© Copyright 1996 American Chemical Society
VOLUME 12, NUMBER 22
Letters Hydrogen-Bonding, Self-Assembled Monolayers: Ordered Molecular Films for Study of Through-Peptide Electron Transfer Robert S. Clegg and James E. Hutchison* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253 Received August 19, 1996X New peptide-containing alkanethiol self-assembled monolayers (SAMs) on gold have been prepared. These monolayers exhibit extended interchain hydrogen bonding and have well-ordered alkane chains. Elemental composition and thickness of the monolayer are obtained by X-ray photoelectron spectroscopy. External reflective IR spectroscopy shows that the SAMs are uniaxial and possess microcrystalline, densely packed methylene chains with hydrogen bonding between neighboring amide moieties. These highly ordered monolayers form excellent electrochemical spacers as characterized by electrochemical blocking studies and double-layer capacitance measurements. The well-defined structure makes this system a promising prototype for systematic investigations of long-range electron transfer through amide bonds.
Introduction The dependence of long-range electron transfer on the medium separating two redox sites is an important and unresolved problem in biology1 (e.g., in photosynthesis and respiration) and in molecule-based electronic systems.2 Intramolecular electron transfer between two redox centers has been investigated in modified proteins1b,c and through polypeptide-based bridging ligands3 in an effort to develop a better understanding of the effects of the intervening protein structure upon biological longrange electron transfer. Systematic variation of the molecular composition between two redox sites while * To whom correspondence should be addressed. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, October 1, 1996. (1) (a) Bertini, I.; Gray, H. B.; Lippard, S. J.; Valentine, J. S. Bioinorganic Chemistry; University Books: Mill Valley, CA, 1994; pp 315-333. (b) Winkler, J. R.; Gray, H. B. Chem. Rev. 1992, 92, 369-379. (c) Langen, R.; Chang, I.-J.; Germanas, J. P.; Richards, J. H.; Winkler, J. R.; Gray, H. B. Science 1995, 268, 1733-1735. (2) Molecular ElectronicssScience and Technology: AIP Conference Proceedings; Aviram, A., Ed.; American Institute of Physics: New York, 1992; Vol. 262. (3) Gretchikhine, A. B.; Ogawa, M. Y. J. Am. Chem. Soc. 1996, 118, 1543-1544 and references therein.
S0743-7463(96)00825-6 CCC: $12.00
maintaining a stable, well-defined structure and avoiding intermolecular electron transfer has proven to be difficult.3 We are developing peptide-containing, alkanethiolderived self-assembled monolayers (SAMs) on gold,4 as versatile systems for developing a detailed understanding of how the amino acid composition and structure between redox sites influence electron transfer rates. Alkanethiol monolayers on gold surfaces5 offer access to highly ordered, surface-confined molecular structures with a variety of demonstrated applications,5b,6 including use as stable, well-defined spacers for electron transfer studies.5e,7-9 The densely packed monolayer maintains a precise separation (4) (a) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (b) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. A.; Jeon, N.; Nuzzo, R. A. Langmuir 1995, 11, 4371-4382. (c) Wagner, P.; Hegner, M.; Gu¨ntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867-3875. (d) Lenk, T. J.; Hallmark, V. M.; Hoffman, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610-4617. (5) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (b) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481-4483. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (d) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (e) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.
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between a gold electrode surface and the pendant redox center and effectively eliminates conformational mobility that can complicate electron transfer rate studies. Through control of the surface concentration of the redox center in the monolayer, intermolecular electron transfer can be eliminated.8 Although systematic variation of the molecular composition of the spacer is synthetically feasible,9 most studies have been limited primarily to allalkane chain spacers. There have been few reports of amide-containing nalkyl SAMs. A primary amide-functionalized alkanethiol film was formed containing ordered alkane chains, but the terminal amides showed no evidence of hydrogen bonding.4a An amide group separated from the thiolate by a methylene linker was introduced into alkanethiol SAMs, but the hydrocarbon chains were poorly ordered.4b An amide-containing SAM derived from cystamine was reported,4c but these films remain uncharacterized. Ordered amide-containing perfluoroalkane monolayers were also reported, but electron transfer investigations were not performed.4d Herein we report the preparation and structural characterization of a highly ordered, hydrogen-bonding, peptide-containing alkanethiol monolayer (Figure 1). Characterization by contact angle goniometry, X-ray photoelectron spectroscopy (XPS), external reflection infrared spectroscopy (FTIR-ERS), and cyclic voltammetry (CV) demonstrates that this monolayer provides a structurally well-defined foundation for developing systems for modeling electron transfer through polypeptide chains. The extended interchain hydrogen bonding and high degree of order among the methylene chains make this the first well-characterized system to show promise as a useful spacer for studies of electron transfer through peptides assembled on electrode surfaces. Experimental Section A general synthetic strategy for preparation of alkanethiols containing one or more amide bonds has been developed.10 The monolayer precursor required for the present study, 3-mercaptoN-pentadecylpropionamide (1), was synthesized in three steps.
