Hydrogen-Bond-Induced Chiral Discrimination in Monolayers of

The insertion and systematical displacement of two vicinal hydroxyl groups along the hydrophobic alkyl chain of methyl octadecanoates generates bipola...
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Langmuir 2000, 16, 8937-8945

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Hydrogen-Bond-Induced Chiral Discrimination in Monolayers of Bipolar Methyl Dihydroxyoctadecanoates†,‡,§ Marina Fix,| Robert Lauter,⊥ Christian Lo¨bbe,| Gerald Brezesinski,⊥ and Hans-Joachim Galla*,| Institut fu¨ r Biochemie der Universita¨ t Mu¨ nster, Wilhelm-Klemm-Strasse 2, 48149 Mu¨ nster, Germany, and Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Am Mu¨ hlenberg 2, 14476 Golm, Germany Received April 1, 2000. In Final Form: July 3, 2000 The insertion and systematical displacement of two vicinal hydroxyl groups along the hydrophobic alkyl chain of methyl octadecanoates generates bipolar amphiphiles. These molecules offer the possibility to investigate the influence of the position and the stereochemistry of the hydroxyl groups on monolayer properties and intermolecular interactions. Monolayers of enantioenriched and racemic vicinally dihydroxylated methyl octadecanoates (DHOs) were investigated using fluorescence microscopy, fluorescence spectroscopy, and grazing incidence X-ray diffraction (GIXD). The monolayers of racemic threo-2,3-DHO, erythro-9,10-DHO, and 17,18-DHO show heterochiral discrimination in fluorescence microscopy investigations due to a preferential interaction between the enantiomers with the opposite configuration. GIXD measurements exhibit rectangular unit cells for the racemic films, while the enantiomers form oblique and therefore chiral structures. Erythro-9,10-DHO exhibits an extraordinarily large lattice spacing due to the disturbing effect of the hydroxyl groups. The formation of rectangular lattices by racemic films strongly suggests that chiral phases such as in the case of the enantiomers are prevented by the dimerization due to stereospecific hydrogen bonds between the two antipodal enantiomers. Accordingly, the individual enantioenriched amphiphiles are able to distinguish between the same and antipodal enantiomers, and thus they are able to recognize chemical structures at a molecular level. Fluorescence spectroscopy at the air-water interface revealed whether the hydroxyl groups or the methyl ester of the R,ω-bipolar amphiphile 17,18-DHO is directed to the water surface in the condensed state at subphase temperatures of 278 and 293 K, respectively.

Introduction The complex structure of biologically relevant molecules such as enzymes guarantees stereospecific recognition and fast chemical transformation of different substrates. Intermolecular interactions play an important role in such recognition processes. Therefore, the investigation of intermolecular forces such as hydrophobic and electrostatic interactions, van der Waals interactions, or hydrogen-bond formation is the key to understanding recognition processes from a physical-chemical point of view. Due to the complexity of biological molecules we have chosen a simplified system consisting of bipolar octadecanoic acid methyl esters with two vicinal hydroxyl groups as the second polar moiety to investigate interactions at a molecular level. The monopolar fatty acid esters are known to form stable monolayers at the air-water interface.1-7 The insertion of a second polar moiety into * To whom correspondence should be addressed. Phone: +49251-83 33200. Fax: +49-251-83 33206. E-mail: [email protected]. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. ‡ This work is part of the Ph.D. thesis of M.F. submitted to the science faculty of the University of Mu¨nster. § Abbreviations used: DHO, methyl dihydroxyoctadecanoate; LE, liquid-expanded; LC, liquid-condensed; DiO, 3,3′-dioctadecyloxacarbocyanine iodide; NBD-C12-Acid, 12-(N-(7-nitrobenz-2-oxa-1,3diazol-4-yl)amino)dodecanoic acid; BAM, Brewster angle microscopy; GIXD, grazing incidence X-ray diffraction; PSD, positionsensitive detector; NN, nearest neighbor; HASYLAB, Hamburger Synchrotronstrahlungslabor; DESY, Deutsches Elektronen Synchrotron; fwhm, full width at half-maximum. | Universita ¨ t Mu¨nster. ⊥ Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. (1) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848.

