Langmuir 1998, 14, 6167-6172
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Macrodipole Interaction of Helical Peptides in a Self-Assembled Monolayer on Gold Substrate Katsuhiko Fujita,*,†,‡ Natascha Bunjes,‡ Ken Nakajima,† Masahiko Hara,† Hiroyuki Sasabe,† and Wolfgang Knoll†,‡ Frontier Research Program, The Institute of Physical and Chemical Research (Riken), Hirosawa 2-1, Wako, Saitama 351-0198, Japan, and Max-Planck-Institute for Polymer Science, Ackermannweg 10, 55021 Mainz, Germany Received January 29, 1998. In Final Form: August 4, 1998 Monolayers of helical peptides on gold substrates were prepared by a self-assembly approach aiming at fabricating a regular structure by the use of the specific interaction between neighboring helices. Two kinds of helical peptides were synthesized. One has a disulfide group at the N-terminal of the helix part, and the other, at the C-terminal of the same helix. Self-assembled monolayers (SAMs) were prepared by dipping a gold substrate into each peptide solution. The thickness and the molecular orientation of each monocomponent SAM indicated that the helical peptides were adsorbed on the surface with a preferred orientation parallel to the surface. However, those of an equimolar mixed SAM showed that the peptide took an appreciably vertical orientation on the surface. These observations indicated that an antiparallel helix packing is significantly more favorable than a parallel one. It is strongly suggested that the SAM structure is regulated by a dipolar interaction between helical peptides since the geometric fitting among the molecules in the parallel packing could be hardly different from that in the antiparallel packing.
Introduction Self-assembled monolayers (SAMs) of organosulfur compounds tethered on gold substrates via gold-sulfur interactions have been widely studied as a simple approach for modifying surface properties.1,2 Many alkanethiol and dialkyl disulfide derivatives are known to be arranged spontaneously in well-ordered molecular arrays when gold substrates are exposed to the solutions containing the compounds.3,4 Recently, a practical interest began to be paid to SAMs composed of helical peptides5-8 in an attempt to utilize their unique properties such as the rigid structure, the macrodipole moment, and the possibility to place a desired functional group at a predetermined position along the helix axis. In naturally occurring proteins, helix bundle structures are often found.9 The packing manner of the helices in the bundles is regulated by several interactions: for example, electrostatic interaction between charged groups, dipolar interaction between macrodipoles of helices, and steric interlocking * To whom correspondence should be addressed. Present address: Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan. E-mail:
[email protected]. † The Institute of Physical and Chemical Research (Riken). ‡ Max-Planck-Institute for Polymer Science. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Reviewed in: Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Part 3, Academic Press: San Diego, 1991. (3) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (4) Widrig, C.; Alves, C. A.; Porter, M. J. J. Am. Chem. Soc. 1991, 113, 2805. (5) Whitesell, J. K.; Chang, H. K. Science 1993, 261, 73. (6) Worley, C. G.; Linton, R. W.; Samulski, E. T. Langmuir 1995, 11, 3805. (7) Sano, K.; Machida, S.; Sasaki, H.; Yoshiki, M.; Mori, Y. Chem. Lett. 1992, 1477. (8) Heise, A.; Menzel, H.; Yim, H.; Foster, M. D.; Wieringa, R. H.; Schouten, A. J.; Erb, V.; Stamm, M. Langmuir 1997, 13, 723. (9) for example Fermi, G.; Perutz, M. F. Atlas of Molecular Structures in Biology. 2. Haemoglobin and Myoglobin; Clarendon Press: Oxford, England, 1981.
