Reversible Photoisomerization of Self-Organized Cylindrical Peptide

A new photoswitchable peptide system composed of two flat, ring-shaped cyclic ... sequence cyclo-[(L-Phe-D-MeN-Ala)3-L-Cys-D-MeN-Ala] tethered via an ...
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Langmuir 1999, 15, 3956-3964

Reversible Photoisomerization of Self-Organized Cylindrical Peptide Assemblies at Air-Water and Solid Interfaces Claudia Steinem, Andreas Janshoff, Martin S. Vollmer, and M. Reza Ghadiri* Departments of Chemistry & Molecular Biology and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California 92037 Received November 19, 1998. In Final Form: March 12, 1999 A new photoswitchable peptide system composed of two flat, ring-shaped cyclic octapeptides with the sequence cyclo-[(L-Phe-D-MeN-Ala)3-L-Cys-D-MeN-Ala] tethered via an azobenzene moiety was investigated at the air-water interface, on mica, germanium, and quartz glass supports. Surprisingly, both the E- and Z-forms of the peptide system form very stable layers at the air-water interface. The surface pressurearea isotherms of each isomer are readily distinguished by plateau regions at 12.5 mN/m (E-isomer) and 14.0 mN/m (Z-isomer). Scanning force microscopy (SFM) was employed to scrutinize the structure of transferred Langmuir-Blodgett (LB) films on mica. The SFM and film balance measurements as well as the results from ATR-FT-IR spectroscopy indicate that the peptide cylinders are oriented predominantly perpendicular to the surface normal. SFM images demonstrate that at higher surface pressure a second peptide layer atop the first one is formed. The proposed model was further supported by temperaturedependent isotherms and high-resolution SFM images, revealing that the stacking process occurs in the plateau region. Remarkably, the azobenzene subunits could be reversibly isomerized at the air-water interface. The area per molecule increased upon isomerization from the E- to Z-isomer by 70 Å2 at constant pressure. At constant area an increase in surface pressure of 1.3 mN/m was detected by switching from E to Z. As confirmed by UV/vis spectroscopy, peptide LB films transferred onto quartz glass also retain the ability to isomerize.

Introduction The demand for organic materials suitable for reversible optical data storage has led to the design of a variety of molecular systems based on photoisomerization, photocyclizations, photochromic keto-enol tautomerism, and chirooptical switches.1,2 The main objective in this field of research is the design of molecular systems that undergo large changes in structure,3,4 conductivity,5 transmission,6 or helicity7 upon photoisomerization. Among the many different photochromic molecules, azobenzene derivatives are particularly desirable for use in supramolecular, polymeric,8 and especially liquid crystalline systems9,10 due to their highly efficient, reversible photoisomerization and the attendant large changes in molecular * To whom correspondence should be addressed. (1) Rau, H. Photochromism, Molecules and Systems; Elsevier: Amsterdam, 1990. (2) Feringa, B. L.; Jager, W. F.; de Lange, B. Tetrahedron 1993, 49, 8267. (3) (a) Menzel, H.; McBride, J. S.; Weichart, B.; Ru¨ther, M. Thin Solid Films 1996, 284-285, 640. (b) Bu¨chel, M.; Sekkat, Z.; Paul, S.; Weichart, B.; Menzel, H.; Knoll, W. Langmuir 1995, 11, 4460. (c) Menzel, H. Macromol. Chem. Phys. 1994, 195, 3747. (4) (a) Shinkai, S. Pure Appl. Chem. 1987, 59, 425. (b) Shinkai, S.; Minami, T.; Kusano, Y.; Manabe, O. J. Am. Chem. Soc. 1983, 105, 1851. (c) Ueno, A.; Osa, T.; Fukushima, M. Chem. Lett. 1991, 709. (5) (a) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J. Am. Chem. Soc. 1989, 111, 3080. (b) Tachibana, H.; Goto, A.; Nakamura, T.; Matsumoto, M.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. Thin Solid Films 1989, 179, 207. (c) Tachibana, H.; Manda, E.; Azumi, R.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Appl. Phys. Lett. 1992, 61, 2420. (6) Shinikai, S.; Aoki, M.; Ikeda, A.; Murata, K.; Nishi, T. J. Chem. Soc., Chem. Commun. 1991, 1715. (7) (a) Goodman, M.; Kossoy, A. J. Am. Chem. Soc. 1966, 88, 5010. (b) Goodman, M.; Falxa, M. L. J. Am. Chem. Soc. 1967, 89, 3863. (c) Ueno, A.; Anzai, J.; Osa, T.; Takahshi, K. J. Am. Chem. Soc. 1981, 103, 6410. (8) (a) Willner, I.; Rubin, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 367. (b) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915.

