Langmuir 1995,11, 4577-4582
4577
FTIR Study of Redox Switching of z-Interactions and Reorientation of Adsorbed Ubiquinone-10 on Gold Simon N. Port and David J. Schiffrin" Department of Chemistry, University of Liverpool, Liverpool L69 3BX, U.K.
Theodros Solomon Chemistry Department, Addis Ababa University, P.O. Box 11 76, Addis Ababa, Ethiopia Received June 20, 1995. In Final Form: August 18, 1995@ The reduction of ubiquinone-10 adsorbed onto a gold electrode in borax and sodium hydroxide solutions has been studied by subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS). A reorientation of the adsorbed ubiquinone molecule on reduction could be observed. The SNIFTIRS spectra show that the molecule is anchored t o the electrode surface through the isoprenoid groups,with the quinone ring away from the surface. Upon reduction, the aromatic ring formed is adsorbed by n interactions with the metal surface and is aligned flat to the surface, slightly lifting the isoprenoid chains.
Introduction Ubiquinones are dimethoxytoluquinones with polyisoprenoid side chains which act as both electron and proton transporters in biologicalmembranes. Although generally believed to exist in the hydrocarbon region of the inner mitochondrial membranes, their exact location is still the subject of investigation. Their function as electron transport mediators has attracted considerable attention over the last three decades, since the understanding of the redox properties of Ubiquinone-10 (UQlo)is essential for a knowledge of its function. For this reason their electrochemical properties have been studied. UQlo (with 10isoprenoid side chains) adsorbed on apyrolytic graphite electrode was investigated by Schrebler et al. l y 2 who determined the acid-base constant and the equilibrium potential at pH = 0 for the various ubiquinone couples. Similar studies were carried out by Takehara et al. on glassy carbon3and thiol-derivatized gold4electrodes. They proposed that two different charge transfer processes controlling the redox reaction occur, one assigned to the inter- andor intramolecular proton transfer at monolayer sites and the second to adjacent UQlo molecules a t the inner layer of multilayer sites within the film.
'""G CH,O
(CH$X=C-CH2)loH I
cH3
0
Ubiquinone 10
Recently, Gordillo and Schiffrin5 investigated the redox properties of ubiquinone-10 adsorbed on a mercury electrode in aqueous electrolyte solutions a t coverages smaller than a monolayer. Stability regions, acid-base @Abstract published in Advance A C S Abstracts, November 1,
1995. (1)Schrebler, R. S.; Arratia, A.; Sanchez, S.;Haun, M.; Duran, N. Bioelectrochem. Bioenerg. 1990,23,81. (2) Sanchez, S.; Arratia, A.; Cordova, R.; Gomez, H.; Schrebler, R. Bioelectrochem. Bioenerg. 1996,36,67. (3) Takehara, K.; Ide, Y. BioeZectrochem. Bioenerg. 1991,26,297. (4) Takehara, K.;Takemura, H.;Ide, Y.; Okayama, S . J.Electroanal. Chem. Interfacial Electrochem. 1991,308,345. (5) Gordillo, G. J.; Schiffrin, D. J. J. Chem. SOC.,Faraday Trans. 1994,90,1913.