1
Reaction of 3-bromopropionic acid and potassium thioacetate11a yields S-acetyl-3-mercaptopropionic acid. Dehydration with dicyclohexylcarbodiimide to yield the corresponding anhydride is followed by reaction with 1-pentadecylamine to form the amide bond.11b The thioester is hydrolyzed with NaOH to yield the thiol 1.12 (6) (a) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (b) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277-3283. (c) Zak, J.; Yuan, H.; Ho, M.; Woo, K.; Porter, M. D. Langmuir 1993, 9, 2772-2774. (7) (a) Chidsey, C. E. D. Science 1991, 251, 919-922. (b) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173-3181. (8) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (9) Cheng, J.; Sa`ghi-Szabo´, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680-684. (10) The general synthetic strategy and procedures will be detailed in a future publication. (11) (a) Bonner, W. A. J. Am. Chem. Soc. 1951, 73, 2659-2666. (b) Schu¨ssler, H.; Zahn, H. Chem. Ber. 1962, 95, 1076-1080. (12) 1 was obtained as a white microcrystalline solid (mp 69.2-69.7 °C), in 52% yield with respect to 3-bromopropionic acid. 1H-NMR (300 MHz, CDCl3): δ 0.88 (t, J ) 6.6 Hz, 3H), 1.23-1.35 (overlapping resonance, 24H), 1.51 (m, 2H), 1.62 (t, J ) 8.4 Hz, 1H), 2.48 (t, J ) 6.8 Hz, 2H), 2.82 (m, 2H), 3.27 (m, 2H), 5.54 (br, 1H). Anal. Calcd: C, 68.51; H, 11.82; N, 4.44. Found: C, 68.72; H, 11.56; N, 4.47.
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Figure 1. Idealized view of 1/Au. Gold substrates on clean glass microscope slides were prepared by evaporation of 100 Å of a chromium adhesion layer followed by 2000 Å of gold.6b,c The gold substrates were cleaned by ozonolysis for 15 min in a UV-Clean (Boekel Industries), rinsed with copious amounts of Nanopure water and degassed absolute ethanol, and dried in a stream of argon immediately prior to formation of monolayers. SAMs of 1 on gold (1/Au) were formed by immersing the gold substrates in 1 mM 1 in degassed absolute ethanol for at least 24 h at room temperature. Contact angles (Nanopure water) were measured by Method A in ref 5c on a contact angle goniometer constructed in our laboratory. FTIR-ERS was performed with polarized light at an 80° angle of incidence using a Spectra-Tech reflectance accessory in a Nicolet Magna-IR 550 Spectrometer, using 256-1024 signalaveraged scans with Happ-Genzel apodization, 1 cm-1 data spacing, and no zero filling. Background spectra were taken on gold substrates coated with biphenyl mercaptan SAMs.13 Spectra shown in Figures 2 and 3 are unsmoothed. XPS was performed on a Kratos HSi analytical spectrometer using a monochromatic Al ΚR source (13.5 kV, 15 mA), pass energy of 20 eV, and 0.1 eV data spacing. Multiplexed scans were collected with total scan times as follows: C(1s), 20 min; N(1s), 40 min; O(1s), 40 min; S(2p), 60 min. Sensitivity factors were calculated from a powder sample of S-acetyl-3-mercaptoN-pentadecylpropionamide. Argon ion sputtering was performed using a Minibeam I ion gun at 5.0 kV beam energy and 20 mA emission current for 5 s intervals. Argon ion sputtering for monolayer thickness determinations was alternated with single scans on C(1s) for 120 s (100 data points) and Au(4f) for 60 s (120 data points). Electrochemical measurements were performed using a BAS 100 B/W electrochemical analyzer. The cell consisted of a bare gold or SAM-derivatized working electrode, a platinum wire auxillary electrode, and a saturated calomel reference electrode at 21 ( 1 °C. The electrolyte was 1.0 M potassium chloride in Nanopure water. For electrochemical blocking studies, the analyte was 1.0 mM potassium ferricyanide. The geometrical area of the working electrode was 0.95 ( 0.03 cm2.