geometry.1,3,11-24 However, photoinduced mesoscopic structural changes in LB films are rarely observed.11,25 The primary drawbacks of azobenzene derivatives that have hampered their utility in molecular devices stem from (9) (a) Seki, T.; Tamaki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K. Macromolecules 1989, 22, 3505. (b) Schmidt, A.; Sawodny, M.; Knoll, W.; Urban, C.; Ringsdorf, H.; Ahuja, R. C.; Mo¨bius, D. Acta Polym. 1994, 45, 217. (c) Sawodny, M.; Schmidt, A.; Stamm, M.; Knoll, W.; Urban, C.; Ringsdorf, H. Polym. Adv. Technol. 1991, 2, 127. (10) (a) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (b) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature 1993, 361, 428. (c) Yu, H. Z.; Wang, Y. Q.; Cheng, J. Z.; Zhao, J. W.; Cai, S. M.; Inokuchi, H.; Fujishima, A.; Liu, Z. F. Langmuir 1996, 12, 2843. (d) Ka¨mpf, G. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1179. (e) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (11) Scho¨nhoff, M.; Chi, L. F.; Fuchs, H.; Lo¨sche, M. Langmuir 1995, 11, 163. (12) (a) Yamazaki, I.; Ohta, M. Pure Appl. Chem. 1995, 67, 209. (b) Iwamoto, M.; Noguchi, T.; Fuwa, H.; Majima, Y. Jpn. J. Appl. Phys. 1991, 30, 1020. (c) Iwamoto, M.; Majima, Y.; Naruse, H.; Noguchi, T.; Fuwa, H. Nature 1991, 353, 645. (d) Higuchi, M.; Minoura, N.; Kinoshita, T. Langmuir 1997, 13, 1616. (e) Sato, T.; Ozaki, Y.; Iriyama, K. Langmuir 1994, 10, 2363. (f) Durbin, M. K.; Malik, A.; Richter, A. G.; Yu, C.-J.; Eisenhower, R.; Dutta, P. Langmuir 1998, 14, 899. (g) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, E.; Kang, H.; Michel. B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (h) Mazumoto, M.; Tachibana, H.; Sato, F.; Terrettaz, S. J. Phys. Chem. B 1997, 101, 702. (i) Zhao, C.-X.; Zhang, J.; Liu, Z.-F. Chem. Lett. 1997, 473. (j) Zhao, J.; Wu, Z. Y; Zhang, J.; Zhu, T.; Ulman, A.; Liu, Z. F. Langmuir 1997, 13, 2359. (13) Maak, J.; Ahuja, R. C.; Tachibana, H. J. Phys. Chem. 1995, 99, 9210. (14) Wang, Y. Q.; Yu, H. Z.; Mu, P.; Luo, Y.; Zhao, C. X.; Liu, Z. F. J. Electroanal. Chem. 1997, 438, 127. (15) Ahuja, R. C.; Maak, J.; Tachibana, H. J. Phys. Chem. 1995, 99, 9221. (16) Higuchi, M.; Minoura, N.; Kinoshita, T. Colloid Polym. Sci. 1995, 273, 1022. (17) Ve´lez, K.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (18) Wang, R.; Iyoda, T.; Jiang, L.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 438, 213. (19) Maak, J.; Ahuja, R. C.; Mo¨bius, D.; Tachibana, H.; Matsumoto, M. Thin Solid Films 1994, 242, 122.

10.1021/la981624+ CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999

Peptide Assemblies at Air-Water and Solid Interfaces Scheme 1

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composed of an even number of alternating D- and L-Ramino acids can adopt flat, ring-shaped structures and under appropriate conditions self-assemble to form β-sheetlike hydrogen-bonded tubular structures.26 The solution phase structural and optical properties of 1 have been recently characterized, highlighting a number of salient features, as demonstrated by 1H NMR, IR, and UV/vis studies.27 The E-isomer was found to exist as an “assembly pool” of various intermolecularly hydrogen-bonded aggregation states (E-1). Irradiation with UV light promoted quantitative E f Z-isomerization to form a discrete intramolecularly hydrogen-bonded species Z-1. Furthermore, visible light treatment results in Z f E-isomerization producing a completely reversible photoswitchable system. These results prompted investigation of the selforganization and switching properties of 1 at both the air-water interface and on solid supports. The results outlined below describe the use of surface pressure-area isotherms, scanning force microscopy (SFM), IR and UV/ vis spectroscopy to probe the orientation and isomerization ability of peptide monolayers at interfaces. Reversible changes in surface tension and area/molecule upon photoisomerization were observed by illuminating a monolayer at the air-water interface and subsequently monitoring the surface pressure at constant area or alternatively by recording the area/molecule at constant surface pressure. In addition, reversible isomerization of the LB films on quartz glass was also demonstrated by UV/vis spectroscopy. Results

