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Behavior of the Long-chain Bifunctional Molecule r,ω-13,16-Dimethyloctacosanedioate Dimethylester at the Air-Water Interface Junggeun Lee,† Hyun Joo,† Sang Gil Youm,† Seok-Ho Song,‡ Seunho Jung,§ and Daewon Sohn*,† Department of Chemistry, Hanyang University, Seoul 133-791, Korea, Microoptics NRL in Department of Physics, Hanyang University, Seoul 133-791, Korea, and Department of Microbial Engineering & Bio-Molecular Informatics Center, Konkuk University, Seoul 143-701, Korea Received July 31, 2002. In Final Form: March 29, 2003 The conformational change of bifunctional C30DME, R,ω-13,16-dimethyloctacosanedioate dimethylester, with changes of surface pressure at the air-water interface was investigated. The surface pressure (π)area (A) isotherm indicates that the conformation of C30DME at the air/water interface changes from linear to reverse U-shape as surface area is compressed. The two hydrophilic ends anchored on the water surface and the hydrophobic chains rise up in the air and generate a reverse U-shape as the surface pressure increases. To confirm the formation of a reverse U-shape, we examined the thickness of the Langmuir-Blodgett film with a surface plasmon resonance spectrometer and an ellipsometer. Theoretical calculations coupled with experimental observations showed that the conformational change depends on the rotational sites in the middle of the structure, the 13 and 16 positions of the hydrocarbon, and the energy of the conformational transition is about 1 kcal/mol.
Introduction Bolaform amphiphilic molecules which have two hydrophilic sites separated by a relatively great distance have not received all the attention they deserve despite their physiological activity, association in water, and selfassembled characters.1 R,ω-13,16-Dimethyloctacosanedioate dimethylester (C30DME) is a bolaamphiphile, in which two polar headgroups are linked covalently at the ends of a long hydrophobic saturated hydrocarbon.2 Compared to single-headed amphiphiles, the introduction of a second headgroup generally induces a higher solubility in water, an increase in the critical micelle concentration (cmc), and a decrease in the aggregation number. C30DME is prepared from a major structural fatty acyl component of the membrane lipids in a strict anaerobic thermophilic bacterium, Thermoanaerobacter ethanolicus, which contains R,ω-dicarboxylic fatty acyl components.2 Under environmental stresses such as high temperature and low pH, this unusual bipolar transmembrane structure is formed by the coupling process, in which the long-chain fatty acids are formed by ω or ω-1 tail-to-tail coupling of the fatty acid chains from opposite leaflets of the membrane.3 Figure 1 shows the structure of C30DME with a diester on both ends and methyl groups at the 13 and 16 carbons. In the transition state, C30DME gives rise to a stereochemistry having two centers of asymmetry, and * Corresponding author. E-mail:
[email protected]. Tel: +82-2-2290-0933. Fax: +82-2-2299-0762. † Department of Chemistry, Hanyang University. ‡ Department of Physics, Hanyang University. § Department of Microbial Engineering, Konkuk University. (1) (a) Fuhrhop, J. H.; Fritsch, D. Acc. Chem. Res. 1986, 19, 130. (b) Escamilla, G. H.; Newkome, G. R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1937. (2) Jung, S.; Zeikus, J. G.; Hollingsworth, R. I. J. Lipid Res. 1994, 35, 1057. (3) (a) Jung, S.; Hollingsworth, R. I. J. Lipid Res. 1994, 35, 1932. (b) Moril, H.; Eguchi, T.; Nishihara, M.; Kakinuma, K.; Konig, H.; Koga, Y. Biochim. Biophys. Acta 1998, 1390, 339.