ionization constants, and standard potentials for redox equilibria between conjugate stable species were determined and reaction mechanisms proposed. It was found that the stability of the semiubiquinone intermediate is very strongly pH dependent. The disproportionation of the radical at pH 8-9 is very fast, whereas the radical anion appears to be quite stable in alkaline solution. The reduction of UQlo follows a two-electron, two-proton reaction similar to other quinones. It has been shown from electron paramagnetic resonance spectroscopy that in biological membranes the intermediate semiubiquinone radical is significantly stable.6 Nevertheless, disagreement over the values for the redox potentials measured for the UQ/U&'- and UQ'-/UQH2 couples still exists.6-8 It is clear that the environment in which the quinone is present greatly affects the redox potentials measured, and the conditions chosen to study electron transfer can strongly influenced these. Classically, electrochemical studies have used nonaqueous solvents, for example a~etonitrile,~ although the results from aprotic solvents cannot be easily related to the membrane case. Under normal conditions the coenzyme is present in a lipidic environment with water freely available to partake in intermediate chemical reaction pathways. From this point of view, the work of Petrova et al.1° who studied the redox properties of ubiquinone in films of phospholipids spread on carbon electrodes, gives a more realistic model of the behavior of UQlo in membranes. A similar modelistic approach was taken by Laval and Majda'l who studied the structure and electron transfer properties of phospholipid bilayers incorporating ubiquinone on a solid electrode. Bilayer assemblies consisting of a thiol and a phospholipid formed a t the gold-solution interface were used as model biological membranes to investigate the electron transfer and lateral mobility of ubiquinone. (6) Ohnishi, T.;Salerno, J. C.; Blum, H.; Leigh, J. S.; Ingledew, W. I. InBioenergetics ofMembranes;Parker, L.;Papageorgoiu, G .C.,Trebst, A.; Eds.; Elsevier: Amsterdam, 1977; p 209. (7) Urban, P. F.;Klingenberg, M. Eur. J. Biochem. 1969,9,519. (8)Crane, F.L.; Barr, R. In Coenzyme Q.;Lenaz, G., Ed.; Wiley: Chichester, 1985; p 1. (9) Morrison, L. E.; Schelhom, J. E.;Cotton, T. M.; Bering, C. L.; Loach, P. A. In Function of Quinones in Energy Conserving System; Trumpower, B. L., Ed.; Academic Press: New York, 1982; p 35. (10)Ksenzhek, 0.S.;Petrova, S.A.;Kolodyazhy,M. V.Bioelectrochem. Bioenerg. 1982,9,167. (11) Laval, J-M.; Majda, M. Thin Solid Films 1994,244, 836.
0743-746319512411-4577$09.00/0 0 1995 American Chemical Society
4578 Langmuir, Vol. 11, No. 11, 1995
Two different types of voltammetric peaks have been observed with a mercury e l e ~ t r o d eone , ~ corresponding to a pH-independent reorientation process and another to a pH-dependent reversible redox reaction of a n adsorbed species. The difference between the Hg5 and the Aull results is due to the absence of reorientation peaks in the latter, indicating a stronger interaction of the adsorbate with the substrate. The ubiquinone environment in phospholipid bilayers has been studied by IR spectroscopy. Stidham et a1.12 analyzed the location of ubiquinone-10 in phospholipid bilayers using a variety of physical techniques, including light microscopy, 13C NMR, and X-ray diffraction. Ondarroa and Quinn13 studied the interaction between ubiquinone-10 and phospholipid bilayers using difference infrared spectroscopy and concluded that when UQlo is codispersed with dipalmitoylphosphatidylcholinein water the two lipids phase separate. Furthermore, they found no evidence for UQlo intercalating between phospholipid molecules but suggested that the benzoquinone substituent resides in a hydrophobic domain and that aggregates spanning the bilayer are a possible arrangement of the UQlo structure. Clear spectral changes were observed between UQlo in phospholipid layers when compared with UQlo in solutions of dodecane and chloroform. Breton et al. l4 used light-induced FTIR difference spectroscopy and infrared spectroelectrochemistry to investigate models for photosynthetic electron acceptors such as UQlo. The results were used to estimate the contribution of quinone reduction to light-induced infrared difference spectra between the charge-separated and the relaxed state of the photosynthetic reaction