the thermodynamic instability of the Z-isomer and/or difficulties in photoisomerization of solid supported constructs due to the steric congestion and close packing of Langmuir-Blodgett (LB) films.25 Strategies to facilitate E f Z-isomerization in densely packed LB-films have included modification of azobenzenes with bulky hydrophobic or hydrophilic moieties, use of crown-ether derivatized azobenzenes, and fabrication of LangmuirBlodgett-Kuhn films of hairy rodlike azobenzenederivatized polyglutamates.3,14,22,24 In this paper, we describe the structure and function of photoswitchable peptide monolayers composed of two cyclic octapeptides tethered via azobenzene (compound 1, Scheme 1). As described previously, cyclic peptide subunits (20) (a) Anzai, J.; Osa, T. Tetrahedron 1994, 50, 4039. (b) Kinoshita, T. J. Photochem. Photobiol. B: Biol. 1998, 42, 12. (c) Kimizuka, N.; Kawasaki, T.; Kuniktake, T. Chem. Lett. 1994, 1399. (d) Evaraars, M. D.; Marcelis, T. M.; Sudhoelter, J. R. Liebigs Ann. 1997, 21. (e) Evaraars, M. D.; Marcelis, T. M.; Sudhoelter, J. R. Colloids Surf. A 1995, 102, 117. (f) Song, X.; Geiger, C.; Vaday, S.; Perlstein, J.; Whitten, D. G. J. Photochem. Photobiol. A: Chem. 1996, 102, 39. (21) (a) Tazuke, S.; Horiuchi, S.; Ikeda, T.; Karanjit, D. B.; Kurihara, S. Macromolecules 1990, 23, 36. (b) Tazuke, S.; Horiuchi, S.; Ikeda, T.; Karanjit, D. B.; Kurihara, S. Macromolecules 1990, 23, 42. (c) Tazuke, S.; Horiuchi, S.; Ikeda, T.; Karanjit, D. B.; Kurihara, S. Chem. Lett. 1988, 1679. (22) (a) Seki, T.; Ichimura, K. Thin Solid Films 1989, 179, 77. (b) Nishijama, K.; Fujihira, M. Chem. Lett. 1988, 1257. (c) Zang, R.; Zang, X.; Shen, J. Langmuir 1994, 10, 2727. (d) Zang, R.; Zang, X.; Wang, J.; Shen, J. Thin Solid Films 1994, 248, 102. (e) Haitjema, H. J.; Tan, Y. Y.; Challa, G. Macromolecules 1995, 28, 2867. (23) Seki, T.; Ichimura, K.; Fukuda, R.; Tamaki, T. Thin Solid Films 1996, 284-285, 365. (24) Menzel, H.; Hallensleben, M. L.; Schmidt, A.; Knoll, W.; Fischer, T.; Stumpe, J. Macromolecules 1993, 26, 3644. (25) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Joshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479.

Surface Pressure-Area Isotherms. Film balance measurements provide valuable information about the alignment of the peptides at interfaces and reveal twodimensional phase diagrams. The E-isomer of peptide system 1 (which contains about 10% of the Z-form) is spread from a chloroform solution at the air-water interface of a Langmuir trough. The most striking feature of the pressure-area isotherms of the E-isomer at 20 °C is the plateau region at (12.5 ( 0.2) mN/m (Figure 1). It extends from an area/molecule of 205 ( 5 to 140 ( 5 Å2 with almost infinite compressibility. At higher surface pressure, the area/molecule decreases to 130 ( 10 Å2.28 The isotherms display significant hysteresis, which becomes more pronounced at higher compression/expansion speed. Nonetheless, reproducible compression and expansion cycles up to pressures of 25 mN/m indicate film stability at the air-water interface and suggest that solubility of the peptide in the subphase is minimal. Reversible compression is further demonstrated by rapid expansion of an E-film compressed to 25 mN/m. Expansion with a barrier speed of 300 cm2/min to a constant area/ molecule of 175 Å2 results initially in a surface pressure of 6-8 mN/m, with restoration of equilibrium pressure (12.5 mN/m) after 3-5 min, which is expected at an area/ (26) (a) Clark, T. D.; Buriak, J. M.; Kobayashi, K.; Isler, M. P.; McRee, D. E.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 8949. (b) Kim, H. S.; Hartgerink, J. D.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 4417. (c) Clark, T. D.; Ghadiri, M. R. J. Am. Chem. Soc. 1995, 117, 12364. (d) Engels, M.; Bashford, D.; Ghadiri, M. R. J. Am. Chem. Soc. 1995, 117, 9151. (e) Granja, J. R.; Ghadiri, M. R. J. Am. Chem. Soc. 1994, 116, 10785. (f) Ghadiri, M. R.; Granja, J. R.; Buehler, L. K. Nature 1994, 369, 301. (g) Khazanovich, N.; Granja, J. R.; McRee, D. E.; Milligan, R. A.; Ghadiri, M. R. J. Am. Chem. Soc. 1994, 116, 6011. (h) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich, N. Nature 1993, 366, 324. (27) Vollmer, M. S.; Clark, T. D.; Steinem, C.; Ghadiri, M. R. Angew. Chem. Int. Ed. Engl., in press. (28) The value is obtained by extrapolating the linear part of the isotherm at low area/molecule to zero pressure.