Figure 1. Molecular structure of peptide I and peptide II.
between side-chains.10 For the realization of a sophisticated SAM system with a regulated molecular packing and a desired function, it will be necessary to take such interactions into account. A peptide containing R-aminoisobutyric acid (Aib) tends to form a helical structure because of the steric hindrance around the R-carbon atom of Aib residues.11,12 For example, a hexadecapeptide composed of L-alanine (Ala) and Aib protected by a tert-butyloxycarbonyl (Boc) group and a methoxy (OMe) group on each terminal, Boc-(AlaAib)8-OMe (BA16M), has been revealed by X-ray diffraction13 to take an R-helical structure in the crystalline state. The dimensions of the unit cell were 9.3 × 9.3 × 25.7 Å, which are in good agreement with the ideal value for the R-helical structure.14 In the present study, two kinds of BA16M derivatives (Figure 1) with a disulfide group were synthesized to investigate their properties after self-assembling into a monolayer. BA16M has a welldefined structure and minimum side-chains which enable (10) Branden, C.; Tooze, J. Introduction to Protein Structure; Garland: New York, 1991. (11) Burgess, A. W.; Leach, S. J. Biopolymers 1973, 12, 2599. (12) Karle, I. L.; Balaram, P. Biochemistry 1990, 29, 6747. (13) Otoda, K.; Kitagawa, Y.; Kimura, S.; Imanishi, Y. Biopolymers 1993, 33, 1337. (14) Lavigne, P.; Tancrede, P.; Lamarche, F.; Max, J.-J. Langmuir 1992, 8, 1988.
S0743-7463(98)00115-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/19/1998
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to simplify the problem of the helix interaction. Peptide I has two helices connected at the N-terminus with a short spacer containing a disulfide bond, and peptide II has the equivalent disulfide bridge at the C-terminus. SAMs were prepared by immersion of gold substrates into the solutions containing peptide I, peptide II, or an equimolar mixture of both. The resulting two kinds of monocomponent SAMs and an equimolar mixed SAM were investigated with surface plasmon spectroscopy (SPS), X-ray reflectivity (XR), scanning tunneling microscopy (STM), and Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS). The molecular packing of the tethered helical peptides will be discussed in terms of the interactions between the helices based on the determined thickness and the molecular orientation in each SAM. Materials and Methods
Fujita et al. model: glass/gold/SAM/air or ethanol. A Teflon cell with an inlet and an outlet port was attached on the gold substrate to measure the adsorption kinetics. The reflectivity curves were measured before and after SAM formation on the substrate both in ethanol (in situ) and in the air (ex situ). For kinetics measurement, the angle of incidence was fixed at 0.5° lower than the angle corresponding to the minimum of the initial reflectance curve. X-ray reflectivity (XR) measurements were carried out using Cu KR radiation from a 18 kW rotating anode with graphite monochromator.17 The reflectivity curves were recorded before and after SAM deposition. These curves are analyzed in assuming the electron density profile across the film through a distinct layer model of air/gold/SiOx and air/SAM/gold/SiOx with the interfaces broadened by convoluting both electron densities with a Gaussian function characterizing by a mean square roughness of the interfaces. The parameters such as the layer thickness, electron densities, and roughness were varied within the narrow range limited by physical constraints and recently reported results for each layer,18 where the electron density for the SAM was varied around the density of the bulk peptide, until the model adequately simulated the experimental reflectivity curves. Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) was performed on a Mattson Infinity spectrometer with an mercury-cadmium-tellurium (MCT) detector, a Pike reflectance accessory of the fixed angle of incidence at 80°, and a ZnSe polarizer. The measurements were carried out with a resolution of 4 cm-1 using an untreated gold substrate as a reference. The optical path was purged with dry nitrogen before and during measurements. Scanning tunneling microscopy (STM) was carried out on NanoScope III (Digital Instruments) with Pt-Ir tip (Digital Instruments). STM images were obtained in the constant current mode with the bias voltage at 500 mV (sample positive) and the setpoint current at 100 pA.
