Enantiomeric Recognition of Amino Acids by Amphiphilic Crown

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Enantiomeric Recognition of Amino Acids by Amphiphilic Crown Ethers in Langmuir Monolayers Mounia Badis,† Iwona Tomaszkiewicz,†,§ Jean-Pierre Joly,‡ and Ewa Rogalska*,† Equipe de Physico-chimie des Colloı¨des and Groupe SUCRES, UMR 7565 CNRS/Universite´ Henri Poincare´ Nancy 1, Faculte´ des Sciences, BP 239, 54506 Vandoeuvre-le` s-Nancy Cedex, France Received February 16, 2004. In Final Form: May 4, 2004 Four new chiral, amphiphilic crown ethers differing by the hydrophobic tailgroups were synthesized, and their capacity to recognize enantiomeric amino acids was examined using Langmuir films. Surface pressure and surface potential measurements performed on the subphases containing L or D enantiomers of alanine, valine, phenylglycine, and tryptophane indicate that the crown ethers forming the monolayer interact with the amino acids. The effects observed are ascribed to the formation of host-guest complexes. The differences in the magnitude of the effects measured show that the crown ethers are capable of discriminating between different amino acids as well as the enantiomers. Our results demonstrate that the structure of the monolayer plays a decisive role in the molecular recognition process including chiral recognition.

1. Introduction Molecular recognition has been the focus of supramolecular chemistry as one of the fundamental processes in biological systems.1-5 Such phenomena as recognition of substrate by enzyme, antigen by antibody, neurotransmitter by neuroreceptor, or DNA by protein are governed by weak, noncovalent binding forces. Recent achievements in imitating the natural processes using synthetic artificial receptors have shown that biological-like specificity can be introduced into relatively simple molecules6 such as crown ethers. Indeed, crown ethers which are macrocylic polyethers, first introduced in 1967 by Pedersen,7,8 have been studied for their capability to interact with various cations such as metals,9-14 primary ammoniums,15,16 and * To whom correspondence should be addressed. E-mail: [email protected]. † Equipe de Physico-chimie des Colloı¨des, Universite ´ Nancy 1. ‡ Groupe SUCRES, Universite ´ Nancy 1. § On leave from the Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland. (1) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (2) Jolles, B.; Laigle, A.; Priebe, W.; Garnier-Suillerot, A. Chem.Biol. Interact. 1996, 100, 165. (3) Leng, F.; Savkur, R.; Fokt, I.; Przewloka, T.; Priebe, W.; Chaires, J. B. J. Am. Chem. Soc. 1996, 118, 4731. (4) Lampidis, T. J.; Kolonias, D.; Podona, T.; Israel, M.; Safa, A. R.; Lothstein, L.; Savaraj, N.; Tapiero, H.; Priebe, W. Biochemistry 1997, 36, 2679. (5) Frezard, F.; Pereira-Maia, E.; Quidu, P.; Priebe, W.; GarnierSuillerot, A. Eur. J. Biochem. 2001, 268, 1561. (6) Zhang, X. X.; Bradshaw, J. S.; Izatt, R. M. Chem. Rev. 1997, 97, 3313. (7) Pedersen, C. J. Science 1988, 241, 536. (8) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 2495. (9) Steed, J. W. Coord. Chem. Rev. 2001, 215, 171. (10) Gokel, G. W.; Barbour, L. J.; Ferdani, R.; Hu, J. Acc. Chem. Res. 2002, 35, 878. (11) Adam, K. R.; Baldwin, D. S.; Lindoy, L. F.; Meehan, G. V.; Vasilescu, I. M.; Wei, G. Inorg. Chim. Acta 2003, 352, 46. (12) Campagna, M.; Dei, L.; Gambi, C. M. C.; Lo Nostro, P.; Zini, S.; Baglioni, P. J. Phys. Chem. B 1997, 101, 10373. (13) Zawisza, I.; Bilewicz, R.; Luboch, E.; Biernat, J. F. Dalton 2000, 499. (14) Pietraszkiewicz, M.; Kozbial, M.; Pietraszkiewicz, O. J. Membr. Sci. 1998, 138, 109. (15) Rudiger, V.; Schneider, H.-J.; Solov’ev, V. P.; Kazachenko, V. P.; Raevsky, O. A. Eur. J. Org. Chem. 1999, 1847, 7. (16) Zhu, C. Y.; Izatt, R. M.; Bradshaw, J. S.; Dalley, N. K. J. Inclusion Phenom. 1992, 13, 17.