an amphiphilic structure influences molecular properties and therefore the physical characteristics of the monolayer.8-17 Film balance measurements at the air-water interface offer the opportunity to easily control molecular orientation by variation of the surface area. The investigation and comparison of film properties and phase behavior of particular regio- and stereoisomers of vicinally dihydroxylated methyl octadecanoates permit a systematic study of hydrogen-bond formation and hydrophobic interactions.18-20 Variations in the phase behavior of the (2) Gaines, G. L., Jr. Insoluble monolayers at liquid-gas interfaces; John Wiley & Sons: New York, London, Sidney, 1966. (3) Sackmann, H.; Do¨rfler, H.-D. Z. Phys. Chem. 1972, 251, 303. (4) Baret, J. F.; Hasmonay, H.; Firpo, J. L.; Dupin, J. J.; Dupeyrat, M. Chem. Phys. Lipids 1982, 30, 177. (5) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (6) Ulman, A. An introduction to ultrathin organic films. From Langmuir-Blodgett to self-assembly; Academic Press: San Diego, 1991. (7) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779. (8) Tachibana, T.; Hori, K. J. Colloid Interface Sci. 1977, 61, 398. (9) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452. (10) Kellner, B. M. J.; Cadenhead, D. A. Chem. Phys. Lipids 1979, 23, 41. (11) Tachibana, T.; Yoshizumi, T.; Hori, K. Bull. Chem. Soc. Jpn. 1979, 52, 34. (12) Matuo, H.; Rice, D. K.; Balthasar, D. M.; Cadenhead, D. A. Chem. Phys. Lipids 1982, 30, 367. (13) Menger, F. M.; Richardson, S. D.; Wood, M. G., Jr.; Sherrod, M. J. Langmuir 1989, 5, 833. (14) Neumann, V.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1995, 11, 2206. (15) Huda, M. S.; Fujio, K.; Uzu, Y. Bull. Chem. Soc. Jpn. 1996, 69, 3387. (16) Sakai, H.; Umemura, J. Langmuir 1998, 14, 6249. (17) Asgharian, B.; Cadenhead, D. A. Langmuir 2000, 16, 677.

10.1021/la000506v CCC: $19.00 © 2000 American Chemical Society Published on Web 08/26/2000

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amphiphiles during film compression and expansion are expected to correlate directly with distinct interactions between the polar and nonpolar molecular moieties of the amphiphiles. To examine monolayer properties in more detail, film balance experiments can be combined with modern techniques such as Brewster angle microscopy (BAM),21 fluorescence microscopy,22 surface potential measurements,23 and fluorescence spectroscopy.24 Additionally, investigations of monolayers at the air-water interface with grazing incidence X-ray diffraction (GIXD)25-29 afford an actual insight into the monolayer structure at a molecular level. Differences in the lattice structure of regio- and stereoisomers of our model amphiphiles are expected due to different intermolecular interactions. Previous investigations on methyl dihydroxyoctadecanoates (DHOs) at the air-water interface have shown differences in the phase behavior depending on the position and the stereochemistry of the two vicinal hydroxyl groups.18-20 Film balance and Brewster angle microscopy (BAM) measurements on racemic and enantioenriched films exhibit a heterochiral discrimination in the case of threo-2,3-DHO and erythro-9,10-DHO. The surface pressure-area isotherms of 17,18-DHO show no chiral discrimination (congruent isotherms), while BAM examinations reveal different domain shapes and therefore also a heterochiral discrimination. Heterochiral discrimination means the preferred interaction between the two antipodal enantiomers in the racemic film in contrast to the interaction between the enantiomers with the same configuration.30-33 This behavior was investigated on films of different chiral amino acids. Some of the examined amphiphiles show homochiral discrimination34-39 and heterochiral discrimination,35 respectively. Films of 2-hydroxyhexadecanoic acid show homochiral discrimination in the case of Zn2+ cations in the subphase and heterochiral discrimination in the case of Ca2+ and Pb2+ cations in the

Fix et al.

Figure 1. Structure and stereochemistry of the investigated vicinally dihydroxylated methyl octadecanoates. The two antipodal enantiomers of a racemic mixture are shown.

subphase.14,40 In addition to these chiral effects observed in monolayers at the air-water interface, bilayer systems also show chiral discrimination.41 In this paper, we present GIXD measurements at the air-water interface on racemic and enantioenriched films of the dihydroxylated methyl octadecanoates threo-2,3DHO, erythro-9,10-DHO, and 17,18-DHO to obtain further information on the structure of the condensed phases and therefore the reason for chiral discrimination effects. Additionally, monolayers of the enantioenriched bolaamphiphile (R,ω-bipolar amphiphile) methyl 17,18-dihydroxyoctadecanoate (17,18-DHO) were investigated with fluorescence spectroscopy at the air-water interface to determine which of the two headgroups is directed to the water subphase in the condensed state. Fluorescence microscopy was performed to examine the influence of the chiral hydroxyl groups and therefore the OH position and stereochemistry on the domain shapes and sizes. Materials and Methods