Peptide Preparation. Boc-(Ala-Aib)8-OCH2C6H5 (BA16B), TFA‚H-(Ala-Aib)8-OCH2C6H5 (HA16B), and Boc-(Ala-Aib)8OH (BA16OH) were prepared as previously reported.15 HA16B was reacted with 1/2 mol of 3,3′-dithiodipropionic acid in dimethylformamide for 12 h by a coupling reagent, 0-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU), in the presence of a 1.5-fold excess of triethylamine, followed by gel filtration with a Sephadex LH-20 (Pharmacia) column using methanol as the eluant, to obtain peptide I. Peptide II was synthesized by coupling BA16OH with 1/2 mol of cystamine dihydrochloride using HATU in the presence of a 2.5-fold excess of triethylamine and purified in the same procedure as peptide I. Peptide I and peptide II were observed as a single peak on an HPLC with reverse phase ODS column, Mightysil RP-18 (22 cm, Kanto Chemical), using a linear gradient from methanol/ water (7/3, v/v) to methanol. SAM Preparation. Glass slides (3.5 × 2 cm2, LaSFN9 and fused silica) were cleaned in 5% Extran in Milli-Q ultrapure water and rinsed with Milli-Q water carefully. The slides were then dried in a nitrogen stream and placed in an Edwards Auto 360 evaporator. On the LaSFN9 glass, 50 nm of gold was deposited directly and the substrate was used for SPS kinetics measurement immediately. On fused silica glass used for FT-IR and SPS measurements in air, 1 nm of chromium was evaporated first to promote gold adhesion, followed by deposition of 50 nm of gold. For an XR measurement a larger glass plate of 10 × 5 cm2 was used. Then 10 nm of gold was deposited onto the plate after gentle rinse with toluene. The substrates used for STM investigations were prepared by evaporation of gold on freshly cleaved mica plates while keeping the substrate temperature at 570 °C and the pressure below 10-7 Torr. Each peptide SAM was formed by immersion of a substrate in the ethanolic solution of the objective peptide for 2 h. The sample plates for SPS in air and FT-IRAS were rinsed by agitating in pure ethanol, and those for XR and STM were cleaned by pouring ethanol gently onto the plate to prevent damage of the surface. Measurements. Circular dichroism (CD) spectra of the ethanol solutions were recorded at room temperature on a JASCO J-700 CD spectropolarimeter using optical cells of 1 and 0.01 cm path lengths for a 1 µM solution and for a 100 µM solution, respectively. Surface plasmon spectroscope (SPS) measurements were performed by homemade Kretschmann configuration equipment16 with a He-Ne laser (λ ) 633 nm) as the light source. The reflectivity of the p-polarized light from the gold/glass interface was monitored as a function of the incident angle. The reflectivity spectrum shows a sharp reduction as the incident radiation couples with the surface plasmon polariton along the interface and the shape of the spectrum depends on the optical constants and thickness of the gold and of the dielectric adsorbed layer. The optical thickness of the adsorbed SAM can be determined with Fresnel reflectivity calculation16 using a four-layer slab
Conformation and Aggregation in Ethanolic Solution. CD spectra of peptide I, peptide II, and the equimolar mixture of both peptides in ethanol (Figure 2) showed a double-minimum profile, which is characteristic of a helical structure.19 The molar ellipticity per residue at 222 nm of each spectrum was comparable with that of BA16M in ethanol. The modification of the peptides hardly caused disturbance on the conformation. All of these spectra were identical at the concentrations of 1 and 100 µM. This concentration independence indicated that every peptide is dispersed in the solution without significant aggregation. When each ethanolic solution of peptide I, peptide II, or the equimolar mixture at a concentration of 50 µM or 5 mM was applied to a 1.5 m length of a gel filtration column (Sephadex LH-20 with ethanol as an eluent), the peptides were detected at almost the same elution volume in all cases. This observation also indicated the molecular dispersion of the peptides in the solutions. SAM Characterization. Figure 3 displays adsorption isotherms of peptide I, peptide II, and the equimolar mixture from the ethanolic solutions at concentrations of 18 and 90 µM, which were recorded by SPS at an incidence angle of 58.3°. The vertical axis was converted from the recorded reflectivity to the adsorbed layer thickness by calculation from fitting SPS curves taken before and after the adsorption measurement under the assumption that the refractive index of the layer is equal to 1.5, which is often adopted for organic thin layers having no absorption bands in the considered range of wavelength. The thickness increased immediately after the peptide injec-
(15) Fujita, K.; Kimura, S.; Imanishi, Y.; Rump, E.; Ringsdorf, H. Langmuir 1994, 10, 2731. (16) (a) Knoll, W. MRS Bull. 1991, XVI, 29. (b) Phelpes, J. M.; Taylor, D. M. J. Phys. D: Appl. Phys. 1996, 29, 1080.