even neutral molecules.17 It has been demonstrated that the cavity of the crown ether is specific for shape, as well as charge and size.18 Since the lone pairs on the oxygen atoms are available to noncovalently bind electronaccepting guests, crown ethers may distinguish between potassium and sodium or silver ions,19-21 and between amino acids.22 However, the chemical and biological properties of molecules depend not only on the nature of their constituent atoms but also on how these atoms are positioned in space. Indeed, chiral specificity is fundamental in chemistry and biology, and it plays a central role in controlling molecular recognition. Not surprisingly, optically active macrocyclic receptors and their enantioselective recognition of chiral compounds have been attracting much attention.6,23 In this study, four new chiral crown ethers were used as molecular receptors of enantiomeric amino acids.24 The crown ether derivatives were amphiphilic and insoluble in water and could be thus used to form Langmuir monolayers25-29 at the air/water interface. The Langmuir films are systems of choice for studying molecular recognition in membranelike systems.30-38 The practical (17) Chen, H.; Weiner, W. S.; Hamilton, A. D. Curr. Opin. Chem. Biol. 1997, 1, 458. (18) Cram, D. J.; Cram, J. M. Science 1974, 183, 803. (19) Rodriguez, L. J.; Liesegang, G. W.; White, R. D.; Farrow, M. M.; Purdie, N.; Eyring, E. M. J. Phys. Chem. 1977, 81, 2118. (20) Brandt, K.; Seliger, P.; Grzejdziak, A.; Bartczak, T. J.; Kruszynski, R.; Lach, D.; Silberring, J. Inorg. Chem. 2001, 40, 3704. (21) Sureshan, K. M.; Shashidhar, M. S.; Varma, A. J. J. Org. Chem. 2002, 67, 6884. (22) Sousa, L. R.; Sogah, G. D. Y.; Hoffman, D. H.; Cram, D. J. J. Am. Chem. Soc. 1978, 100, 4569. (23) Zhao, H.; Hua, W. J. Org. Chem. 2000, 65, 2933. (24) Pietraszkiewicz, M.; Kozbial, M. J. Inclusion Phenom. 1993, 14, 339. (25) Kele, P.; Orbulescu, J.; Calhoun, T. L.; Gawley, R. E.; Leblanc, R. M. Langmuir 2002, 18, 8523. (26) Wang, S.; Zhang, Q.; Datta, P. K.; Gawley, R. E.; Leblanc, R. M. Langmuir 2000, 16, 4607. (27) Heiney, P. A.; Stetzer, M. R.; Mindyuk, O. Y.; DiMasi, E.; McGhie, A. R.; Liu, H.; Smith, A. B. J. Phys. Chem. B 1999, 103, 6206. (28) Zawisza, I.; Bilewicz, R.; Luboch, E.; Biernat, J. F. Thin Solid Films 1999, 348, 173. (29) Muszalska, E.; Bilewicz, R.; Luboch, E.; Skwierawska, A.; Biernat, J. F. J. Inclusion Phenom. 1996, 26, 47. (30) Rogalska, E.; Ransac, S.; Verger, R. J. Biol. Chem. 1993, 268, 792.

10.1021/la049596k CCC: $27.50 © 2004 American Chemical Society Published on Web 06/19/2004

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Figure 1. Synthesis of crown ethers 1-4 from A via diol B.

advantages of this technique for studying molecular recognition are simplicity and low operational costs. The results obtained with microgram quantities of the chiral hosts/receptors forming Langmuir monolayers may be used for evaluating the utility of these molecules as chiral selectors39 in large-scale processes such as chromatography or extraction. Indeed, some crown ethers described in the literature display sufficient enantioselectivity for practical resolution of racemic protonated amino acids40 or carboxylic acids41 as their metal salts via fractional crystallization18,42 or chromatography.43,44 We report here the synthesis of four new crown ethers obtained by 4,6di-O-alkylation of a common chiral diol and a study of their capacity for differentiating between L and D enantiomers of alanine, valine, tryptophane, and phenylglycine, using a Langmuir technique. 2. Experimental Section Four new chiral, optically pure crown ethers (1, 2, 3, and 4) were synthesized from the precursor A45 via diol B by using a two-step reaction, with the second step being the Williamson ether synthesis46 (Figure 1). 2.1. Synthesis of Crown Ethers. All chemicals were of reagent grade (Acros Organics, France). THF and DMF were distilled over Na/benzophenone and CaH2, respectively, and (31) Ransac, S.; Rogalska, E.; Gargouri, Y.; Deveer, A. M.; Paltauf, F.; de Haas, G. H.; Verger, R. J. Biol. Chem. 1990, 265, 20263. (32) Rogalska, E.; Nury, S.; Douchet, I.; Verger, R. Chirality 1995, 7, 505. (33) Douchet, I.; De Haas, G.; Verger, R. Chirality 2003, 15, 220. (34) Pietraszkiewicz, M.; Prus, P.; Bilewicz, R. Pol. J. Chem. 1999, 73, 1845. (35) Pietraszkiewicz, M.; Prus, P.; Bilewicz, R. Pol. J. Chem. 1999, 73, 2035. (36) Pietraszkiewicz, O.; Pietraszkiewicz, M.; Radecka, H.; Radecki, J. J. Inclusion Phenom. Mol. Recognit. Chem. 2001, 41, 129. (37) Chahinian, H.; Bezzine, S.; Ferrato, F.; Ivanova Margarita, G.; Perez, B.; Lowe Mark, E.; Carriere, F. Biochemistry 2002, 41, 13725. (38) Chahinian, H.; Belle, V.; Fournel, A.; Carriere, F. Eur. J. Lipid Sci. Technol. 2003, 105, 590. (39) Wenzel, T. J.; Thurston, J. E.; Sek, D. C.; Joly, J.-P. Tetrahedron: Asymmetry 2001, 12, 1125. (40) Lingenfelter, D. S.; Helgeson, R. C.; Cram, D. J. J. Org. Chem. 1981, 46, 393. (41) Zinic, M.; Frkanec, L.; Skaric, V.; Trafton, J.; Gokel, G. W. J. Chem. Soc., Chem. Commun. 1990, 1726. (42) Dotsevi, G.; Sogah, Y.; Cram, D. J. J. Am. Chem. Soc. 1975, 97, 1259. (43) Joly, J.-P.; Moll, N. J. Chromatogr. 1990, 521, 134. (44) Dotsevi, G.; Sogah, Y.; Cram, D. J. J. Am. Chem. Soc. 1976, 98, 3038. (45) Joly, J.-P.; Nazhaoui, M.; Dumont, B. Bull. Soc. Chim. Fr. 1994, 131, 369. (46) Baker, R. H. J. Am. Chem. Soc. 1948, 70, 3857.