Results and Discussion To determine their suitability for electron transfer experiments, the monolayers were characterized by contact angle goniometry, FTIR-ERS, XPS, and CV. The contact angle for water on 1/Au is 119 ( 2°, demonstrating that these SAMs have ordered, methyl surfaces comparable to those of well-ordered octadecanethiol-derived monolayers (ODT/Au), which exhibit contact angles of 118 ( 2°.5c XPS measurements confirm the presence of oxygen, nitrogen, carbon, and sulfur near the gold surface. The binding energies and relative atomic intensities (Table 1) support the monolayer structure shown in Figure 1. The S(2p) peaks are shifted to lower binding energies char(13) Gao, Z. Q.; Siow, K. S.; Chan, H. S. O. Synth. Met. 1995, 75, 5-10.
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Langmuir, Vol. 12, No. 22, 1996 5241
Table 1. Composition of 1-Derived SAMs on Gold by XPS
C(1s) (alkyl, CdO) N(1s) O(1s) S(2p3/2, 2p1/2)
E (eV)
atom % by XPS
predicted atom %
285.1, 287.5 399.7 531.9 161.9, 163.1
90.2 4.4 3.8 1.9
85.7 4.8 4.8 4.8
Table 2. FTIR-ERS Data (in cm-1) for 1 in Solution, as the Neat Solid, and Deposited on Au 1 in CCl4
solid 1
amide A amide B CH3(as)
3453 a 2960 (sh)
CH3(Fermi) CH2(as) CH3(Fermi) CH2(sym) amide I amide II
a 2927 2872 2855 1684 1511
3302 3055 2964 (ip) (sh) 2954 (op) a 2919 2872 2850 1633 1541
1 on Au a a 2964 2938 2918 2877 2850 1637 (br) 1561
a Peak not observed (see text). as ) asymmetric stretch; sym ) symmetric stretch; sh ) shoulder; ip ) in-plane; op ) out-of-plane; br ) broad resonance.
Figure 2. FTIR-ERS spectra in the high-frequency C-H region: top, 1 in CCl4; middle, solid 1; bottom, 1/Au (Au electrode modified using a SAM derived from 1). Absorbance bar applies to 1/Au only.
acteristic of sulfur bound to gold.5d Peaks corresponding to free thiols were not observed,5d indicating that physisorption of 1 and multilayer formation do not occur. Signals from atoms deep in the monolayer (S, N, and O) are attenuated.4b As a result, these relative intensities are lower than those calculated from the molecular formula. The accentuation of the C(1s) (alkyl) signal confirms the predominance of carbon near the top of the film.5c These XPS data are consistent with the orientation of the monolayer shown in Figure 1. Film thickness was measured by XPS taking advantage of the well-known attenuation of Au(4f7/2) photoelectrons by the organic overlayer.14 After measurement of the Au(4f7/2) signal through the monolayer, the organic film is removed by argon ion sputtering. The intensity of bare gold is measured when the C(1s) peak disappears.15 The thickness, based on the Au(4f7/2) signals for the bare and monolayer-coated substrates, is calculated as 28 ( 2 Å for 1/Au films.14 This thickness indicates that the monolayers are indeed a single molecular layer thick and suggests a more vertical orientation of the monolayer molecule than found in alkanethiol-derived films.16-18 Evaluation of the crystallinity of the methylene chains was made by using FTIR-ERS (Figure 2 and Table 2).19 The peak frequencies for CH2(as) and CH2(sym) are the same in both the monolayer and in solid 1 and are redshifted compared to the frequencies for 1 dissolved in CCl4. The red shifts are the same as those seen in the liquidto-solid phase transition for n-alkanes.20 The frequencies (14) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 16701673. (15) Additional sputtering after complete removal of carbon did not significantly affect the intensity of the Au peak (1.3% increase after 1.5 times the original sputter time). This suggests that the gold substrate morphology is not significantly altered during the sputtering process. (16) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 11471152 and references therein. (17) Molecular mechanics (Sybyl force field) of the fully extended conformer of 1 on gold suggests a length of ∼26.5 Å. (18) As a control, we measured ODT-derived monolayer thicknesses at 25 ( 2 Å, within experimental error of the 23 ( 2 Å ellipsometric measurement of Bain et al. (ref 5c). (19) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66.