(17) Foster, M.; Stamm, M.; Reiter, G.; Hu¨ttenbach, S. Vacuum 1990, 41, 1441. (18) Stamm, M.; Reiter, G.; Kunz, K. Physica B 1991, 173, 35. (19) Holzwarth, G.; Doty, P. J. Am. Chem. Soc. 1965, 87, 218.
Results
Macrodipole Interaction of Helical Peptides
Figure 2. CD spectra of peptide I (- -), peptide II (s), and the equimolar mixture (- s -) in ethanolic solutions at concentrations of (a) 1 µM and (b) 100 µM.
tion at t ) 0 and reached a quasi-plateau after several minutes in every case. Although the thickness kept increasing gradually to values that varied depending on the peptide concentration after reaching the quasi-plateau, the curves of every peptide system went down into a narrow thickness range after exhaustive rinse with more than 50-fold cuvette volume of fresh ethanol. The thickness ranges after rinse did not change even for much longer immersion times, for example 6 h. These results indicated that the peptide spontaneously formed a layer on the gold surface and that only a weakly adsorbed fraction of the
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Figure 3. Adsorption isotherms of (a) peptide I, (b) peptide II, and (c) the equimolar mixture in ethanolic solutions at the concentration of 18 µM (s) and 90 µM (- - -). Arrows indicate the time at which the cell was rinsed with excess amount of ethanol.
layer increased gradually and was removed by the rinsing process. Notably, peptide II formed a relatively thick weakly adsorbed fraction of the layer compared with the others. The thickness of each SAM determined by SPS is summarized in Table 1. Measurements were repeated three times at peptide concentrations of 18, 50, and 90 µM. Although the SAMs show a thickness in the range of a monomolecular layer, there was a significant difference between the monocomponent SAMs and the equimolar mixed SAM. The former was about 10 Å, which is very close to the diameter of the helix, and the latter was about 20 Å, which is slightly shorter than the molecular length along the helix axis.
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Table 1. Thickness of the Peptide SAMs Determined by SPS (Å) in ethanol (in situ) 18 µM peptide I peptide II equimolar mixture a
in air (ex situ)
50 µM 90 µM
11 ( 2a 8 ( 2a 18 ( 4a
12 10 16
14 9 19
18 µM
50 µM
90 µM
12 ( 3a 13 ( 4a 15 ( 3a 11 ( 2a 10 ( 2a 13 ( 5a 16 ( 4a 18 ( 2a 17 ( 4a
Summary of the measurements repeated three times.
Table 2. Thickness and Surface Roughness of the SAMs Determined by XR (Å)a peptide I peptide II equimolar mixture
thickness
roughness
12.4 27.0 23.5 23.8
8.4 8.6 10.1 7.8
a Two independent measurements were carried out for the equimolar mixture. Variation of the roughness was dependent on the evaporation batch.
Figure 5. FT-IRAS of peptide I (- -), peptide II (s), and the equimolar mixture (- s -) on a gold substrate, where the resolution ) 4 cm-1, accumulation ) 600 scans, and the incident angle ) 80°.
Figure 4. STM image of the equimolar mixed SAM on a gold substrate, where the scan size ) 164.4 nm, the scan rate ) 7.63 Hz, the setpoint current ) 100 pA, the bias voltage ) 500 mV (sample positive), and the imaging mode is constant current.