the dicarboxylic acids generate them to be chiral. In the enzymatic process, the methyl groups are directed away from the face of the enzyme and are on the same side of the bilayer, in which the coupling of the two identical chains occurs on the same side of the bilayer. The resulting product would be racemic of (R,R) or (S,S) conformation prepared from the general tail-to-tail synthesis by enzyme.2,3 In addition to the study of the activities in the membrane including the passage of metabolites out of the cell, the bolaamphiphile has the potential to be used in supramolecular arrays and nanostructures. Thus, study of the behaviors of C30DME will help us to understand not only the biological aspect but also the surface chemistry of these bifunctional molecules. While the monofunctional amphiphilic molecules have been widely investigated with regard to their behavior at the air-water interface, there have been few investigations of bolaamphiphiles.4 So far, several mimicking structures such as bisbenzimidalzole derivatives and dicarboxylic acid at the air/water interface have been studied, but there is still a lack of information about bifunctional molecules. Several researchers have proposed the formation of a reverse U-shape monolayer of bolaamphiphiles by observation of long plateau regions and by calculation of the molecular area from isotherms, but there are some limitations. With these studies, if the skeleton between the two polar headgroups is very long (CH2 chain length > 25),5 it is hard to prove the bending of the main chains because there is mingling of the chains at the air/water interface. On the other hand, a shortchain bolaamphiphile (CH2 chain length < 10) can only (4) (a) Di Meglio, C.; Rananavare, S. B.; Svenson, S.; Thompson, D. H. Langmuir 2000, 16, 128. (b) Lee, J.; Joo, H.; Lee, Y.; Han, O. H.; Jung, S.; Cho, C.-G.; Sohn, D. Bull. Korean Chem. Soc. 2002, 23 (5), 776. (c) Eguchi, T.; Kano, H.; Kakinuma, K. Chem. Commun. 1996, 3, 365. (d) Moss, R. A.; Fujita, T.; Okumura, Y. Langmuir 1991, 7 (11), 2415. (e) Thompson, D. H.; Wong, K. F.; Humphry, B. R.; Wheeler, J. J.; Kim, J. M.; Rananavare, S. B. J. Am. Chem. Soc. 1992, 114 (23), 9035. (5) Franceschi, S.; de Viguerie, N.; Riviere, M.; Lattes, A. New J. Chem. 1999, 23 (4), 447.
10.1021/la0263256 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/03/2003
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Figure 1. Molecular structure of C30DME where numberings and R and ω carbons are indicated.
result in a one-head-on monolayer.6 Menger et al. proposed a “wicket-like” conformation where the C12 bolaform electrolytes fold at the air/water interface having ∼2 kcal/ mol free energies of adsorption.1 Patwardhan et al. reported that bolaamphiphiles bearing macrocyclic transmembrane, C20 hydrophobic chains are conformationally restricted at the air-water interface.7 Herein we aim to report a conformational change of the C30DME molecule at the air-water interface with the increasing of surface pressure, in which the molecule has two rotating sites in the middle of the chain. We predicted the presence of a reverse U-shape through the analysis of the π-A isotherm and confirmed this through the result of the layer thickness measurement with the surface plasmon resonance (SPR) apparatus and by ellipsometry. Atomic force microscopy (AFM) was also used to measure the change of surface roughness with the increase of surface pressure and showed how the conformation of C30DME at the air-water interface changes with the increase of surface pressure. Experimental Section Materials. C30DME was synthesized in the membrane of the cell and isolated from other molecules using flash column chromatography by Jung et al. as described elsewhere.3 In short, the isolated cells, T. ethnanolicus, were grown on a complex medium that contained yeast extract, trypticase, trace salts, and vitamins under stringent anaerobic conditions at 65 °C. Bacterial membranes were isolated as described in ref 2. The resulting products were chiral.3,4 Spectroscopic grades of chloroform were used to make the spreading solution for the isotherm and the deposition. Apparatus. LAUDA FW-2 (LAUDA Instruments, Germany) was used for the π-A isotherms and Langmuir-Blodgett (LB) film preparation. The trough area was 927 cm2, and the compression speed was 20 cm2/min. The compression of the monolayer was started 10 min after spreading to maintain the equilibrium. The C30DME LB films were vertically deposited with a dipping speed of 5 mm/min on hydrophilic substituted silicon wafers for the ellipsometer and AFM measurements and on gold substrates for the SPR measurements. All film depositions were performed at a constant temperature of 20 ( 0.5 °C. The AFM apparatus was a Nanoscope III (Digital Instrments, Inc.) with a pyramidal Si3N4 tip. A 1.5 µm × 1.5 µm section of the LB film was scanned in the noncontact mode.8 The SPR apparatus was constructed in the laboratory.9 The p-polarized light of a 5 mW