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Figure 1. Surface pressure-area isotherms (isocycles) of E-1 (s) and Z-1 (- - -) on water at T ) 20 °C. The arrows indicate the direction of compression and expansion, respectively. The insert represents the increase in pressure at a constant area/ molecule of 175 Å2 after fast expansion of the peptide films compressed to 25 mN/m with a barrier speed of 300 cm2/min: E-isomer (b); Z-isomer (O).

molecule of 175 Å2 (Figure 1, inset). It is noteworthy that the cyclic peptides cyclo-[(L-Phe-D-MeN-Ala)4] and cyclo[(L-Phe-D-MeN-Ala)3-L-(pMeBn)Cys-D-MeN-Ala] without the connecting azobenzene linker do not form stable monolayers at the air-water interface. Spreading a chloroform solution of peptide 1 illuminated for 15 min with 366 nm UV light (under these conditions the chloroform solution contains 100% Z-isomer)27 results in the isotherm of the Z-isomer (Figure 1). Although the isotherms of both isomers display the same general shape, the surface pressure of the Z-isomer plateau (14.0 ( 0.3 mN/m) is 1.5 mN/m higher than that of the E-isomer. The area/molecule of the E- and Z-form at high surface pressures is almost identical. Similarly, a region of near infinite compressibility is observed for the Z-isomer from 195 ( 5 to 125 ( 5 Å2. The extrapolated area/molecule for high surface pressure is 140 ( 10 Å2.28 Several compression and expansion cycles (isocycles) confirm that the peptide films are remarkably stable at the air-water interface and the isotherms are completely reversible. The lack of observed changes in the area/molecule after four isocycles confirms that the peptide films do not dissolve significantly in the subphase. The isotherms obtained by illuminating expanded E-peptide films for 15 min at zero surface pressure are identical to those observed upon spreading the preformed Z-isomer. Temperature Dependence of the Isotherms. To gain further insight about the origin of the plateau in the isotherms of both species, measurements at temperatures ranging from 10 to 50 °C were performed (Figure 2). For the Z-isomer the peptide solution was illuminated for 15 min before spreading and the isotherm was recorded immediately. In the time period of the experiments the isomerization of the Z-isomer in chloroform is negligible, even at 50 °C.27 In both cases, observed plateau surface pressures decrease with increasing temperature ranging from 13.3 (10 °C) to 11.0 mN/m (50 °C) for the E-isomer and from 15.1 (10 °C) to 11.6 mN/m (50 °C) for the Z-isomer. Although the area/molecule at low surface pressure (30 nN) and scan velocities (60 Hz) and subsequently scanning a larger area using low loading forces (1-3 nN) and moderate scan velocities (1-2 Hz). To avoid structural changes in the film due to the relatively strong interaction between tip and sample during contact mode SFM, TappingMode SFM was solely used to image the LB films at different surface pressures (Figures 3 and 4). The height differences between the higher and lower domains are exclusively measured by section analysis of images obtained from TappingMode. At a surface pressure of 8.0 mN/m, below the plateau region, films of both isomers exhibit a homogeneous topography without the occurrence of domains. The beginning of the plateau region is accompanied by formation of small circular domains with an average height of 1.5 ( 0.3 nm (Figures 3A and 4A, Table 1). Increasing pressure leads to an increase in the domain diameter without detectable changes in height. At the center of the plateau region the circular domains aggregate

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Figure 3. TappingMode SFM images of Z-1 LB-films transferred onto mica at different surface pressures: 13.7 mN/m (A); 14.0 mN/m (B); 14.5 mN/m (C). All image sizes are 5 × 5 µm.