The monocomponent and equimolar mixed SAMs prepared in 50 µM ethanol solutions were examined by XR. The determined thickness was summarized in Table 2. The thickness of SAMs consisting of peptide I and of the equimolar mixture was consistent with the corresponding SPS results, but that of peptide II was much thicker than the SPS result. We needed to employ a much milder rinsing procedure in the sample preparation for XR than that for SPS and FT-IR because an agitation of the plate in pure ethanol caused obscure fringes of the XR profile. This might be due to the surface damage of the gold. Since the much thinner gold layer was evaporated on the plate for XR without a support layer of chromium in order to facilitate the evaluation of the fringes shifted by the SAM adsorption, it should be damaged more easily. Peptide II gave the thick weakly adsorbed layer before the exhaustive rinse (Figure 3). Therefore, the significantly large thickness of peptide II layer determined with XR might be caused by the weakly adsorbed portion remaining after the mild rinsing procedure. In the STM images of the equimolar mixed SAM (Figure 4), a fairly smooth surface without significant aggregation, atomic steps of gold terraces, and some depressions having monatomic depth were observed. Such depressions, so-
called etch pits, are usually observed in STM images of alkanethiol SAMs on gold substrates and are believed to be collection of atomic vacancies which are created in the top layer of gold during the self-assembly process.20 The etch pits with clear contours indicate that the resolution of the image is better than several nanometers at the surface. These observations strongly suggested that the peptide molecules were homogeneously adsorbed on the substrate without forming partial multilayer or aggregates. Although the SAMs composed of peptide I showed also smooth surfaces in the STM images, the SAMs composed of peptide II were different from both cases. We hardly observed any reasonable images at the first approach of the tip to the surface of the peptide II SAM, but very noisy images were observed after several scans were repeated. Imaging was not improved if the tunneling current was set as low as possible. This aspect is often observed for contaminated and/or multilayered samples and could be explained as follows. There might be some peptide molecules on the top of the SAM. The weakly bound peptides would be removed by the STM tip during the repeated scans. The contaminated tip produced the very noisy image. Consequently, the STM investigation supported the existence of the weakly adsorbed portion on the SAM of peptide II after the mild rinse. The SAMs prepared by immersion of gold substrates for 2 h in 80 µM ethanolic solutions of peptide I, peptide II, and the equimolar mixture were examined by FTIRAS (Figure 5). Two peaks corresponding to the amide I band and the amide II band were observed at 1669 and 1540 cm-1, respectively, in each spectrum of the monocomponent SAMs and the equimolar mixed SAM, while those peaks were observed at 1664 and 1539 cm-1 in a transmission spectrum of BA16B in a KBr pellet. An amide group free from a hydrogen bond is known to show the absorption of the amide I band at 1680 cm-1.21 The peak shift from 1664 to 1669 cm-1 in the amide I band could be caused by a certain degree of disturbance in the (20) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (21) Kennedy, D. F.; Crisma, M.; Toniolo, C.; Chapman, D. Biochemistry 1991, 30, 6541.
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Table 3. Intensity Ratio of the Amide I Band to the Amide II Band and the Calculated Tilt Angle of the Helix Axes from the Surface Normala peptide I peptide II equimolar mixture
amideI/amideII
tilt angle (deg)
3.6 1.1 5.5
36 66 26
a AmideI/amideII at the random orientation was 1.54, obtained from a transmission spectrum of BA16B in a KBr pellet, and a uniform angle distribution around the helix axis is assumed.