center. The frequencies and the extinction coefficients of the quinone C=O and C=C stretching vibrations which shift upon reduction were obtained for different solvents. Goni et al. l5 studied the physical state of UQlo in both pure form and incorporated into phospholipid bilayers using FTIR and concluded that the C=O stretching vibration is the one most sensitive to phase and environmental change. Alternatively, Bauscher et al. 16,17recently reported on the electrochemical and infrared-spectroscopic characterization of redox reactions of UQlo and concluded that the neutral quinone can be easily detected from the C=O and C=C vibrations, but the reduced product is more difficult to determine. From the above, it is quite clear that IR spectroscopy is a powerful technique for probing the state of UQlo; in particular, the surface selection rules can give additional information on orientation and possible aggregation of UQlo on the surface.l*Jg The purpose of the present work was to study the electrochemical reduction of adsorbed UQlo by infrared reflectance spectroscopy, as well as the possible reorientation of the adsorbed molecule as a function of the applied interfacial potential, using the technique of subtractively normalized interfacial FTIR spectroscopy (SNIFTIRS).20r21For comparison with pre(12) Stidham, M. A.; McIntosh,T. J.; Siedow,J. N. Biochim. Biophys. Acta 1984,767, 423. (13)Ondarroa, M.; Quinn, P. J. Biochem. J . 1986,240,325. (14)Breton,J.; Berthomieu, C.; Thibodeau,D. L.; Nabedryk, E.FEBS Lett. 1991,288,109. (15)Castresana, J.;Alonso,A.;Arrondo, J.-L.R.;Goni,F.M.;Casal, H. Eur. J . Biochem. 1992,204,1125. (16)Bauscher, M.; Nabedryk, E.; Bagley, K.; Breton, J.; Mantele, W. FEBS Lett. 1990,261, 191. (17)Bauscher, M.;Mantele, W. J. Phys. Chem. 1992,96, 11101. (18)Greenler, R. G. J. Chem. Phys. 1966,44,310. (19)Pearce, H. A.;Sheppard, N. Surf: Sci. 1976,59,205. (20) Pons, S.; Davidson, T.; Bewick, A. J . Electroanal. Chem. Interfacial Electrochem. 1984,160, 63. (21) Bewick, A.;Pons, S. B. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. N., Hester, R. E . , Eds.; Wiley: London, 1985; VOl. 12, p 1.
Port et al.
vious work,ll the investigations were carried out in 0.1 M borax and 0.1 M NaOH aqueous solutions.
Experimental Section A polycrystalline gold disk electrode (Goodfellows 99.999% purity) of 2 mm thickness and 12 mm diameter, was glued to a glass holder with epoxy resin (RS Components). This was then polished to a plane mirror finish with successively finer grades of alumina (Buehler 3.0, 0.3, and 0.05 pm) and finally cleaned in a n ultrasonic bath with quadruply distilled water. Solutions were prepared using AnalaR grade borax (BDH Chemicals, Poole) and Microselect NaOH (Fluka Chemicals). A0.03 mg/mL solution of UQlo (Sigma, synthetic) was prepared in redistilled pentane (Fluka, puriss); this was subsequently thoroughly deaerated with pure nitrogen and stored at -20 "C. The latter precaution is essential to ensure stability of the stock solution. A three-electrode cell was employed for all electrochemical and spectroscopic experiments. The counter electrode was a platinum 0.5 mm wire ring surrounding the working electrode, and the reference electrode was a saturated calomel electrode. The potential was controlled with a potentiostat and waveform generator (HiTek, England, DTlOl and PPR1, respectively). A dry air purged FTIR spectrometer, BioRad FTS 40, with an additional second optical compartment was used. The optical path was modified with the addition of one extra mirror to redirect the beam onto the electrode surface. The spectroelectrochemical cell was placed inside a perspex box to allow air purging and to remove subsequent interference from atmospheric moisture and carbon dioxide. A liquid nitrogen cooled mercury cadmium telluride detector was employed. A polarizer (Specac P/N12950) was used to select p or s radiation. Experiments were carried out using both CaFz and ZnSe windows. A normalized spectrum was obtained by subtracting two spectra (Rz - R1) obtained for different potentials and dividing - ~ 3 the by one of them, which was used as the r e f e r e n ~ e . ~ ~Thus normalized change in reflectance is given by ARIR1= (Rz - R1)I R1. Both positive and negative bands can appear, associated with species formed at the first (R1)or second (Rz)potential. In the present work, positive pointing bands correspond to species formed