Figure 4. TappingMode SFM images of E-1 LB films transferred onto mica at different surface pressures: 12.0 mN/m (A); 12.5 mN/m (B); 13.0 mN/m (C). All image sizes are 5 × 5 µm.

to form interconnected stringlike domains resulting in a longitudinally isometric network (Figure 3B/4B). The end of the plateau region is characterized by dominance of the higher domains. The lower phase disappears via stringlike structures that condense to circular domains (Figures 3C and 4C). Images at higher magnification (Figure 5) reveal that the fine structure of the lower and higher phases is indistinguishable.29 Each exhibits a grainlike

topography with a particle size in the range of several nanometers. Height profiles of the two discernible phases were obtained by scratching (500 × 500) nm scan windows in (29) The images presented in Figure 5 are processed by a software (Digital Instruments) providing a filter that equalizes the contrast so that the features occurring in the higher and lower domains become closer to the same height and can be seen simultaneously.

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Table 1. Average Height of Different Structures of LB Films of Peptide 1 (E- and Z-Isomer) Transferred on Mica height/nma lower domain, mica higher domain, mica higher domain-lower domain a

2.0 ( 0.5 3.5 ( 0.5 1.5 ( 0.3

Averaged from section analysis of 50 profile measurements.

Figure 5. Magnifications of LB film domains of the E-isomer (A) and Z-isomer (B). The lateral dimensions are 400 × 400 nm.

LB films of each isomer. A section analysis of 50 measurements provided average height values for each domain type (Table 1). The error in heights obtained after removing the peptide layer by contact mode SFM is higher since the scratching process manipulates the film and sometimes even the mica sheet (in contrast to measurements performed by TappingMode SFM). The height of the lower domain of the Z-film was 2.0 ( 0.5 nm, whereas that of the higher domains was 3.5 ( 0.5 nm. The height parameters of the E-films are essentially the same. Lateral force microscopy measurements with load forces of 3-10 nN were performed in order to investigate differences in friction between domains. Simultaneously acquired topography and friction images of a Z-film deposited on mica at 14.2 mN/m are shown in Figure 6. The dark and light regions in the forward scan image represent low and high frictional forces, respectively (Figure 6B). The contrast is inverted for the backward

Figure 6. Topography (A) and lateral force images (B forward scan direction, C backward scan direction) obtained simultaneously from a Z-film transferred on mica at 14.2 mN/m. The dark and light regions in (B) represent areas of low and high friction, respectively. The contrast in the backward scan direction is inverted.

scan (Figure 6C). We observed essentially the same behavior for E-films (data not shown). ATR-FT-IR Spectroscopy. Supporting information about the orientation of the peptide cylinders with regard to the surface normal in the peptide films was obtained by ATR-IR spectroscopy on germanium. The LB monolayers of peptides deposited in the plateau region exhibit a dichroic ratio (R1630 ) A|/A⊥) of the amide I stretching

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Figure 8. Absorption spectra of a peptide layer on quartz glass in response to irradiation with UV and visible light: before illumination (s); after 15 min illumination with UV light (- - -); after 15 min illumination with visible light (‚ ‚ ‚).

Figure 7. Four isomerization cycles of the peptide film at the air-water interface at a constant pressure of 18 mN/m (A). Four isomerization cycles of the peptide film at the air-water interface at a constant area/molecule of 175 Å2 (B).

vibration at 1630 cm-1 of 1.05 ( 0.05. The analysis of the dichroic ratio of the amide I band was carried out according to the method reviewed by Fringeli and Gu¨nthard,30 using equations modified for films, which are considerably thinner than the wavelength of the evanescent wave.31 Assuming nGe ) 4.03 for the refractive index of germanium, npeptide ) 1.5 for the peptide film and setting the angle between the transition dipole moment of the amide I vibration and the z-axis of the peptide cylinders (molecular axis) to 0°, the measured dichroic ratio translates into an order parameter S of -(0.32-0.12), indicating that the peptide rings are aligned predominantly with the molecular axis parallel to the surface of the support (60°-70° with respect to the surface normal). Isomerization of Peptide System 1 at the AirWater Interface. Isomerization at Constant Pressure. Before isomerization the expanded peptide film was equilibrated for 10 h at 20 °C followed by application of a constant pressure of 18 mN/m. The equilibration period was necessary since the pressure where the plateau occurs increases for both the E- and Z-isomer by a maximum of 6 mN/m each. However, the difference in the plateau pressure between the E- and Z-isomer remains always constant. After equilibration of an E-isomer film the isotherms are constant and the pressure of 18 mN/m corresponds to an area/molecule of 125 Å2. The change in area/molecule upon illumination with either UV or visible light is displayed in Figure 7A. Illumination with UV light (30) Fringeli, U. P.; Gu¨nthard, H. H. Membrane Spectroscopy; Springer-Verlag: New York, 1981; pp 271-332. (31) Axelsen, P. H.; Kaufman, B. K.; McElhaney, R. N.; Lewis, R. N. A. H. Biophys. J. 1995, 69, 2770.