conformation. However, the peptides should practically maintain the helical structure because the peak shift was relatively small. The difference in the intensity ratio of the two peaks can be explained in terms of the orientation of the helix peptide on the substrate.22 In a RAS measurement an absorption ascribed to a transition moment perpendicular to the surface becomes intensive owing to the surface selection rules. Since the transition moment of the amide I band in a helical peptide is directed appreciably along the helix axis and that of the amide II band is directed orthogonal to the axis, the helix peptide should be oriented with a smaller tilt angle away from the surface normal as the intensity ratio of amide I/amide II is larger. It is, therefore, concluded that the helical peptides in the equimolar mixed SAM have the smallest tilt angle among the three SAMs because of the largest intensity ratio. Discussion When a peptide takes the R-helical conformation, transition moments of amide I and amide II should be located in a plane formed by the CONH atoms of the amide group and the angles of the moments with respect to the helix axis are reported to be 39° and 75°, respectively, as was determined in an R-helical peptide, poly(γ-benzylL-glutamate).23 In assuming a perfect R-helical structure and a uniform orientation of the BA16M derivatives on the substrate, the tilt angle of the helix axis with respect to the surface normal can be estimated by a RATIO method24 from the intensity ratio of amide I/amide II. The intensity ratios and the calculated tilt angles were summarized in Table 3, where the intensity ratio for a random orientation distribution is 1.54, as obtained from a transmission spectrum of BA16B in a KBr pellet, assuming a uniform angle distribution around the helix axis. We did not take into account for this estimation any disorder of the orientation. In particular, when the intensity ratio is small and close to the ratio at the random orientation, a uniform orientation with the estimated tilt angle gives an intensity ratio similar to that for a disordered orientation with a significant degree.25 Although the layer thicknesses expected from the tilt angles and the molecular length are relatively larger than those estimated by SPS (Table 1) or XR (first line in Table 2), it should result from experimental errors and overestimation of the refractive index and the electron density for the SAM layers due to the disorderness or the anisotropy. In the case of alkanethiol SAMs, a maximum van der Waals contact between aliphatic chains and a molecular packing commensurate with the surface lattice of the substrate are accommodated by a tilted molecular ori(22) Fujita, K.; Kimura, S.; Imanishi, Y.; Okamura, E.; Umemura, J. Langmuir 1995, 11, 1675. (23) Tsuboi, M. J. Polymer Sci. 1962, 59, 139. (24) Debe, M. K. J. Appl. Phys. 1984, 55, 3354. (25) Enriquez, E. P.; Samulski, E. T. Mater. Res. Soc. Symp. Proc. 1992, 255, 423.
Figure 6. Schematic illustration of the peptide SAM model: (a) a cylinder of the same dimensions as the helix peptide; (b) the arrangement of the hollow sites on Au(111) and top view of the helices with the centers located on the hollow sites; (c) a cylinder arrangement when the contact area among the cylinders is maximized.
entation.26 To discuss the molecular packing in the SAM, let us consider a simple model (Figure 6) consisting of a substrate, Au(111), and a cylinder with a radius of 4.65 Å and a length of 25.7 Å, which correspond to the dimensions of BA16M determined by X-ray scattering. It has been pointed out that a 3-fold hollow site is the most stable binding site for the chemisorption of a thiol on Au(111).2 The hollow sites available for the chemisorption are arranged in a hexagonal fashion 4.99 Å apart from each other on the surface. Since the cylinder radius is almost comparable to the distance between the hollow sites, the cylinders can be arranged in a hexagonal fashion with twice as long periodicity as the hollow sites when every cylinder axis is perpendicular to the surface and the centers of the cylinders are located on the hollow sites. In this case, the slight difference between the radius of the cylinder base and the distance between the hollow sites causes vacancies among the cylinders. Suppose the cylinders are tilted in a same way to establish optimum van der Waals contacts, which should be realized by maximizing the contact area among the sides of the cylinders. Then the tilt angle with respect to the surface normal and the thickness of the layer are calculated to be 24° and 23.5 Å, respectively. The experimental results of the thickness (Tables 1 and 2) and the tilt angle (Table 3) for the equimolar mixed SAM are in good agreement with the model calculation, and the surface of the SAM was observed to be fairly homogeneous. The tilt angles (26) Ulman, A. Chem. Rev. 1996, 96, 1533.