at the first (reference) potential or present in greater amounts than those at the second potential, the opposite is the case for negative bands. Individual spectra, R1 or Rz, were regularly checked in order to ensure the absence of any ghost bands in the normalized spectra. These can arise mainly from absorptions due to the window or the electrolyte layer between the electrode and the window.23 The signal-to-noiseratio was improved by collecting a number of interferograms. However, averaging over a large number of interferograms can lead to poor results due to drift in the electrochemical system. It was found that 200 interferograms collected a t each potential were sufficient to obtain good results. This took typically 60-80 s after which the normalized spectrum (hRIR1)was aalculated between the selected and the reference potential. This procedure was then repeated five times, and the final normalized spectrum was obtained by averaging the data. The solutions were deoxygenated with OFN nitrogen (BOC Ltd.). Reproducibility of the electrochemical response was monitored by repeated potential sweeps between +0.2 and -0.85 V a t 10 mV s - l . The potential was held at +0.2 V before spectral collection, and then the electrode was positioned against the cell window prior to measurement. For the study of the spectral response of adsorbed UQlo, the electrode underwent the above procedure in the base electrolyte, was then removed from the cell, was washed with quadruply distilled water, and was dried in a current ofnitrogen, and finally UQlo was spread on the surface. Typically, 400 pL of the stock UQlo solution was applied directly onto the surface in 100 ,uL aliquots with a micropipet. The electrode surface was dried in a stream of nitrogen between additions. The amount of UQlo spread was well in excess ofthat required for monolayer coverage. After repeated scanning, the currents observed on cycling rapidly decreased to a constant value corresponding to monolayer (22)Jayasooriya, M. A,; Chester, M. A,; Howard, M. W.; Kettle, S. E . A.; Powell, D. B.; Sheppard, N. Surf: Sci. 1980,93, 526. (23)Bewick, A,; Kalaji, M.; Larramona, G. J. Electroanal. Chem. Interfacial Electrochem. 1991,318, 207.
FTIR Study of Adsorbed Ubiquinone-IO on Gold
Langmuir, Vol. 11, No. 11, 1995 4579
2oz a
a
10
I
40
-
30 N
’5
0 -
4
2 I
-10
N
5
-
4
E
52
20
-
\
-20
.
-30
-
10
0
-0.90 -0.70 -0.50
-0.30
-0.10
0.10
0.0
0.30
0.2
PotentialIV
b N
0.6
0.4
0.8
1.o
s w e e p r a t e / V s-’
80
40
‘ 1
40
5 4
a
\
w
0
E
2 a 0
-40
-80 -1.00
-0.80
-0.60
-0.40
-0.20
PotentiaVV
Figure 1. (a) Cyclic voltammogram of UQlo adsorbed onto gold in 0.1 M borax. Sweep rate: (a) 0.1, (b) 0.2, (c) 0.3, (d) 0.4, (el0.5,(00.6, (g)0.8, and(h) 1.OVs-l.(b)Cyclicvoltammogram of UQio adsorbed onto gold in 0.1 M NaOH. Sweep rate: (a) 0.05, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4, (0 0.5, and (g) 0.6 V s-l.
coverage, indicating that excess quinone was lost from the electrode surface to yield a reproducible coverage.
Results and Discussion Electrochemistry. Typical cyclic voltammograms of adsorbed UQlo in 0.1 M borax and in 0.1 M NaOH are shown in parts a and b of Figure 1. As previously observed,ll reduction in borax is more irreversible than in NaOH; although a clear reduction peak is observed, the reoxidation is very irreversible. This behavior is in significant contrast with that on mercury5 and is a clear indication of the large differences in interaction energy of the components of the redox couple with these two metal surfaces. Irreversible behavior on solid electrodes in this pH range has been previously observed on glassy carbon3 and on g ~ l d . ~Laval Jl and Majdall observed a more reversible redox behavior than that reported here. The difference with this previous workis in the method ofUQlo deposition. These authorsi1 used a Langmuir-Blodgett (LB) deposition technique which resulted in a much greater surface concentration of UQlo, corresponding to an adsorbed charge of40.5pC cm-2. Fromintegration ofthe areaunder the peaks, the results shown in Figure la,b indicate a of 8.1 and 7.3 pC cm-2 for borax and surface charge (QT) NaOH solutions, respectively. The subtraction of the capacitance current in the measurement of QT is only
0.00
0.1
0.2
0.3
0.4
0.5
0.6
Sweep rate1Vs-l
Figure 2. Dependence of current peak height on sweep rate of U Q ~ adsorbed O onto gold in (a) 0.1 M borax and (b) 0.1 M NaOH.