results in an increased area/molecule of about 70 Å2 after approximately 25 min. The area/molecule returns to a value of 125 Å2 upon exposure to visible light after approximately 18 min. A slight decrease in area/molecule (17 Å2) was observed for the formation of the Z-isomer after four isomerization cycles. Isomerization at Constant Area/Molecule. To isomerize the peptide film at constant area/molecule, it was again first equilibrated for 10 h at the air-water interface and then compressed to an area/molecule of 175 Å2. This value corresponds to the center of the E-isomer plateau. Illumination of the peptide film with UV light results in an increase in surface pressure from 17.4 to 18.7 mN/m over 10 min (Figure 7B). Z f E-isomerization with visible light for 20 min returns the pressure to its original value of 17.4 mN/m. The reproducibility of these transitions over four switching cycles demonstrates the reversibility of photoisomerization at the air-water interface. Isomerization of LB Films of Peptide 1 on Quartz Glass. Peptide E-1 was spread on the water surface and compressed to a surface pressure of 20 mN/m. One layer was transferred vertically to each side of a quartz glass plate with a transfer ratio of 1. The resulting UV/vis spectra (Figure 8) display typical azobenzene absorption bands. The maximum absorption at 340 nm is assigned to the π-π* electronic transition. The significantly weaker n-π* transition band at 450 nm was not observed in the monolayer spectra. Irradiation with UV light for 20 min caused a decrease of the π-π* transition band consistent with E f Z-photoisomerization. The conversion rate of the photostationary state was estimated to be 60% using the difference spectra method32 and considering that the film of the E-isomer contains 10% of Z-1. Illumination with visible light isomerized the peptide film to E-1. These switching cycles were repeated three times. Discussion The experiments described above employ film balance, IR, and SFM measurements to obtain information about the orientation of the peptide system 1 at the air-water interface. Recently, we reported that 1-D- and 2-D 1H NMR data of Z-1 in chloroform revealed that this molecule forms (32) Brode, W. R.; Gould, J. H.; Wyman, G. M. J. Am. Chem. Soc. 1952, 74, 4641.

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Figure 9. Schematic representation of a conceivable assembly of the E- and Z-isomer of peptide system 1 at an interface as a monolayer (A) and a bilayer (B). The illustration is meant to emphasize the relative orientation of the close packed cylindrical assemblies with respect to the air-water or solid interface. The position of the azobenzene moiety and the average number of hydrogen-bonded oligomers in the case of the E-isomer is as yet unclear and remains to be elucidated.

a single defined species with intramolecular hydrogen bonding.27 In contrast, E-1 forms an “assembly pool” of intermolecularly hydrogen-bonded aggregates. The theoretical area occupied by the peptide system 1 can be estimated from X-ray data33 and molecular modeling.34 Since the structure and molecular area of E and Z are essentially dominated by the hydrogen-bonded cyclic peptides and not by the conformation of the azobenzene moiety, the area/molecule is almost independent of the isomeric form. It is known that one cyclic peptide ring composed of eight amino acids has an outer diameter of =20 Å and a height of =5 Å. Assuming a peptide orientation with molecular axes (z-axis of the peptide cylinder; see Scheme 1) parallel to the surface normal and a high packing density (hexagonal unit cell) gives an area/molecule of 346 Å2 for both isomers. An arrangement of the peptides rotated 90° to the surface normal results in a theoretical value of 200 Å2 for close packing. A comparison of these values with experimental data obtained from the film balance measurements demonstrates that at p > 0 mN/m (at an area/molecule of 350 Å2) the molecules might already have a closely packed structure with their molecular axes parallel to the surface normal. The fact that the area/molecule is only 195-205 Å2 immediately before the plateau region suggests an orientation of the molecular ring axis perpendicular to the surface normal. These findings are in good agreement with the angle of the peptide cylinders relative to the surface normal obtained by ATR-FT-IR spectroscopy. Since the smallest area/molecule at 25 mN/m is much smaller than the calculated value for any close packing in a peptide monolayer, we conclude that multilayers are formed upon compression of the film. The formation of multilayers might occur in the region of near infinite compressibility (plateau region). To support this hypothesis, we studied LB films of the peptide in the two-phase region by SFM. The SFM images clearly demonstrate that a second structure with a well-defined height occurs upon compression of the film to surface pressures above the plateau (33) Hartgerink, J. D.; Granja, J. R.; Milligan, R. A.; Ghadiri, M. R. J. Am. Chem. Soc. 1996, 118, 43. (34) Molecular modeling was performed using the program Insight II.