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for alkanethiol SAMs, which are densely packed and well ordered, have been reported to be about 20-30°. Therefore, the helical peptides should be packed densely and oriented basically perpendicular to the surface in the equimolar mixed SAM. Since the layer thickness and the tilt angle indicated a relatively flat lying or disordered orientation in the monocomponent SAMs, an antiparallel packing of the helices is considerably more favorable than a parallel packing on the substrate when the tilt angle is small. From the fact that aggregate formation was not observed in the solutions, it is concluded that individual peptides are organized in the SAMs after the adsorption on the gold surface. It is hardly conceivable in terms of sidechain interactions or an interlocking due to unevenness on the side of the helix that an antiparallel packing is significantly more favorable, because the peptide molecules have smooth surfaces without significant bulges of side chains or depressions on the side of the helix. The helical peptides carry a macrodipole moment27 along the helix axis of 55 D per helix. The molecular packing in the SAMs could be discussed in terms of the dipolar interaction. In particular, a dipole placed near a conductor surface is stabilized by an image dipole and the vector summation of the dipole and its image dipole strongly depends on their orientation with respect to the surface. A vertically oriented dipole is enhanced by the image aligned in the same direction though the enhancement is getting smaller as the tilt angle is getting larger.28 Antiparallel dipoles placed in a certain distance interact attractively with each other when the tilt angle is small enough. Thus, an antiparallel packing of the helices is energetically more favorable and that the dipolar interaction between the thin helices can be dominant for the molecular packing on a conductor surface though it has been reported that such dipolar interactions cause little energy difference between a parallel and an antiparallel helix packing in solution.29 There were significant differences between the monocomponent SAMs. Peptide II had more tilted or more disordered orientation in the monocomponent SAM and a larger amount of weakly adsorbed fraction after the mild rinse than peptide I. Peptide II has a disulfide group connected to an Aib residue via the short spacer while (27) Wada, A. Adv. Biophys. 1976, 9, 1. (28) Iizuka-Sakano, T.; Fujita, K.; Ishoshima, T.; Wada, T.; Sasabe, H. Abstract for Kerea-Japan Joint Forum 98, Organic Materials for Electronics and Photonics; Sapporo, Japan, 1998; p 73. (29) Gilson, M. K.; Honig, B. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 1524.
Fujita et al.
peptide I has that connected to an Ala residue. Since the dihedral angle around the R-carbon of Aib is restricted due to the steric hindrance more than in case of Ala, the distance and the angle between the connected two helices should be restricted to small and large values, respectively. They might prevent a flexural structure turned in the spacer. If the cleavage rate of the disulfide bond or the diffusion of the helices after the cleavage on the surface is relatively slow compared with the adsorption, the large angle between the helices should be kept for a while after the adsorption and should cause the disorder of the helix orientation on the surface. This is probably why the monocomponent SAM of peptide II shows the larger average tilt angle and the thicker weakly adsorbed fraction. In the present study, we showed a practical example where a molecular assembly structure of helix peptides is regulated by interaction between helix macrodipoles. The largest intensity ratio of amide I/amide II among the recently reported SAMs of helix polypeptides6,30 was observed in FT-IRAS of the equimolar mixed SAM, indicating that the helices were mostly oriented perpendicular to the surface. Notably, the dipolar interaction between helices and the molecular dimensions commensurate with the surface lattice should be taken into account for the molecular design of helix peptides to fabricate more sophisticated SAM systems with a finely regulated structure. However, it was also found that an appropriate molecular design avoiding steric hindrance on adsorption allows for the preparation of a directionally aligned SAMs with a uniform dipolar sense if a certain degree of disordered or tilted orientation is acceptable for that purpose, as seen in the monocomponent SAM of peptide I. To construct a more sophisticated organic nanostructure, we need to develop methods to place a desired function group at a predetermined position in the nanostructure and to manipulate electrostatic properties such as capacitance and pyroelectricity. The oligopeptides are expected to widen the range of possibilities for the sophisticated nanomaterials on account of the regulated molecular arrangement in the SAMs and the capability of chemical modification. Acknowledgment. We express our sincere thanks to the support from a Special Postdoctoral Researchers Program at Riken and from the Max-Planck-Society through a grant from the Schloessmann Foundation. LA9801155 (30) Frey, B. L.; Corn, R. M. Anal. Chem. 1996, 68, 3187.