for the 1/Au peaks are also the same as those found in crystalline methylene chains such as in ODT/Au monolayers.4a,5d,19 The narrow band widths for 1/Au in the highfrequency C-H region (full widths at half-maxima: CH2(as), 15 cm-1; CH2(sym), 9 cm-1) are characteristic of tightly packed chains with minimal gauche defects.21 Taken together, the peak frequencies and band widths are indicative of a microcrystalline array of tightly packed methylene chains in all-trans conformations. Information regarding the orientation of 1 on the gold surface is also contained in reflective IR spectra.5c The relative intensities of the in-plane and out-of-plane methyl stretches provide a qualitative indicator of anisotropy of the methyl groups in 1/Au. In the monolayers, the band for CH3(as) contains only the in-plane component, whereas the out-of-plane resonance dominates the band in the isotropic solid. The relative intensities of CH3(as) to CH3(sym) are well-known to follow an even-odd dependence on the number of methylene units present.4a The spectrum of 1/Au is qualitatively similar to the pattern seen in n-alkanethiols containing even numbers of methylenes.4a The ratio of CH2(sym) to CH2(as) is decreased in 1/Au (1:2) relative to that ratio in ODT/Au (1:3), indicating a difference in tilt and twist of the methylene chain for 1/Au,22 perhaps due to templating of the microcrystalline oligomethylene matrix by the amide groups in 1/Au4b versus the thiol moieties in the case of ODT/Au. Uncertainty about the conformation of the ethylene linker between the amide group and the thiolate23 is a complicating factor which precludes quantitative determination of the tilt and twist angles of the hydrocarbon chain at this time. The presence of hydrogen bonding in 1/Au monolayers is evident from FTIR spectra of the amide stretching and bending modes. The nature and extent of hydrogen bonding in the monolayers, as well as the orientation of the amide groups with respect to the substrate, can be determined by comparing 1/Au spectra to data collected for 1 in solution and in the solid phase as well as to appropriate literature spectral values. In a CCl4 solution of 1 (Figure 3 and Table 2), where hydrogen bonding is insignificant, the amide A band (N-H stretch) appears as an unassociated peak at 3453 cm-1, amide I (primarily CdO stretch with a small component of OdC-N bend) occurs at 1684 cm-1, and amide II (20) Snyder, R. G.; Strauss, H. L.: Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150. (21) Bent, S. F.; Schilling, M. L.; Wilson, W. L.; Katz, H. E.; Harris, A. L. Chem. Mater. 1994, 6, 122-126. (22) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A. Langmuir 1991, 7, 2700-2709. (23) (a) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (b) Ro¨thlisberger, U.; Klein, M. L.; Sprik, M. J. Mater. Chem. 1994, 4, 793-803.
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Figure 3. FTIR-ERS spectra in the amide I and II region: top, 1 in CCl4; middle, solid 1; bottom, 1/Au. Absorbance bar applies to 1/Au only.