He-Ne laser (λ ) 632.8 nm) was directed through the prism onto a thin metal layer (54 nm gold), and the intensity of the reflected light was monitored as a function of the incident angle (θ) with a photodiode. The LB film was built up on a 50 nm thick gold layer evaporated onto a clean glass substrate, which was attached on the bottom of the prism using index-matching oil for optical coupling. The representative complex dielectric constant of the evaporated gold layer was determined to be ∈* ) -12.3 + 1.1i at a wavelength of 632.8 nm (He-Ne laser). An AutoEL nulling ellipsometer (Rudolph Scientific) was also used for measuring the layer thickness. The angle of the incidence beam was 70° to the sample, and the resolution of the instrument was (0.04° in ∆ and Ψ.10 (6) (a) Menger, F. M.; Wrenn, S. J. Phys. Chem. 1974, 78 (14), 1387. (b) Sohn, D.; Kitaev, V.; Kumacheva, E. Langmuir 1999, 15, 1698. (7) Patwardhan, A. P.; Thompson, D. H. Langmuir 2000, 16, 10340. (8) Mori, O.; Imae, T. Langmuir 1995, 11, 4779. (9) (a) Yamamoto, S.; Tsujii, Y. Langmuir 1996, 12, 3671. (b) Do, Y. H.; Lee, G.; Song, S. H.; Sohn, D.; Lee, S. S. Hankook Kwanghak Hoeji 2002, 13 (1), 9.
Figure 2. (a) Surface pressure-area (π-A) isotherms of stearic acid (open squares) and C30DME (open circles) monolayers at the air-water interface, 20 °C. The insert is the hysteresis of the C30DME isotherm. (b) Surface pressure-area isotherms of C30DME at four different temperatures, 10, 20, 30, and 40 °C.
Results and Discussion The Behavior of C30DME at the Air/Water Interface. The π-A isotherm of typical monofunctional amphiphiles, stearic acid (STA, 0), on pure water at 20 °C is shown in Figure 2a. The isotherm exhibits the gas/LE (liquid expanded) and LE/LC (liquid expanded/liquid condensed) phase transitions with increasing surface pressure.11 The collapse pressure of ∼50 mN/m at the area of 19 Å2/molecule agreed well with the calculated value of 22 Å2/molecule.11 Figure 2a also shows the π-A isotherm of the C30DME monolayer (O). The C30DME isotherm shows the characteristic features such as an increase of surface pressure in a wide region from ≈110 to 65 Å2/molecule and the long plateau region below 65 Å2/molecule. Also, the collapse pressure of the C30DME monolayer is about 13.5 mN/m at the (10) (a) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Springer: Berlin, 1988. (b) Yamaguchi, H.; Sakamoto, Y. J. Am. Chem. Soc. 1990, 112, 3188. (11) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; John Wiley & Sons: New York, 1997.
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area of 65 Å2/molecule. The broad increase of surface pressure indicates an inherently large flexibility of the long carbon chain in C30DME. The molecular area of ≈110 Å2/molecule and the collapse area of 65 Å2/molecule correspond to the area of the molecule for the fully expanded conformation of the C30DME in which the length and the width are 32.1 and 2-4 Å, respectively. The variation of the width depends on how the molecules are packed at the air/water interface without the conformational change. While the surface area was smaller than 65 Å2/molecule, there was no surface pressure change, but the surface area decreased. The 65 Å2/molecule is the region where the hydrophilic ends of C30DME are anchored in the water subphase and compressed to raise up the middle of the chain. The molecules follow the linear expanded to linear condensed phase transition (LNELNC transition) at ≈110 Å2/molecule and the linear to reverse U-shape transition (LN-RU transition) at 65 Å2/ molecule. The molecules maintain their linear conformation at low surface pressure and start to bend at this transition point. Hysteresis measurements confirmed that these processes are reversible as shown in the insert of Figure 2a.12 Consider the geometry of the linear shape of C30DME; there are free volumes in the middle of the C30DME chains which come from the bulky esters on the ends of the molecules and two methyl substituents at 13 and 16 carbons. The monolayer formed at the surface area around 110 Å2/molecule has a large free volume, but the monolayer at 65 Å2/molecule has a relatively condensed phase with linear shape conformation. In the LN-RU transition region, the two methyl substituents have the important role to bend (or rotate) around the 13 and 16 carbons. (See the molecular modeling section.) During this bending process, the surface area monotonically decreases by compression, but the surface pressure is unchanged. Figure 2b shows isotherms of C30DME at four different temperatures. The LNE-LNC transitions are little affected by the thermal energy because these processes are entropically driven packing processes of the linear molecules at the air/water interface.13 The configurations or the arrangements of the linear molecules at the air/water interface minimize their free energies by lowering the excluded volume of the molecules. The LN-RU transitions, however, are sensitive to temperature-dependent