(18) Jacobi, S.; Chi, L. F.; Plate, M.; Overs, M.; Scha¨fer, H.-J.; Fuchs, H. Thin Solid Films 1998, 327-329, 180. (19) Fix, M. Thesis, University of Mu¨nster, Germany, 2000. (20) Overs, M.; Fix, M.; Jacobi, S.; Chi, L. F.; Sieber, M.; Scha¨fer, H. J.; Fuchs, H.; Galla, H.-J. Langmuir 2000, 16, 1141. (21) (a) He´non, F.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (b) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (22) (a) Lo¨sche, M.; Sackmann, E.; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 848. (b) Weis, R. M.; McConnell, H. M. Nature 1984, 310, 47. (23) Brockman, H. Chem. Phys. Lipids 1994, 73, 57. (24) (a) Teissie´, J.; Tocanne, J. F.; Baudras, A. FEBS Lett. 1976, 70, 123. (b) Teissie´, J. Chem. Phys. Lipids 1979, 25, 357. (25) Leveiller, F.; Jacquemain, D.; Leiserowitz, L.; Kjaer, K.; AlsNielsen, J. J. Phys. Chem. 1992, 96, 10380. (26) Jacquemain, D.; Grayer Wolf, S.; Leveiller, F.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. Angew. Chem. 1992, 104, 134. (27) Als-Nielsen, J., Jaquemain, D., Kjaer, K., Leveiller, F., Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (28) Kjaer, K. Physica B 1994, 198, 100. (29) Kaganer, V. M.; Peterson, I. R.; Kenn, R. M.; Shih, M. C.; Durbin, M.; Dutta, P. J. Chem. Phys. 1995, 102, 9412. (30) Stewart, M. V.; Arnett, E. M. Top. Stereochem. 1982, 13, 195. (31) Andelman, D. J. Am. Chem. Soc. 1989, 111, 6536. (32) Andelman, D.; Orland, H. J. Am. Chem. Soc. 1993, 115, 12322. (33) (a) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131. (b) Rose, P. L.; Harvey, N. G.; Arnett, E. M. Adv. Phys. Org. Chem. 1993, 28, 45. (34) Stine, K. J.; Uang, J. Y.-J.; Dingman, S. D. Langmuir 1993, 9, 2112. (35) Stine, K. J.; Whitt, S. A.; Uang, J. Y.-J. Chem. Phys. Lipids 1994, 69, 41. (36) Parazak, D. P.; Uang, J. Y.-J.; Turner, B.; Stine, K. J. Langmuir 1994, 10, 3787. (37) Parazak, D. P.; Uang, J. Y.-J.; Whitt, S. A.; Stine, K. J. Chem. Phys. Lipids 1995, 75, 155. (38) Uang, J. Y.-J.; Parazak, D. P.; Stine, K. J. Chem. Phys. Lipids 1995, 75, 163. (39) Vollhardt, D.; Emrich, G.; Gutberlet, T.; Fuhrhop, J.-H. Langmuir 1996, 12, 5659.

Materials. DHOs with two vicinal hydroxyl groups at different positions along the hydrophobic alkyl chain were synthesized as described elsewhere20,42,43 and kindly provided by Prof. Scha¨fer (Institute of Organic Chemistry, University of Mu¨nster, Germany). The chemical structure and stereochemistry of the examined amphiphiles are shown in Figure 1. Film Balance Measurements. Surface pressure-area isotherms were obtained using film balances with different systems for surface pressure measurements with an accuracy of 0.1 mN/m (Wilhelmy plate system, total surface area of 144 cm2 (Riegler&Kirstein, Wiesbaden, Germany); Langmuir system, total surface area of of 927 cm2 (Lauda, Lauda-Ko¨nigshofen, Germany)). The troughs were temperature controlled. Ultrapure water (pH 5.5) was obtained from a Millipore system (Millipore, Bedford, MA) comprising reverse osmosis, ion exchange, and 0.22 µm ultrafiltration cartridge. Monolayers were prepared by spreading appropriate chloroform solutions (HPLC grade CHCl3 was purchased from Sigma-Aldrich, Steinheim, Germany) onto ultrapure water using a 100 µL syringe (ILS, Stu¨tzerbach, Germany). After spreading, 10 min was allowed for solvent evaporation, and then the monolayer was compressed with a velocity of 5.8 cm2/min. Fluorescence Microscopy. For the fluorescence microscopy investigations the film balance (144 cm2, Riegler&Kirstein) was mounted onto the stage of the Olympus microscope BX-FLA (Hamburg, Germany) equipped with a mercury lamp (USH-102D 100 W, power supply unit BH2-RFL-T3), three objectives (UMPLFL10x, LMPLFL20x, LMPLFL50x), and a CCD camera with a camera controller (Hamamatsu C4742-95, Hamamatsu, (40) Hu¨hnerfuss, H.; Gericke, A.; Neumann, V.; Stine, K. J. Thin Solid Films 1996, 284-285, 694. (41) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565. (42) Plate, M. Thesis, University of Mu¨nster, Germany, 1998. (43) Plate, M.; Overs, M.; Scha¨fer, H. J. Synthesis 1998, 9, 1255.