possible at low sweep rates, and the values of QTare only approximate. These results should be compared with those obtained with H$ for which QT = 5.7 pC cm-2. Considering that a solid electrode has a n effective area greater than that of Hg, these results indicate that the deposition and post-treatment employed in the present work result in a monolayer coverage with UQlo molecules lying flat on the ~ u r f a c e .The ~ LB technique probably leads to a more compact layer with a packing ofisoprenoid chains standing away from the metal surface, a t a tilt angle determined by the deposition surface pressure. Evidence for a flat orientation dependent on the redox state is presented later. Thevoltammetry in 0.1 M NaOH shows sharp reductionoxidation peaks, with a small peak separation of 15 mV a t low sweep rates (50 mvs-l), in agreement with the results of Laval and Majda.’l Also, the present results are similar to those of Schrebler et al.,1,2although the voltammograms a t a pyrolytic graphite electrode showed a significant peak separation for pH values greater than 11. The measured reversibility of the UQ/UQH2 couple observed for more alkaline solutions is similar to the behavior observed on Hg,6 where the standard rate constant increases by a n order of magnitude between pH 9.2 and 13. The dependence of peak height on sweep rate for adsorbed UQlo in 0.1 M borax and in 0.1 M NaOH is shown in Figure 2a,b. These results were corrected for the capacitance charging current contribution by subtraction of the current a t the foot of the wave. It is not easy to
Port et al.
4580 Langmuir, Vol. 11, No. 11, 1995 1643
-0.lOV
-0.35V
-0.20v
-0.30V -0.40V 10''
-0.AOV -0.45V
-0.50V
-0.45V
: 10-4
-0.5ov
-0.55V 1
1410 1320
-0.65V -0.65V
1800
1400
1600
1200
I 1800
1000
I
I
I
1600
1400
1200
I 1000
Wavenumber (cm-')
Wavenumber (cm-'l
Figure 3. SNIFTIRSdifference spectra of a gold electrode in 0.1M borax solution (reference potential: 0.2 V).
Figure 4. SNIFTIRS difference spectra of UQlo adsorbed onto a gold electrode in 0.1M borax solution (reference potential: 0.2 V).
obtain values of total monolayer coverage charge from these results for two reasons. First, if reorientation occurs on electron transfer, a significant contribution to the capacitative currents will result, which cannot be easily quantified. Second, the reduction reactions are complex. For borax it probably follows the sequence5 UQ
+ H20 + e- - UQH' + H 2 0 2UQH'
-
UQ
+ UQH,
(1) (2)
where UQH' is the protonated semiubiquinone radical and UQHz is ubiquinol. The disproportionation reaction (2) is very fast, with a n equilibrium constant of 2 x 1014.5 Since in this case all the chemical steps are very fast, the reaction appears as a n irreversible process with n = 2. For 0.1 M NaOH, the mechanism is different, probably following5
-
+ e- UQ'UQ'- + eUQ2UQ2- + H20 UQH- + OHUQ
-0.55'4
-0.75V
(3)
-0.8OV
(4) (5)
IS00
1600
1400 Wavenumber
Since reactions 3-5 are very fast, the process appears as a 2e- reversible surface reaction. However, differences in the standard potentials of reactions 3 and 4 and the quasi-reversible behavior observed do not allow for a single interpretation of the sweep rate dependence of the peak current. For these reasons, only the results at low sweep rate were used for estimating the surface coverage. Infrared Spectroscopy. Before spectral collection the potential was applied for 5 s, thus ensuring complete oxidation of hydroquinone to quinone, as ascertained by the voltammetric results. Subtractively normalized interfacial FTIR spectra with the reference potential chosen at 0.2 V are shown in Figures 3 and 4 for the experiments in borax and in Figures 5 and 6 for those in NaOH. In both cases, new bands appear in the presence of adsorbed ubiquinone (compare Figures 3 and 4 and Figures 5 and
1200
1000
Icm-'l
Figure 5. SNIFTIRS difference spectra of a gold electrode in 0.1 M NaOH solution (reference potential: 0.2 V).
