutamic acid, which is consistent with the ET behavior and molecular dynamics characterization of Doneux et al.69 for H(EL)5C. We now turn to single molecule conductance measurements on H(EL)5C. These measurements rely on the terminal groups of H(EL)5C being able to contact with the respective gold electrodes

Scheme 2. Structure of (A) H(EL)5C and (B) the Ru-Tagged H(EL)5C

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Figure 2. Cyclic voltammograms of a Ru
H(EL)5C SAMs in phosphate buffer solution at pH 6.9 (black curve) and pH 2 (red curve). The scan rate was 50 mV s
1 in both cases.

Figure 3. I(s) scans for H(EL)5C at a bias voltage of
0.2 V and a set point current, I0 = 10 nA. Data recorded for a Au STM tip and substrate and a pH 2 electrolyte. These scans have been staggered along the x-axes to aid clarity of presentation.

Figure 4. Upper: All current data I(s) histogram for H(EL)5C at Vbias ( 0.2 V bias voltage and I0 = 10 nA. The blue curve is a Gaussian fit to the peak region of the histogram showing a peak at 1.7 nS. Lower: A histogram of break off distance values (stotal = s0 þ Δs) taken from I(s) traces with the addition of the initial distance between tip and surface (s0) at the start of the I(s) scan. Solution pH = 2.

to form Au|H(EL)5C|Au two-terminal junctions. One terminus of the molecular bridge is histidine, while the other one is cysteine. There have been several studies focusing on the interaction of histidine with a Au(111) surface.74,75 In a very recent study, Feyer et al.74 have used soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) to characterize the adsorption of histidine and histidine-containing peptide sequences, for both monolayer coverage and a low coverage phase. Their XPS data showed chemisorption of the histidine to gold, via the imino nitrogen atom (imidazole ring), with only a weak interaction existing between the other nitrogen and the surface. In these UHV-based experiments, deprotonation of the carboxylic acid group of histidine was also apparent, with surface coordination also occurring through this moiety. The adsorption of cysteine has also been well characterized, with STM studies under electrochemical conditions and also in UHV characterizing several different monolayer structures.76
81 Cysteine groups at the end of peptide chains have been shown to be effective for linking groups in gold|peptide|gold junctions formed in STM BJ experiments.46,48,82 Note that H(EL)5C, with a histidine group at one end and cysteine at the other, provides a “dual binding motif”. However, because the STM-based method relies on the repeated formation and breakage of gold|peptide|gold junctions from lower coverage phases, it is likely that there is a random orientation in the junction, when averaged over the large number of junctions formed.

For a given bias voltage, the set-point (I0) current can be used to control the initial tip
substrate distance prior to the I(s) retraction experiment. A set-point current of 10 nA was chosen by systematically increasing I0 from 5 nA to the point where plateau formation was observed in retraction curves. Figure 3 shows examples of I(s) scans of H(EL)5C in pH 2 solution with a bias voltage of
0.2 V and I0 = 10 nA. Steps in these I(s) curves are related to the breaking of the molecular junctions as it is stretched. Figure 4 (upper) shows a histogram of 200 I(s) curves with a conductance peak at 1.7 ( 0.6 nS. Because these measurements were carried out in pH 2 solution to maintain the helical structure throughout, this conductance value is attributed to Au|H(EL)5C| Au junctions with an R-helical structure. The conductance value for these junctions is comparable to that for heptanedithiol, demonstrating the enhanced ET properties of the R-helix conformation, which is more than double the length of heptanedithiol with a length difference of about 1.2 nm between the two. This ability of the R-helix conformation to allow long-range ET has been discussed in a number of recent publications and attributed to a hopping mechanism with the amide groups acting as hopping sites (see ref 40 and references therein). Figure 4 (lower) shows plots of the junction breaking distance for H(EL)5C. This junction break-off distance is obtained using the procedure explained in the Experimental Section. The average break off distance is 1.6 nm, while the 95%ile break off distance is 2.4 nm. A molecular model of the peptide in an 8365

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Figure 5. Molecular model of H(EL)5C produced with Spartan (molecular mechanics). For the purposes of this molecular modeling length estimation of the R-helical form, the glutamic acid residues are protonated (promotes R-helix formation), while the histidine terminus is not protonated, so the computations are performed on the uncharged peptide. Protonation of histidine terminus is not expected to have a large impact on the overall length of the R-helical form.

Figure 7. (a) Selection of I(s) scans for H(EL)5C recorded at pH 6.9 with
0.2 V bias voltage. (b) Current histogram for H(EL)5C at pH 6.9,
0.2 V, and I0 = 10 nA. All data points taken from 1000 I(s) scans used in the construction of this histogram.