conformational changes, so the surface pressures are dramatically influenced by the temperature change of the subphase. The surface pressure during this linear to reverse U-shape transition is 16 mN/s at 10 °C and drops 12 mN/s at 40 °C as shown in Figure 2b. As the temperature of the subphase of fixed surface area increases, the surface pressure decreases. When the external pressure acting on the linear C30DME molecules spread at the air/water interface is increased, the surface area decreases. The reasons for the decreased surface pressure are not entropic but enthalpic, due to the conformational transition of the linear to reverse U-shape. Grazing angle IR studies at the air/water interface indicate a shift of the CH2 asymmetric stretching band from 2926 to 2920 cm-1 as the surface area is compressed, which suggests the conformational change to the highly ordered structure by increasing surface pressure.14 (12) Tachiba, T.; Yoshizumi, T. Bull. Chem. Soc. Jpn. 1979, 52, 34. (13) Crisp, D. J. In Surface Phenomena in Chemistry and Biology; Danielli, J. F., Pankhurst, K. G. A., Riddiford, A. C., Eds.; Pergamon: New York, 1958. (14) (a) Hoffmann, F.; Huhnerfuss, H.; Stine, K. J. Langmuir 1998, 14, 4525. (b) Han, S. W.; Kim, C. H.; Kim, K. Langmuir 1999, 15, 1579. (c) Hwang, M. J.; Kim, K. Langmuir 1999, 15, 3563. (d) Mao, L.; Ritcey, A. M. Langmuir 1996, 12, 4754. (e) Lee, J.; Kang, S.; Kim, S.; Sohn, D. Mol. Cryst. Liq. Cryst. 2001, 371, 25.
Lee et al. Table 1. Surface Roughness Determined by Atomic Force Microscopy with the C30DME Monolayer Deposited on a Silicon Wafer Substratea surface pressure surface roughness layer thickness
1.5 1.7 8.0
6.5 3.8 11
14 2.3 16
a The roughness was determined by averaging three different points. Layer thickness was measured from the bottom of the plate to the peak of the mountain on AFM images. Units are surface pressure, mN/m; surface roughness, Å; and layer thickness, Å.
Surface Roughness Measurement. Figure 3a,b shows AFM images of the C30DME monolayer at 1.5 mN/ m. The height difference from the bottom to on the image is 8 Å, which is comparable with the SPR and ellipsometry data (see the next section). The other AFM images at different pressures, 6.5 and 14 mN/m, show height differences from the bottom to top of 11 and 16 Å, respectively. These are also similar to the SPR and ellipsometry data. Table 1 shows the surface roughness determined by AFM with the C30DME monolayer deposited on a silicon wafer substrate. The surface roughness was averaged at three different points with the change of surface pressure.15 Table 1 summarizes the root-mean-square deviation of the height from the standard plane determined by averaging the height of the monolayer. Our interest in this experiment is the change of the roughness with the increase of surface pressure. From Table 1, it can be seen that the surface roughness becomes larger with increasing surface pressure up to 6.5 mN/m, but it decreases with increasing surface pressure from 6.5 to 14 mN/m. At 14 mN/m, we observed a mostly smooth surface. In the gas phase, C30DME molecules may have a mixed conformation in their long carbon chain at the air-water interface. At low surface pressure, the C30DME monolayer has a small surface roughness because C30DME molecules might have a low height regardless of their structures. With the increase of surface pressure, C30DME molecules have different compression types according to the initial conformations of the carbon chains, which result in a large surface roughness of the C30DME monolayer. At greater surface pressure, the long carbon chain goes from a zigzagged or/and twisted conformation to a reverse U-shape, which occupies a smaller molecular area. Consequently, the increased ordering of the reverse U-shape molecules minimizes the surface roughness. This is why the surface roughness changes with increasing surface pressure (Table 1). Figure 4 illustrates the conformational change of the C30DME molecules at the air-water interface: (a) the molecules stretch their conformations but have many different shapes at the low surface pressure, (b) the number of different shapes increases with increasing surface pressure, and (c) the uniformity of the reverse U-shape increased at very low surface area. To measure the surface roughness and to confirm the presence of the reverse U-shape, we adapted SPR and ellipsometry experiments. Layer Thickness Measurements. Figure 5a-c shows the results of layer thickness measurements as a function of the surface pressure by SPR. The C30DME monolayer was deposited on a gold substrate. The surface pressure was increased from 1.5 to 14.5 mN/m for the SPR experiment. This figure shows the abrupt growth of the layer thickness from 7.1 to 17.8 (15) (a) Simpsom, G. J.; Sedin, D. L.; Rowlen, K. L. Langmuir 1999, 15, 1429. (b) Fielden, M. L.; Claesson, P. M.; Verrall, R. E. Langmuir 1999, 15, 3924. (c) Kago, K.; Matsuoka, H.; Yoshitome, R.; Yamaoka, H. Langmuir 1999, 15, 4295.