6). The origins of the bands observed are discussed below in detail and are listed in Table 1. However, it should be noted that for all applied potentials in the absence of ubiquinone only the 0 - H bending of water a t 1643 cm-I is observed, see Figures 3 and 5 . In Figure 3 a small broad positive band is seen between approximately 1410 and 1320 cm-l, which corresponds to the desorption at the negative potentials of tetraborate that has been adsorbed a t the reference potential.24 Due to the high sensitivity, noise is apparent in some spectra between (24)Rao, C.N. R. In ChemicalApplicationsoflnfrared Spectroscopy; Academic Press: New York, 1963.
FTIR Study of Adsorbed Ubiquinone-10 on Gold -0.1ov
-0.45'4
-0.50V
-0.55V
-0.65V
-0.75V
1465
1430 -0.BOV
1800
1600
1400
1200
1000
Wavenumber Icm-'l
Figure 6. SNIFTIRS differencespectra of UQlo adsorbed onto a gold electrode in 0.1 M NaOH solution (reference potential: 0.2V). Table 1. SNIFTIRS Bands Observed for the Reduction of Adsorbed UQlo freauencvhm-' assignment band direction 1650 C=O (quinone) positive C=C conj with C=O 1610 positive C-H bending negative 1465 1430 CHZasym deform negative B-0 str positive 1410 B-0 str 1320 positive C - 0 methoxy positive 1260 methoxy positive 1202 1150 1100
methoxy
positive
methoxy
positive
1800 and 1400 cm-l. The noise observed results from interference due to residues of atmospheric water in the optical path. 1410 and 1320 cm-'. These positive bands appeared only in borax (Figures 3 and 4) a t potentials negative of -0.35Vand are due to the B-0 stretchingvibrationwhich normally occurs between 1320 and 1410 cm-1.24 These positive bands imply a potential dependent specific adsorption of the tetraborate ion a t positive potentials and correspond to a surface species. In order to confirm the specific adsorption of tetraborate, a comparable experiment was carried out using s- instead of p-polarized light a t the same potential.ls A clear absence of any adsorptions due to tetraborate or ubiquinone in this wavenumber region was observed, which confirms that these IR bands correspond to adsorbed tetraborate and not to species in solution. 1650 em-'. In Figures 4 and 6 a positive band a t 1650 cm-l, assigned to the C=O bond,13-17first begins to appear a t approximately -0.35 and -0.45 V, respectively. It gradually increases in intensity a t more negative potentials, indicating a surface concentration of the quinone ring a t the reference potential (0.2 V) greater than that a t more negative potentials. This is a clear indication that the voltammetric response corresponds to the reduction of the quinone moiety. The assignment of this band to the C=O stretching vibration is in agreement with previously reported results.13-17
Langmuir, Vol. 11, No. 11, 1995 4581 1610 cm-l. In addition to the 1650 cm-l band in Figures 4 and 6 a positive band is observed a t 1610 cm-l, appearing
at the same potentials, which is assigned to the stretching vibration of the C=C bond conjugated to C=0.15 This, again, increases for more negative potentials, confirming the reduction of the quinone ring to yield the aromatic quinol. The 1650 and 1610 cm-l bands are detected in the bulk spectrum of UQlo (spectra not shown)13-17and are not due to changes in the water- bendingmode with potential. Additional evidence for the above assignment is their absence when the SNIFTIRS experiment is carried out with no UQlo present (Figures 3 and 5). 1430 and 1465 em-'. These are negative bands which can only be observed in the NaOH solution (Figure 6). Their absence in borax (Figure 4) is due to the large absorption bands in this region that result from the B-0 stretching vibration.24 The bands appeared a t -0.55 V and grow for more negative potentials; the most likely IR assignment is to the in-plane ethylene C-H bending vibration and the CH2 asymmetric deformation vibration.24!25 This indicates that a t increasing negative potentials the normally flat and therefore IR-inactive isoprenoid group may be slightly lifted and, hence, becomes IR active as the quinone is reduced to a flatly adsorbed and aligned aromatic ring. The surface selection rule predicts the absence of infrared activity for the quinol ring in a flat