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Figure 2. Surface pressure-area isotherms of different vicinally dihydroxylated methyl octadecanoates on ultrapure water at 293 K. The isotherms of the racemates (A) are compared with those of the corresponding enantiomers (B).

Figure 3. Scattering intensity as a function of in-plane scattering vector component Qxy for different Qz intervals (indicated) for racemic (A) and enantioenriched (B) monolayers of threo-2,3-DHO at 293 K on ultrapure water. Japan). The spreading solutions contained 1 mol % fluorescence probe 12-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoic acid (NBD-C12-Acid). The fluorescence probe was a product of Molecular Probes Inc., Eugene, OR. Fluorescence Spectroscopy. The measurements were performed on monolayers prepared at the air-water interface of a temperature-controlled film balance (NIMA model 601 M, Coventry, U.K.). According to Teissie´,24 the spectrofluorimeter Fluorolog II (Spex, New York) was modified with quartz lightguides and lenses (Oriel, Stratford, CT) in order to couple the excitation and emission light to the air-water interface. A coated gray filter was put into the trough as a light trap to diminish stray light. To measure the weak fluorescence emitted from the monolayer, a cooled photomultiplier (Products for research, Danvers, MA) was used. The surface-active dye 3,3′-dioctade-

cyloxacarbocyanine iodide (DiO; the synthesis is described elsewhere)44 was used as a fluorescent probe for the presence of polar groups at the air-water interface.45 The amphiphiles were mixed with the dye in a molar ratio of 5:1 (amphiphile:DiO) in a chloroform solution. This fluorescence dye associates preferentially with the condensed phase46 in contrast to the fluorescence dye NBD-C12-Acid, which is preferentially distributed in the liquid-expanded phase of the amphiphile. Grazing Incidence X-ray Diffraction. GIXD measurements were performed using the liquid surface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, (44) Sondermann, J. Justus Liebigs Ann. Chem. 1971, 749, 183. (45) Vogel, V.; Mo¨bius, D. Thin Solid Films 1985, 132, 205. (46) Nag, K.; Keough, K. M. W. Biophys. J. 1993, 65, 1019.

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Figure 4. Scattering intensity as a function of in-plane scattering vector component Qxy for different Qz intervals (indicated) for racemic (A) and enantioenriched (B) monolayers of erythro-9,10-DHO at 293 K on ultrapure water. Two peaks are an indication of the centered rectangular unit cell in the case of the racemic monolayer (A), and three peaks are due to an oblique unit cell in the case of the enantioenriched film (B). Germany. The experimental setup and evaluation procedures are described in detail elsewhere.26-28 The X-ray beam was made monochromatic by Bragg reflection at the (002) plane of a beryllium crystal. It hits the air-water interface at a grazing incidence angle Ri ) 0.85Rc, where Rc is the critical angle for total external reflection. The intensity of the diffracted beam is detected by a position-sensitive detector (PSD) (OED-100-M, Braun, Garching, Germany). The resolution (1.5 mrad) of the horizontal scattering angle 2θhor is given by a Soller collimator located in front of the PSD. The scattering vector Q has an in-plane component Qxy ≈ 2k sin θhor and an out-of-plane component Qz ≈ k sin Rf, where k is the wave vector and Rf the vertical scattering angle. The accumulated position-resolved scans were corrected for polarization, footprint area, and powder-averaging (Lorentz factor). Model peaks taken to be Lorentzian in the inplane direction and Gaussian in the out-of-plane direction were fitted to the corrected intensities. The lattice spacings are obtained from the in-plane diffraction data. The lattice parameters can be calculated from the lattice spacings.

Results and Discussion Figure 2 shows the comparison of surface pressurearea isotherms of racemic DHOs containing vicinal hydroxyl groups at the positions 2,3; 9,10; and 17,18 (Figure 2A) with those of the corresponding enantiomers (Figure 2B). At 293 K, all DHOs investigated show a plateau region, indicating a first-order phase transition from a liquid-expanded to a condensed monolayer state. A model for the conformational changes occurring during the film compression19,20 was presented earlier: At large molecular areas all amphiphiles form liquid-expanded phases with both hydrophilic groups attached to the water surface. The formation of a condensed monolayer phase