Figure 6. I(s) scans for H(EL)5C displaying low current plateaus. Utip =
200 mV, and I0 = 10 nA. Solution pH = 2.

R-helical conformation is shown in Figure 5. The model gives an estimated length of this peptide of 2.22 nm. The 95%ile break-off distance correlates well with the length of the peptide. The observation of a large spread of break-off distances to lower values indicates that many junctions break at shorter distance, where either the molecule is tilted in the Au 3 3 3 Au junction at the break-off point or that the molecule
gold contact occurs along the STM tip rather than at the very end. Au-carboxylate binding is unlikely at this pH where this chemical moiety is protonated (
COOH), since it has been previously shown that
COOH is a relatively ineffective Au contact binding group for forming junctions, when compared to its deprotonated form (
COO
).83 Close examination shows that some I(s) scans exhibit a very low current plateau, which typically follows the larger current plateaus. Figure 6 shows some examples of such I(s) scans, with plateaus around 0.05 nA, corresponding to a conductance of ∼0.3 nS. Because the frequency of such plateaus is low, they are obscured in the all-current data histograms. Such low current plateaus may be linked to conformation changes as the peptide is further extended in the junction. Several groups have proposed

that the mechanical extension of an R-helical peptide proceeds via 310 helices,84,85 and the low current plateau may be indicative of such an occasional structural conformation change as the tip is retracted. The 310 helix is about 0.4 Å per residue longer than the R-helix.86 An analysis of break-off distances over 1.2 nm and the associated current value reveals that the longer break-off distances correlate with low conductance value of around 0.15 nS. Sek et al. also proposed a similar explanation for their observed nonzero current drops when performing I(s)-like measurements on helical peptides. They concluded that low-conductance conformations of the peptide may be responsible.46 Figure 7a shows representative I(s) scans for H(EL)5C recorded at pH 6.9. No plateaus were observed during these measurements. Figure 7b shows an all data histogram constructed from over 1000 I(s) measurements, and there is clearly no discernible peak. The minimum conductance that we can detect is about 0.05 nS, meaning that the conductance decreases at least ∼40 fold on going from the R-helical form of H(EL)5C at pH 2.0 to the random coil at pH 6.9. The length of H(EL)5C from molecular modeling is 2.22 nm in its R-helical form (Figure 5). The random coil will have many possible configurations and is generally formed from bends and turns. As such, it is not possible to attribute a defined length to it. However, in its fully extended form (which may be achieved in an STM “pulling” experiment), its length is 4.23 nm. Taking for the purposes of illustration a constant attenuation factor of 0.5 Å
1, a length increase from 2.22 (R-helical form) to 4.23 nm would lead to a conductance decrease of more than 4000-fold. These experimental observations are consistent with the model presented by Cristancho et al.87 They performed theoretical calculations on Rhelical forms of 15-mer polyglycine, poly-L-alanine, and the corresponding linear configuration of the polyglycine. They 8366

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The Journal of Physical Chemistry C predicted that the linear configurations would have extremely small conductance values and show no current
voltage dependence due to the extended ET pathway. The ability of the Rhelical form to promote long-range electron transport (ET) is also consistent with the recent observation of Arikuma et al.39,40 of long-range ET over 10
12 nm with an R-helix peptide. As in our current study, we did not observe ET at pH 6.9 in a fully extended conformation, and this comparison confirms that conformational “rigidity” promotes long-range ET. In conclusion, we demonstrate that long-range ET in peptide single molecular wires can be controlled by solution pH. In these experiments, Au|H(EL)5C|Au two-terminal junctions are formed within a pH-controlled environment, and their conductance is determined by the STM-based I(s) method. At low pH, the H(EL)5C bridge exists in its more compact R-helical state, which is a more efficient conduit for long-range ET between the two gold contacts. In this state, a single molecule conductance of 1.7 nS is recorded. On the other hand, at pH 6.9, H(EL)5C adopts an extended configuration with the conductance dropping below the limit recordable with our instrumentation.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ44 151 794 3533. Fax: þ44 151 794 3588. E-mail: R.J. [email protected]. Present Addresses §

)

Chimie Analytique et Chimie des Interfaces, Faculte des Sciences, Universite Libre de Bruxelles, Boulevard du Triomphe, 2, CP 255, B-1050 Bruxelles, Belgium. NSYSA, Institut des Sciences Moleculaires, UMR 5255, ENSCBP, 16 Avenue Pey Berland, 33607 Pessac, France.

’ ACKNOWLEDGMENT We gratefully acknowledge funding from the European Union (Marie-Curie Fellowship
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