orientation.18J9 1260 cm-'. In both Figures 4 and 6 this positive band appears with increasing negative potential. The infrared spectra of ubiquinone has been known to contain such a band,26 although its assignment has been somewhat ambiguous. Morton et aLZ7proposed that this band was due to the methoxy groups. However, since it also appears in other naturally occurring quinones containing no methoxy groups, it was suggested that it must be due to the quinone group.26 Recent work attributes this to a combination of the C-C stretching of the quinone ring and to C - 0 modes.14 Bands a t 1260 cm-l have been known to appear due to the asymmetric C-0-C stretching vibration of ethers in conjugated systems.25 Therefore, the assignment of the bands is unclear. One possible reason is that these are due to reorientation ofthe adsorbed ubiquinone upon reduction. At positive potentials, the ubiquinone molecule is adsorbed with the quinone ring away from the electrode, with the isoprenoid group anchored to the gold surface.l On reduction, the quinol aromatic ring is aligned and adsorbed flat on the electrode surface due to the stronger interaction of the x electrons with the metal, slightly raising the isoprenoid groups close to the quinone moiety. From the surface selection rules,ls the methoxy groups, which are in the same plane as the aromatic ring, should become IR inactive when the quinone is reduced to the aromatic quinol. A positive band should therefore appear in the difference spectra as is indeed observed. 1202, 1150, and 1100 cm-'. These are weak positive bands but can be clearly observed in Figures 4 and 6, characteristic of the methoxy which increased in intensity with increasing negative potentials. Their appearance as a function of potential can be explained in the light of the reorientation proposal given above. All of the above bands were also observed if ZnSe windows were used instead of CaF2; however, the light ( 2 5 ) Jones, R. N.; Sandorfy, C. in Chemical Applications of Spectroscopy. West, W., Ed.; Interscience: New York, 1956; p 247. (26) Pennock, J. F. InBiochemistry ofQuinones; Morton, R. A., Ed.; Academic Press: London, 1965; p 67. (27) Morton, R. A.; Gloor, U.; Schindler, 0.;Wilson, G. M.; Choparddit-Jean, L. H.; Hemming, F. W.; Isler, 0.;Leat, W. M. F.; Pennock, J. F.; Ruegg, R.; Schwieter, U.; Wiss, 0.Helu. Chim. Acta 1958,41,2343.
Port et al.
4582 Langmuir, Vol. 11, No. 11, 1995
intensities were much smaller. When using s-polarized light, none of the above bands were observed, confirming that these were only due to adsorbed and not solution species. It is proposed that the isoprenoid chain is adsorbed on the metal surface and that the quinone moiety is not present in a flat orientation. On reduction, the stronger n bonding of the quinol with electronic levels in the metal results in a change of orientation of the redox center to make it parallel to the surface. This also causes a slight change in the orientation of the isoprenoid chain next to the quinol group and, hence, in the appearance of C-H bending and CH2 asymmetric deformation vibrations.
Conclusions The electrochemical reduction in aqueous borax solutions of ubiquinone-10 adsorbed on a gold electrode is irreversible. Only a reduction peak can be observed between the potential limits 0.2 to -0.85 V. In sodium
hydroxide solutions sharp reduction-oxidation peaks are observed with a separation of ea. 15 mV a t low sweep rates. Although no reorientation peaks were observed in the voltammograms, the IR spectra show that there is a reorientation of the adsorbed molecule on reduction as a result of the formation of an aromatic ring, which strongly interacts with Au through x bonding. On mercury, reorientation of the adsorbed quinol and quinone occurs a t potentials more negative and positive respectively, compared to that with electron t r a n ~ f e rwhereas ,~ the IR evidence shows that simultaneous electron transfer and reorientation occur on gold.
Acknowledgment. T.S. thanks the British Council for financial assistance that made possible his research visit to Liverpool. Funding for the purchase of the FTIR spectrometer by the University of Liverpool is gratefully acknowledged. LA950494R