(primarily N-H bend with a small component of C-N stretch) arises at 1511 cm-1. These observations correlate well with published data for secondary amides in solution.24 In solid 1 (Figure 3 and Table 2), amide A shifts to 3302 cm-1, and amide B (first harmonic of N-H in-plane bend) arises as a weak resonance at 3055 cm-1, indicating that hydrogen bonding occurs in solid-phase 1.25 The occurrence of hydrogen bonding in the solid is substantiated by the shift of amide I to 1633 cm-1 and amide II to 1541 cm-1, both of which are nearly identical to the amide I (parallel component) and amide II bands seen in Nylon 66.26 Additionally, these frequencies are similar to those seen in solid disordered polypeptides26 and are within the range of literature values for solid secondary amides.24c Although solid 1 exhibits hydrogen bonding between the amide groups, the sample is still disordered as indicated by the breadth of the methylene peak21,22 (see Figure 2). Examination of the amide peaks in the 1/Au spectrum (Figure 3 and Table 2) provides compelling evidence of interchain hydrogen bonding and yields information about the average orientation of the amide groups which is consistent with the formation of hydrogen bonds as depicted in Figure 1. The absence of amide A in the reflective spectrum of 1/Au suggests a parallel orientation of the N-H bond with respect to the surface, and the very broad and unresolved amide I peak likewise infers a similar orientation of the CdO bond.4b,d This signal is identical to the resonance at 1637 cm-1 for LB films of octadecanoyl tri- through pentaglycines 25 and is close to that seen at ∼1650 cm-1 for parallel hydrogen bonding in polyglycine II.27 The red-shift compared with the solution spectrum is consistent with a decrease in electron density of the CdO bond as would be seen in hydrogen bonding.26 In contrast to the aforementioned amide peaks, the amide II appears as a strong peak in the 1/Au monolayers, confirming the orientation of its dipole with a large component perpendicular to the surface.4b Hydrogen (24) (a) Nyquist, R. A. Spectrochim. Acta A 1963, 19, 509-519. (b) Letaw, H.; Gropp, A. H. J. Chem. Phys. 1953, 21, 1621-1627. (c) Richards, R. E.; Thompson, H. W. J. Chem. Soc. 1947, 1248-1261. (25) Cha, X.; Ariga, K.; Kunitake, T. Bull. Chem. Soc. Jpn. 1996, 69, 163-168. (26) Bamford, C. H.; Elliott, A.; Hanby, W. C. Synthetic Polypeptides: Preparation, Structure, and Properties; Academic: New York, 1956; pp 147-148, 153-159, 193-194. (27) Frushour, B. G.; Painter, P. C.; Koenig, J. L. J. Macromol. Sci.sRev. Macromol. Chem. 1976, C15, 29-115.
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bonding is evidenced by the blue-shift of this peak to 1561 cm-1, identical to that observed for LB films of octadecanoyl triglycine ethyl ester25 and greater than the blue shift observed for polyglycine II.27 The progression toward higher frequencies on going from the solution spectrum (1511 cm-1) to the solid (1541 cm-1) to the monolayer (1561 cm-1), suggests an increasing restriction on N-H bending consistent with hydrogen bonding.26 As seen in an earlier study of structurally related monolayers,4b the amide II peak exhibits some asymmetry, but for 1/Au the amide II is much narrower and occurs at a higher frequency. The appearance of a strong amide II peak in concert with the absence of amide A and amide I in the 1/Au spectra indicates that the dipoles of amide A and amide I are oriented parallel to the gold surface and that the amide is present in the expected trans conformation,4b,d,26 as would be required for facile hydrogen bonding as shown in Figure 1. Results from FTIR-ERS indicate that 1/Au possess crystalline, tightly packed methylene chains and extended interchain hydrogen bonding. These data also suggest that the hydrogen bonding amide template perturbs the orientation (tilt and twist) of the hydrocarbon chains. The order seen in 1/Au contrasts with the closely related films of Tam Chang et al., where methylene linkers and dodecyl tails were employed, versus the ethylene linker and pentadecyl tail in 1.4b In that study, hydrogen bonding was found to occur among the amides, but the methylene chains were disordered.4b We can only speculate at this time as to the structural requisites for establishing order in the hydrocarbon region. Order has been observed with as few as 11 methylenes in alkanethiol SAMs,5c but templating the hydrocarbon matrix upon the amide moiety4b may increase the threshold value for ordering. The hydrocarbon linker between the amide and the thiolate may be subject to a certain minimal length in order to minimize free space, or to an even-odd effect to provide optimal amide group orientation for hydrogen bonding. We are systematically investigating the structural requirements for polycrystallinity in the hydrocarbon chain region, as this characteristic is certainly required for the passivation needed in a rigid electrochemical spacer. The analytical techniques described above all give measures of the bulk properties of 1/Au. To probe further the bulk properties of 1/Au, as well as to investigate the microscopic order, functional characterization was performed by the electrochemical technique, cyclic voltammetry (CV). As outlined by Porter et al., electrochemistry can occur on a microscopic scale at a modified electrode by any of three mechanisms: (a) permeation of the film by electrolyte; (b) charge transfer at defect sites; (c) electron transfer through monolayer molecules.5e Mechanisms a and c may occur across the whole of an ordered film, while (b) is a microscopic event. The contributions of these three mechanisms are discussed below. Double-layer capacitance measurements allow the films to be probed for their permeability to electrolyte.5e Figure 4 (dashed lines) shows the capacitive envelopes for both a bare gold electrode and an electrode coated with 1. At 100 mV/s scan rate, the charging current for a bare gold electrode is ∼15 µA at +185 mV. For 1/Au the charging current at the same potential is reduced to 96 nA. The double-layer capacitance28 for 1/Au is1.0 µF/cm2. This measurement is the same as that found for ODT/Au and indicates low permeability of the monolayer with respect to electrolyte.28 (28) Chen, C.-h.; Hutchison, J. E.; Postlethwaite, T. A.; Richardson, J. N.; Murray, R. W. Langmuir 1994, 10, 3332-3337.