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Figure 3. AFM images of C30DME monolayers on the silicon wafer transferred at three different surface pressures: (a) at 1.5 mN/m, (b) at 6.5 mN/m, and (c) at 14 mN/m. (A) AFM image, (L) lateral force microscopy (LFM) image, and (H) height profile of a line in the image.
Å with increasing surface pressure (Figure 5b). At the plateau region of the isotherm, the figure shows identical layer thickness. The layer thickness of 17.8 Å coincides with the height of the reverse U-shape of C30DME molecules according to the data of molecular modeling. (See the next section.) The layer thickness was also examined by ellipsometry, and the results are shown in Figure 5c. These results agree well with the result of the SPR experiment where we can see the abrupt growth of the layer thickness at the middle of the surface pressure. The layer thickness from 1.5 to 11 mN/m increases linearly, but at high surface pressures, 12 < π < 14, we observed a sudden steep increase in the layer thickness, which is what was seen from the surface roughness measurements of AFM. In Figure 4, at the LNE phase, the C30DME
molecules have various conformations: zigzagged, twisted, linear, and so forth. With increasing surface pressure, each of these conformations decreases its molecular area by a different compression type. The area of C30DME molecules decreases but the height is identical regardless of the compression type. The reverse U-shape makes the highest conformation at the air/water interface except for the one-head-on monolayer. Both SPR and ellipsometry results prove that the one-head-on structure is not possible in this case. Since there are a variety of conformations at the low surface pressure, the mean height of the monolayer should be relatively low and the surface roughness should be high. The increase in the height would be slowed by a decrease in the surface area, which is the region from 1.5 to 11 mN/m (Figure 5c). Above 11 mN/m, the molecules
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Figure 4. The schematic representation for the formation of the reverse U-shape of C30DME with the increase of surface pressure at the air-water interface.
undergo conformational change. The reverse U-shape can occupy the minimal molecular area against the decreased surface area. This conformational change is continuously produced with increasing surface pressure until the maximum surface pressure is reached. The conformational change of the C30DME monolayer starts at about 12 mN/ m. The isotherm shows that the increment of the surface pressure (∂π/∂A) decreases at about 12 mN/m. This indicates that the conformational changes occur in this direction in order to stabilize the surface pressure. Molecular Modeling. Since C30DME has two chiral centers at the 13 and 16 carbons, there could be (R,S), (R,R), and (S,S) isomers. The linear and U-shape forms of (R,S)-C30DME are shown in parts a and b of Figure 6, respectively. Parts a and b of Figure 7 represent the linear and U-shape of (S,S)-C30DME, respectively. The (R,S) form has a fully expanded conformation, and its bending (or rotating) is more difficult than that of the (S,S) form due to the orientation of the methyl substitutes at the middle of the backbone. Conformational analysis of the (S,S) stereoisomers, assuming rotation of the methyl substituents at the 13 and 16 carbons, shows small energy differences between the linear and bent conformations. When the dihedral angle of ∠C12-C13-C14 in the middle of the backbone is changed, one of the methyl substitutes is placed on the side of the main chain and this increases the steric hindrance. The (R,S) form of the linear chain shows relatively a bent structure, and its U-shape also has greater closed ester functionality. We used semiempirical AM1 and HF (Hartree-Fock) methods for molecular modeling analysis in the Gaussian 9416 program package. Full geometry optimization was performed in the gas phase without any constraint at AM1, HF/3-21G*, and HF/6-31G* levels, and the parameters are summarized in Tables 2 and 3. R1 represents the height and R2 is the distance between two ester carbons in the optimized geometry. R1 was estimated by using trigonometry, R1 ) x sin θ as shown in Figure 6b. The structural (16) Gaussian is a registered trademark of Gaussian, Inc., Carnegie, PA.