requires the removal of one polar group from the water surface. The removal takes place in the two-phase coexistence region. This model is supported by the molecular area at the onset of the plateau region, which increases with increasing distance between the two polar groups. Additionally, the transition pressure decreases with increasing distance between the polar groups (Figure 2). The transition pressures of the racemic mixtures of threo-2,3-DHO and erythro-9,10-DHO are higher than for the corresponding enantiomers, while the racemate and enantiomer of 17,18-DHO exhibit similar transition pressures. To obtain further information about the structure of the condensed phases, the monolayers were studied using GIXD. Figures 3-5 depict the diffracted intensity as a function of the in-plane scattering vector component Qxy of the racemic (A) and enantioenriched (B) monolayers of threo-2,3-DHO (Figure 3), erythro-9,10-DHO (Figure 4), and 17,18-DHO (Figure 5). The diffracted intensity is plotted in Qz intervals with increasing Qz from bottom to top. The enantiomers of erythro-9,10-DHO and 17,18-DHO exhibit three out-of-plane diffraction peaks, indicating an oblique and hence chiral unit cell at all pressures investigated. The three diffraction peaks are well resolved for erythro-9,10-DHO (Figure 4B), whereas the two peaks at higher Qz strongly overlap in the case of 17,18-DHO (Figure 5B). However, the third peak is well above the horizon what proves the existence of a chiral lattice. The diffraction signal of the enantioenriched threo-2,3-DHO is very weak. The diffracted intensity in Figure 3B was measured at a high surface pressure of 45 mN/m. The

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Figure 5. Scattering intensity as a function of in-plane scattering vector component Qxy for different Qz intervals (indicated) for racemic (A) and enantioenriched (B) monolayers of 17,18-DHO at 293 K on ultrapure water. The two peaks at higher Qxy strongly overlap for S17,18-DHO (B).

diffracted intensity is distributed over a broad Q range, indicating a very short coherence length. This monolayer was examined by Chi et al. with atomic force microscopy after transfer to mica.47 They found an extraordinary supermolecular periodic structure with variable tilt angles and directions of the alkyl chains. Therefore, a monolayer structure could not be determined by GIXD. The films of the racemic mixtures of all dihydroxylated methyl octadecanoates exhibit two diffraction peaks characteristic for a centered rectangular unit cell. threo-2,3-DHO and 17,18-DHO exhibit one peak at Qz ) 0 and a 2-folddegenerated peak at higher Qz (Figures 3A and 5A). Such an intensity distribution indicates a tilt direction of the alkyl chains toward nearest neighbors (NNs).29 The diffraction peaks of threo-2,3-DHO exhibit a very small full width at half-maximum (fwhm), indicating a large coherence length. The fwhm of the peak at 1.626 Å-1 is even resolution limited (0.008 Å-1). In the case of erythro9,10-DHO both diffraction peaks are located at zero Qz characteristic for a rectangular unit cell of untilted molecules (Figure 4A). Increasing surface pressure shifts the diffraction peaks to lower Qz and larger Qxy values, indicating a decrease of the tilt angle t and a closer packing in the water plane (decreasing molecular area Axy). Table 1 lists the structural data of the enantioenriched and racemic bipolar amphiphiles investigated. The tilt angles of the racemic and enantioenriched 17,18-DHO are comparable at similar pressures, while the tilt of the racemic erythro-9,10-DHO is drastically reduced compared (47) Chi, L. F.; Jacobi, S.; Anczykowski, B.; Overs, M.; Scha¨fer, H. J.; Fuchs, H. Adv. Mater. 2000, 12, 25.

with the corresponding enantiomer. As shown in Figure 2, chiral discrimination with respect to surface pressurearea isotherms and therefore on a thermodynamical level is also more pronounced for the threo-2,3- and erythro9,10-dihydroxylated amphiphiles compared with 17,18DHO. The reduced tilt angle of the racemic erythro-9,10DHO can be explained by intermolecular interactions of the vicinal hydroxyl groups, which are strongest in the case of a dimer formation of optical antipodes. The hydroxyl groups are in the closest position to each other if the molecules are untilted. In this nontilted orientation hydrophobic interactions are less important. The crosssectional area is the largest if the hydroxyl groups are located in the middle of the alkyl chain (9,10-DHO). As described previously the two hydroxyl groups and the ester group of threo-2,3-DHO form a single extended headgroup.19,20 A complete upright orientation of the molecule is here not essential for hydrogen-bond formation. A tilt of the alkyl chains should increase the hydrophobic interactions. In the case of 17,18-DHO only one chiral hydroxyl group contributes to chiral discrimination. The hydrophobic interactions are therefore more important, and the tilt angle of the molecules mainly due to increased hydrophobic interactions is the same for both the enantiomer and racemate. Nevertheless, the amphiphiles in monolayers of both 17,18-DHO and threo2,3-DHO are tilted toward NNs in the case of the racemic mixtures, which is also a hint for the formation of antipodal dimers. The observed properties of the monolayers agree with the assumption of stereospecific formation of hydrogen