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Langmuir, Vol. 12, No. 22, 1996 5243
more facile than through simple alkanethiols or previously studied tethers.5e,7,8,30 Current work in our laboratory is directed toward development of electrochemically reversible mixed monolayers containing tethered redox centers for the determination of ket through 1.7,8 In summary, the electrochemical results show that gold electrodes derivatized with 1 exhibit low double-layer capacitance, absence of reduction or oxidation waves in the presence of analyte, and an i-E response characteristic of electron transfer through a highly ordered barrier containing minimal defects. Taken together with the FTIR, XPS, and contact angle data, these strongly support a highly ordered monolayer with extended hydrogen bonding.
Figure 4. Cyclic voltammograms for 1.0 mM potassium ferricyanide (K3Fe(CN)6) in 1.0 M KCl electrolyte (solid line) and 1.0 M KCl electrolyte only (dashed line): A, bare Au electrode; B, 1/Au. Scan rates were 100 mV/s.
Electrochemical blocking studies give an estimate of possible defect sites, the presence of which could allow facile reduction of ferricyanide.5e Figure 4 (solid lines) shows i-E responses for ferricyanide in solution at a bare gold electrode and at a gold electrode modified with 1/Au. At 100 mV/s scan rate, the peak cathodic current at a bare electrode is +205 µA. For 1/Au, the cathodic current at the same potential is reduced to 113 nA, corresponding to a blocking effect of 99.5%.28 Using the quantitative treatment of Porter et al.,5e the maximum possible fractional pinhole area is calculated to be 0.2%. While the passivation is high, it is not as complete as that of ODT-covered gold.5e The shape of the CV in Figure 4B suggests that this incomplete blocking is due to electron transfer through the monolayer itself as opposed to reduction of ferricyanide at defect sites. If a significant number of defect sites were present, a sigmoidal CV would result with a sharp current response near E°′ and a limiting current independent of potential. The rapidly increasing cathodic current which occurs beyond Eo′ (+245 mV vs SCE5e,29) is characteristic of kinetically limited electron transfer through the film.5e,30 This increase is more pronounced than in ODT/Au, suggesting that electron transfer through the amide-containing films is (29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 1980. (30) Slowinski, K.; Chamberlain, R. V., II; Bilewicz, R.; Majda, M. J. Am. Chem. Soc. 1996, 118, 4709-4710.
Conclusions We have described the first amide-containing alkanethiol monolayer that has the order and stability required for the study of electron transfer through peptide bonds. The methylene chains reside in microcrystalline environments, and the amide groups engage in extended, interchain hydrogen bonding. The extended hydrogen bonding seen here between neighbors containing single amide groups implies that the registry will allow hydrogen bonding between multiple amide- (peptide-)containing precursor molecules as well. Our synthetic route to amidecontaining alkanethiols follows a versatile strategy which allows an iterative approach toward generating a series of monolayer molecules containing multiple amide bonds. Ongoing synthetic efforts include the development of these oligopeptides and study of their stability and electrontransfer properties. The low double-layer capacitance reflects the insulating quality of films of 1 on Au and is a necessary attribute of coatings for modified electrodes.28 Electrochemical investigations using these and related monolayers, including films derived from molecules with multiple amide bonds, should expand the understanding of long-range electron transfer through peptide-containing media and may provide fundamental information about the effect of primary structure upon the electron-transfer properties of redox proteins. Acknowledgment. We thank the Camille and Henry Dreyfus Foundation and the University of Oregon for financial support. Support from the National Science Foundation for X-ray photoelectron spectrometry (CHE9512185) and NMR (CHE-9421882) facilities is gratefully acknowledged. We also thank Dr. Eric Schabtach for technical assistance in preparing gold substrates and Dr. Seth C. Rasmussen and Scott M. Reed for critical reading of the manuscript. LA960825F