Figure 5. (a) The change of refractivity and the incident angle on the SPR measurement. Monolayers of C30DME were deposited on the gold substrate with various surface pressures at 25 °C. (b) Plot of the layer thickness for the C30DME monolayer as a function of the surface area measured with the SPR apparatus. (c) Plot of the layer thickness for the C30DME monolayer as a function of surface pressure measured by ellipsometry. Table 2. Parameters for the AM1 Calculationa AM1 & HF calculations
(R,S) linear (R,S) reverse U-shape (S,S) linear (S,S) reverse U-shape
R1
R2
19.8 9.1 18.1
37.6 14.4 32.1 10.4
a R1 represents the height of the C30DME, and R2 is the distance between two ester carbons. The R2 value of the (R,S) linear conformation is ambiguous because it varies at different positions. The unit is Å.
difference of the two conformations is small in the bent form, but the distance between the esters is large in the (R,S) conformation. The energy differences of the rotation around the 13 and 16 carbons for the change from bent to linear conformations were determined by HF calcula-
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Table 3. Parameters for the HF/3-21G* and HF/6-31G* Base Seta conformer
parameter
HF/3-21G*
HF/6-31G*
(R,S) reverse U-shape
R1 R2 R1 R2
19.0 15.7 18.9 13.4
18.9 17.3 18.8 14.4
(S,S) reverse U-shape a
The unit is Å.
Figure 7. (a) The linear and (b) the reverse U-shape forms of (S,S)-C30DME.
Figure 6. (a) The linear and (b) the reverse U-shape forms of (R,S)-C30DME. Table 4. Energy Differences between the Bent and the Linear Conformations of C30DME Calculated by HFa level of calculation
∆E of (R,S) conformer
∆E of (S,S) conformer
HF/3-21G* HF/6-31G*
0.75 0.78
2.44 2.30
a
The unit is kcal/mol.
tion. These are summarized in Table 4. The dihedral angle has two energy minima at 0° (linear) and 180° (bent), and the energy barrier to rotation is about 0.78 kcal/mol for the (R,S) conformer and 2.30 kcal/mol for the (S,S) conformer. The conformational energy for the LN-RU transition in the π-A isotherm of Figure 2a is around 0.87 kcal/mol, 13.5 mN/m × 45 Å2/molecule × 6.02 × 1023/mol × 1 cal/ 4.18 J. The energy differences for each conformer are summarized in Table 4.
Conclusion We propose the reverse U-shaped conformation of bolaamphiphiles at the air-water interface. General bolaamphiphiles having esters on both ends did not show the linear to reverse U-shape transition. The transition of C30DME, however, strongly depends on two alkyl substitutions in the middle of the chain. The long alkyl esters could rotate centered on two chiral centers of the molecule and substantially shrink the size of the molecule under increasing surface pressure. The trajectorial area of the linear shape of C30DME was ∼70 Å2 at the air/ water interface, and the reverse U-shape was completed by increasing surface pressure up to 20 Å2. The results of π-A isotherms agree well with the dimensions of the simulated molecules depicted in Figures 6 and 7. Though the stereochemistry of C30DME that is naturally selected in the stress of the biomolecules is not clear, the linear to reverse U-shape transition of C30DME is obvious at the air/water interface. Regioselective bolaamphiphilic molecules are designed and are under investigation. Acknowledgment. This work was supported by the ABRL program of the Korean Science and Engineering Foundation (KOSEF Grant R-14-2002-004-01002-0). D.S. thanks Professor Doo Wan Boo at Yonsei University for measurements of AFM images. LA0263256