region. With increasing surface pressure, the area occupied by the higher structure increases as well. The 20 ( 5 Å height of the lower structure corresponds well with the assumption of predominately perpendicularly oriented molecules (the molecular axes of the peptide cylinders are lying perpendicular to the surface normal) in a monolayer at the interface. In addition, the height of 35 ( 5 Å for the higher structures with respect to the substrate indicates that a second well-defined layer exists atop the first one. Considering a packing geometry of the stacked peptide rings as depicted in Figure 9, the height of the layer with two peptide rings on top of each other should be 32 ( 5 Å depending on the location on the stacked cylinder and the shape of the tip, which is close to the obtained average value of 35 ( 5 Å. From the model we conclude that in the plateau region the molecules of the monolayer roll on top of each other, building up the second peptide layer. This hypothesis is supported by the decrease in surface pressure of the plateau with increasing temperature. The higher temperature facilitates formation of the second layer. The magnifications of Z and E LB films, respectively, suggest that the fine structures of the lower and higher structures are similar, supporting the hypothesis that both are peptide layers with the same orientation and structure. An interesting observation is the complete reversibility in the formation of the second layer. As shown in the inset of Figure 1, a fast expansion of the peptide film to an area/molecule value in the center of the plateau leads to much smaller surface pressures than expected from equilibrium values. Equilibrium is reached after 2-5 min, indicating that the monolayer-bilayer formation is relatively slow. Lateral force microscopy of Z- and E-films reveals that the two domains, although exhibiting indistinguishable morphology, bear different friction coefficients. The contrast of the images obtained from forward and backward scan directions might be interpreted in terms of different interaction forces between the hydrophilic tip and the peptide molecules. The higher domains (bilayer) seem to be more hydrophilic than the lower ones (monolayer) which may be the consequence of differences in the orientation of azobenzene moieties (Figure 9). Similar observations

Peptide Assemblies at Air-Water and Solid Interfaces

were made by Solletti et al.35 for transferring 1,2dipalmitoyl-sn-glycero-3-phosphoethanolamine from the air-water interface onto mica and Ve´lez et al.17 for amphiphilic azobenzene derivatives deposited onto mica by the LB technique. The influence of the azobenzene moieties in orienting the cylindrical peptide structures might be an essential element for its successful spreading at the air-water interfacescyclic peptides without the azobenzene subunit do not form stable monolayers at the air-water interface. The most important feature of the peptide system 1 is its ability to switch between inter- and intramolecular hydrogen-bonded structures with either UV (E f Z) or visible light (Z f E). The E f Z-photoisomerization at the air-water interface at a constant pressure of 18.0 mN/m (equivalent to an area/molecule of 125 Å) after equilibration of an E-film for 10 h results in an increase in the area/molecule. The peptide film of E-1 at a pressure of 18.0 mN/m is composed mainly of a peptide bilayer. However, isomerization to the Z-isomer results in a peptide monolayer, thereby increasing the area/molecule dramatically. It should be pointed out that the actual area/ molecule does not increase but rather a structural change from a bilayer to a monolayer is induced. An increase in the real area/molecule upon illumination with UV light was observed by Seki et al.23 using poly(vinyl alcohol) azobenzene derivatives as well as an azobenzene derivative of a fatty acid. The formation of the Z-isomer in their study led to a molecular kink, which resulted in a larger area/molecule. In contrast, Higuchi et al.,16 investigating a polypeptide containing azobenzene in the main chain found that isomerization from E to Z led to a decrease in area/molecule. In addition to the change in area/molecule, we investigated the alteration of surface pressure upon illumination. The surface pressure increases upon UV irradiation (E f Z) of the peptide film at the air-water interface. This might be due to changes in molecular structure at the interface. The assembly pool of E-1 has a higher surface tension than the discrete species Z-1. Changes in surface properties upon isomerization have also been described by Ahuja and co-workers.15,19 They studied monolayers of the amphiphilic molecule N-[[(p-((p-octylphenyl)azo)phenyl)oxy]eicosyl]pyridinium bromide at the air-water interface and on glass and could demonstrate that isomerization led to a change in resonant reflection, surface potential, and the Brewster angle. Changes in reflectivity of Langmuir-Blodgett-Kuhn multilayers of polyglutamates with azobenzene moieties by surface plasmon resonance have been reported by Bu¨chel et al.3 The transfer of azobenzene derivatives onto solid support often complicates photoisomerization due to limited free volume for conformational changes.14,17 The close packing of azobenzene subunits is reflected in the position of the π-π* transition band. The more the band is shifted to lower wavelengths the closer the azobenzene units are packed. In our system the π-π* transition band on solid support is not shifted relative to the band observed in chloroform (340 nm), indicating that the peptide molecules act as adequate spacers for the azobenzene moieties. The percentage conversion from E- to Z-isomer of peptide films deposited on glass is estimated to be 60% compared to a conversion efficiency of only 30% in LB films of long-chain fatty acid azobenzene derivatives.36 Wang et al.14 demonstrated that a crown ether coupled to (35) Solletti, J. M.; Botreau, M.; Sommer, F.; Brunat, W. L.; Kasas, S.; Duc, T. M.; Celio, M. R. Langmuir 1996, 12, 5379. (36) Liu, Z. F.; Loo, B. H.; Baba, R.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1990, 1023.