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Table 1. Unit Cell Dimensions a, b, and γ, Tilt Angle t, and Projected Area per Molecule Chain Axy of the Molecular Area A0, As Derived from In-Plane Diffraction Data at Different Surface Pressures for the Chiral Enantiomers E and the Racemates R of 17,18-DHO, erythro-9,10-DHO, and threo-2,3-DHOa molecule 17,18-DHO

π (mN/m) 10 20

erythro-9,10-DHO

30 35 10 15 20

threo-2,3-DHO

17

E R E R E R E R E R R

a (Å)

b (Å)

γ (deg)

Ψ*10 (deg)

Ψ*01 (deg)

Ψ*11 (deg)

t (deg)

Axy (Å2)

A0 (Å2)

4.80 5.45 4.77 5.37 4.73 5.38 4.84 5.11 4.80 5.05 5.63

4.89 4.92 4.86 4.86 4.82 4.76 4.91 4.98 4.90 4.95 4.78

112.6 123.7 111.9 123.5 112.0 124.4 112.8 120.9 113.1 120.7 126.1

205.1

137.7 b 134.6 b 135.3 b 200.9 b 188.9 c b

262.1

26 28 25 25 23 22 13 3 11 0 23

21.7 22.3 21.5 21.7 21.2 21.1 21.9 21.9 21.6 21.5 21.7

19.6 19.8 19.6 19.7 19.5 19.6 21.3 21.8 21.2 21.5 20.0

202.7 203.3 268.2 265.8

259.5 260.1 325.2 323.3

a

Ψ*hk means the angle between the lateral tilt direction and the corresponding reciprocal lattice vector. b Orthorhombic unit cell, NN tilt direction. c Hexagonal unit cell, untilted amphiphiles.

bonds in the condensed phase of the enantioenriched and racemic films at the air-water interface. Our hypothesis is that the dimerization of the optical antipodes in the racemic mixture causes a mirror plane between both molecules. Thus, the monolayers of the racemic mixtures can be described as effectively formed by nonchiral doublechain amphiphiles. In contrast, the enantiomers are not able to interact with their hydroxyl groups in a way like folded hands, giving rise to the formation of a twodimensional hydrogen-bond network in which each molecule interacts with two or more different molecules. This leads to the formation of chiral phases with chiral morphology. In the case of methyl 17,18-dihydroxyoctadeacanoate, very weak differences in thermodynamical properties and monolayer structures between the enantiomer and racemate were observed. The surface pressure-area isotherms of enantiomer and racemate are identical at 293 K (Figure 2) and at 278 K (fully condensed isotherms, not shown). GIXD measurements at 293 K revealed that the lattice of the enantiomer is only slightly distorted compared with that of the racemate (Table 1). For the bolaamphiphile 17,18-DHO the question arises whether the ester or the hydroxyl groups are directed to the water subphase in the condensed state. Molecular models show that a molecular area of nearly 0.2 nm2 for the upright oriented molecules is possible in both cases. This value is close to the experimentally determined crosssectional area of 0.197 nm2 (Table 1). The enantioenriched films of 17,18-DHO were investigated by fluorescence spectroscopy at subphase temperatures of 278 and 293 K to determine which of the polar groups are in contact with the subphase in the condensed state. DiO was used as a fluorescent probe, which can form monomers or dimers depending on the microenvironment.45 Monomers of DiO show a strong fluorescence emission peak at 505 nm, while dimers show a weak fluorescence emission peak at 590 nm (Figure 6). Vogel and Mo¨bius examined a bipolar amphiphile with a methyl ester group and a carboxyl group at the opposite ends of the molecule by reflection spectroscopy at the air-water interface. They discovered an orientation of the acid group to the water subphase. The ester located at the water surface hinders the formation of DiO dimers, so that consequently the fluorescence emission peak of the monomer is found. In accord with these results, we obtained at a subphase temperature of 293 K a dimer peak in the fluorescence emission spectra of a mixture of DiO with hexadecanoic acid. The results are shown in Table 2. As expected, the mixture of DiO with methyl octadecanoate exhibited only the monomer peak and the mixture with 1,2-octadecanediol showed the dimer peak in the condensed phase. The mixed monolayer

Figure 6. Excitation and emission spectra of the fluorescence dye DiO showing both the monomer and dimer peaks. Table 2. Fluorescence Spectroscopy Measurements on Monolayers of Mixtures of Different Amphiphiles with DiO (5:1) at 278 and 293 Ka monomer peak sample