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two azobenzene chromophores provides enough free volume to allow E f Z-photochromic reaction. In our system the self-organization of supramolecular cylindrical assemblies enables facile photoisomerization in thin films on solid supports. Conclusions We have demonstrated that monolayers of a rationally designed azobenzene-derivatized supramolecular structure undergo dramatic and reversible structural changes at the air-water interface upon illumination. Large and reversible changes in surface pressure at constant area and in surface area at constant pressure were triggered by UV or visible light. These remarkable changes can be attributed to photochromic switching between structures composed of an assembly pool of intermolecularly hydrogen-bonded aggregates (E-isomer) and a unique intramolecularly hydrogen-bonded species (Z-isomer). The described peptide provides a new photoswitchable system, which permits a reversible change in self-assembly at interfaces by controlling the supramolecular structure. The change in self-organization on solid substrates upon illumination may provide an avenue toward the design of novel photoactive materials. Experimental Section Surface Pressure-Area Measurements. Peptide 1 was dissolved in chloroform at a final concentration of approximately 0.5 mM. 1H NMR data indicated that a chloroform solution of so termed E-1 contains 90% E- and 10% Z-isomer.27 The exact peptide concentration of the solution was calculated by UV/vis spectroscopy using an extinction coefficient of 340 ) 19 800 M-1‚cm-1 for E-1, which was determined by a combination of amino acid analysis and UV/vis spectroscopy. To isomerize the peptide from E-1 to Z-1 the chloroform solution was irradiated with UV light (UV lamp, 4 W, λmax ) 366 nm) for 15 min. Illumination provides 100% of the Z-isomer.27 The Z f Eisomerization was performed by irradiation with visible light (fluorescence lamp, Phillips F15T8/CW, 15 W) for 15 min. Surface pressure-area isotherms of peptide 1 were measured with a Nima film balance (Nima Technology, Ltd.) equipped with a Wilhelmy system and a Teflon trough with an area of 175 cm2. The trough temperature was controlled with a water bath. After spreading the peptide solution on the water subphase (MilliQ, 18 MΩ‚cm) and evaporation of the chloroform, the film was compressed with a rate of 20 cm2/min. Photoinduced surface area changes of the peptide films were measured by compressing the monolayer to the desired surface pressure and keeping it constant during the illumination cycles. Surface pressure changes upon illumination of the peptide film were measured at constant area. LB Transfer of Peptide Films on Solid Support. The transfer of the peptide films onto solid supports was performed by the Langmuir-Blodgett technique. The films were equilibrated at the adjusted film pressure for 30 min. Freshly cleaved mica sheets (10 × 5 × 1 mm) were used for scanning force microscopy, a germanium crystal (θ ) 45°, Spectra-Tech) for ATR-FT-IR spectroscopy (Nicolet 550 Magna Series II FT-IR instrument), and clean quartz plates (40 × 10 × 1 mm) for UV/ vis spectroscopy (Milton-Roy). The substrates were dipped into the subphase without transfer at a speed of 50 mm/min. The peptide films were transferred by withdrawing the substrates out of the water subphase with a speed of 2 mm/min. Scanning Force Microscopy. Scanning force microscopy (SFM) images were obtained under ambient conditions using a Nanoscope IIIa Multimode scanning probe microscope (Digital Instruments, Santa Barbara, CA) operating in TappingMode and contact mode. For TappingMode, silicon NanoProbe tips (TESP, Digital Instruments) were used as purchased. Several tips were used to ensure reproducible results. Contact mode/lateral force microscopy was performed using microfabricated oxide sharpened V-shaped silicon nitride tips with a nominal tip radius of about 5-20 nm and a nominal spring constant of 0.06 N/m (NP-S, Digital Instruments).

3964 Langmuir, Vol. 15, No. 11, 1999

Acknowledgment. We thank the Office of Naval Research (N000149810731 and N000149511293) through the Multidisciplinary University Research Initiative (MURI-95) of the Department of Defense for financial support of this work. We also thank the DFG (C.S.), the Fonds der Chemischen Industrie (A.J.), and the DAAD

Steinem et al.

(M.S.V.) for postdoctoral fellowships. We thank our colleagues Alan J. Kennan and Krishna Kumar for their valuable input. LA981624+