278 and 293 K

S17,18-DHO 1,2-octadecanediol methyl octadecanoate hexadecanoic acid

X X X X

dimer peak 293 K

278 K

X

X X

X

X

a

The X indicates the existence of the corresponding peak in the spectrum.

of DiO and enantioenriched 17,18-DHO exhibited the typical monomer peak in all states of compression. Therefore, the monomer peak could not serve as an indicator for the orientation of the bolaamphiphile (Table 2). The dimer peak was not observed. The results give some hints that the ester group of 17,18-DHO is directed to the water phase in the condensed state of the monolayer. Corresponding infrared-spectroscopy measurements at the air-water interface refer also to an orientation of the ester groups toward the water subphase at 293 K.48 As the subphase temperature is lowered to 278 K, fluorescence spectroscopy investigations show an opposite result (Table 2). At this temperature the dimer peak is found in the condensed state. Decreasing the temperature led to an inverted orientation of the molecule. In contrast to the situation at 293 K, the interactions between the hydroxyl groups and the water subphase seem to be stronger at lower temperature than the interaction between the methyl ester group and the water subphase. (48) Overs, M.; Hoffmann, F.; Scha¨fer, H. J.; Hu¨hnerfuss, H. Langmuir, in press.

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Figure 7. Typical fluorescence micrographs of enantioenriched (A) and racemic (B) monolayers of 17,18-DHO during film compression at 293 K at molecular areas of (A1) 1.03 nm2, (A2) 0.95 nm2, (A3) 0.55 nm2, (A4) 0.4 nm2, (A5) 0.3 nm2, (B1) 1.0 nm2, (B2) 0.8 nm2, (B3) 0.6 nm2, (B4) 0.5 nm2, and (B5) 0.25 nm2. The monolayers contain 1 mol % fluorescence probe NBD-C12-Acid.

Fluorescence microscopy was performed to investigate the influence of different intermolecular interactions between equal and antipodal enantiomers on the monolayer structure at a microscopic level. Fluorescence microscopy investigations of films of 17,18-DHO showed different domain shapes for the enantiomer and racemate (Figure 7). The fluorescence micrographs depicted in Figure 7A show the enantioenriched 17,18-DHO monolayer in the two-phase coexistence region at a subphase temperature of 293 K. Figure 7B shows the corresponding fluorescence micrographs of the racemic monolayer. Although the enantiomer and racemate possess congruent surface pressure-area isotherms, the fluorescence micrographs exhibit differences in shape and size of the domains. In addition, the enantioenriched film forms dendritic domains with a preferential rotation direction of the domain branches (clockwise). The racemic domains also have a dendritic shape, but a preferential orientation of the domain branches is not observed (Figure 7B). The behavior of 17,18-DHO can be explained as follows: The interaction of the terminal, nonchiral hydroxyl group with another hydroxyl group is expected to supply no contribution to chiral discrimination in contrast to erythro-9,10-DHO and threo-2,3-DHO. Therefore, the weaker chiral interactions lead to different monolayer lattices and different domain shapes for the enantioenriched and racemic films, but these interactions are not strong enough to influence the thermodynamical behavior (surface pressure-area isotherms). Fluorescence microscopy investigations of monolayers of 17,18-DHO at the lower subphase temperature of 278 K (Figure 8) also reveal the formation of a chiral phase for the enantiomer (Figure 8A) in contrast to the racemic mixture (Figure 8B). This means that the chiral hydroxyl group at the C17 atom still has an influence on the domain shapes despite the hydration of this group due to the opposite arrangement of the molecule at the lower temperature. In contrast to 293 K the domain branches show a preferential rotation direction counterclockwise. As intermolecular interactions are responsible for the structure of the monolayers and thus for the domain shapes, the mirrorlike change of the rotation direction of the domain branches can only be due to the different orientations of the polar groups of 17,18-DHO toward the subphase in the condensed state at 278 and 293 K. Therefore, the fluorescence microscopy measurements

support the results of the fluorescence spectroscopy investigations. The fluorescence micrographs depicted in Figure 9 show the behavior of erythro-9,10-DHO (A) and threo-2,3-DHO (B) during film compression at 293 K. The enantiomers (Figure 9A1,B1) and racemates (Figure 9A2,B2) form condensed phase domains with differences in shape and size. The chiral phase of the enantioenriched erythro9,10-DHO could not be imaged due to a small observation segment and large domains. However, the chirality of this monolayer has been shown by Brewster angle microscopy.18 Due to the formation of antipodal dimers in racemic monolayers, the chirality of the particular amphiphiles is lost at a molecular level. This can also be observed in nonchiral domains at a microscopic level (Figure 9A2,B2). The enantioenriched films of threo-2,3DHO show nonchiral domains (Figure 9B1) in contrast to the other enantioenriched amphiphiles (Figures 7A, 8A, and 9A1), which show chiral structures due to intermolecular interactions based on the chiral hydroxyl groups. The methyl ester group of the enantioenriched threo-2,3DHO seems to disturb the interaction between the hydroxyl groups in a chiral way due to steric effects. In conclusion the enantioenriched monolayer forms no chiral structure as was also shown by GIXD (Figure 3B). Nevertheless, the hydroxyl group interactions between two antipodal enantiomers lead to the formation of nonchiral dimers due to hydrogen bonds; steric effects seem not to play any role. In conclusion the strong chiral discrimination for threo2,3-DHO and erythro-9,10-DHO at a molecular level as deduced from grazing incidence X-ray diffraction measurements correlates well with large differences in surface pressure-area isotherms. The influence on the monolayer structures at a microscopic scale, as investigated by fluorescence microscopy, is largest for 17,18-DHO despite only one chiral hydroxyl group. Conclusion The vicinal dihydroxylation of octadecanoic acid methyl ester in 2,3; 9,10; and 17,18 positions leads to amphiphiles which form chiral condensed phases with the pure enantiomers and nonchiral phases with their racemic mixtures. This observation agrees well with the assumption of a dimerization of optical antipodes in the monolayers of the racemic mixtures. The dimerization causes

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Figure 8. Typical fluorescence micrographs of enantioenriched (A) and racemic (B) monolayers of 17,18-DHO during film compression at 278 K at molecular areas of (A1) 1.4 nm2, (A2) 1.0 nm2, (A3) 0.8 nm2, (A4) 0.6 nm2, (A5-7) 0.5 nm2, (A8) 0.33 nm2, (B1) 1.00 nm2, (B2) 0.88 nm2, (B3) 0.8 nm2, (B4) 0.5 nm2, (B5) 0.4 nm2, (B6,7) 0.33 nm2, and (B8) 0.26 nm2. The monolayers contain 1 mol % fluorescence probe NBD-C12-Acid.

Figure 9. Fluorescence micrographs of enantioenriched (1) and racemic (2) monolayers of methyl erythro-9,10-dihydroxyoctadecanoate (erythro-9,10-DHO) (A) and methyl threo-2,3-dihydroxyoctadecanoate (threo-2,3-DHO) (B) at 293 K at molecular areas of (A1) 0.75 nm2, (A2) 0.6 nm2, (B1) 0.35 nm2, and (B2) 0.4 nm2. The monolayers contain 1 mol % fluorescence probe NBD-C12-Acid.

a mirror plane in the middle of the dimer molecules due to the stereospecific formation of hydrogen bonds between only two molecules. In contrast, the monolayers of the pure enantiomers form chiral lattices in which one molecule forms hydrogen bonds with two or more other molecules, giving rise to the formation of a two-dimensional chiral hydrogen-bond network. The individual enantioenriched amphiphiles are able to distinguish between the same and antipodal enantiomers, and thus they are able to recognize chemical structures at a molecular level. The position of the vicinal hydroxyl groups along the hydrophobic alkyl chain influences the phase behavior and lattice structures. Mainly, the tilt angle and tilt direction are influenced by the position of the hydroxyl

groups. Additionally, the hydroxyl groups in the middle of the amphiphiles (erythro-9,10-DHO) lead to an extraordinarily large lattice spacing, which reveals a prevented strong approximation of the alkyl chains to one another. Not only do the hydroxyl groups possess a connecting function, but they also act as disturbance groups. The orientation of the bolaamphiphile 17,18-DHO was investigated in the condensed phase using fluorescence spectroscopy. The results show the removal of the hydroxyl groups from the water subphase at 293 K during film compression, while at 278 K the hydroxyl groups are directed to the water subphase in the condensed phase. Due to the opposite orientation of the amphiphiles, the branches of the condensed domains show contrary pref-

Chiral Discrimination in Monolayers of DHOs

erential directions at both subphase temperatures as observed by fluorescence microscopy. Additionally, the fluorescence micrographs show different domain shapes for the enantioenriched and racemic monolayers and therefore heterochiral discrimination of all investigated DHO. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (Grants SFB 424/B2

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and Br 1378/3). We thank Prof. Dr. Scha¨fer and his group from the Institute of Organic Chemistry (University of Mu¨nster) for the supply of the dihydroxylated methyl octadecanoates and the fluorescence dye DiO, and Prof. Dr. Fuchs, Dr. Chi, and Dr. Jacobi (Institute of Physics, University of Mu¨nster) for helpful discussions. LA000506V