Badis et al. stored over 4 Å molecular sieves. Column chromatography was performed with Kieselgel 60 silica gel, particle size 32-63 µm (Merck). NMR spectra were recorded in CDCl3 at 298 K with a Brucker AM 400 spectrometer (400 MHz 1H, 90 MHz 13C), unless stated otherwise. Melting points were determined using a Bu¨chi apparatus and are uncorrected. Optical rotation was measured using a Perkin-Elmer 141 at 20 °C (units are 10-1 deg cm2 g-1). Elemental analysis was performed using a Thermo Finnigan CHNSO-analyzer (1112 Series). Synthesis of Methyl [2,3-b]{11,12-(2,3-naphtho)-1,4,7,10,13,16hexaoxacyclooctadeca-11-ene}-4,6-di-O-benzyl-2,3-dideoxy-R-Dglucopyranoside (1). A solution of 5.826 g (10.0 mmol) of crown ether A45 in 250 mL of a mixture of MeOH/CHCl3 (4:1) was stirred under argon for 10 min. The catalyst (10% Pd/C, 250 mg, 0.235 mmol) was added and the suspension immediately shaken in a Parr hydrogenator under H2 (around 4 × 105 Pa) for 3 h. The resulting mixture was filtered through a Celite pad which was then rinsed with 260 mL of CH2Cl2/EtOH (1:1), and the combined filtrates were concentrated under reduced pressure to yield diol B (4.75 g, 96%); this viscous product solidified on standing at 4 °C. The resulting white solid product was exhaustively washed with n-hexane on a sintered glass and finally dried under vacuum: mp 134-135 °C; [R]D +44.7 (c 1.2, CHCl3); 1H NMR δ 3.39 (s, 3H, OCH3), 3.41 (dd, 1H, J1-2 ) 3.5 Hz, J2-3 ) 9.3 Hz, H-2), 3.56 (m, 2H), 3.625 (dd, 1H, J3-4 ∼ 9 Hz, H-3), 3.72-4.12 (m, 15H, 3 × OC2H4, H-4, -6, -6′), 4.23-4.32 (m, 5H, H-5, OC2H4), 4.77 (s, 1H, H-1), 7.1 (d, 2H, H-1, -4), 7.32 (m, 2H, H-5, -8, naphth), 7.66 (m, 2H, H-6, -7, naphth). Anal. Calcd for C25H34O10: C, 60.72; H, 6.93. Found: C, 60.7; H, 6.9. A mixture of diol B (0.985 g, 2.00 mmol), NBu4HSO4 (1.36 g, 4 mmol), and benzyl bromide (1 mL, ∼4 equiv) in toluene (50 mL) was vigorously stirred for 12 h at room temperature with 50% aq NaOH (40 mL). Chilled water (200 mL) was added, the organic phase isolated, and the aqueous phase extracted with CH2Cl2 (3 × 80 mL). The organic phases were combined, washed with water (3 × 20 mL), dried over MgSO4, and finally purified by column chromatography (CH2Cl2/Et2O, 9:1) to yield the bis-ether 1 (958 mg, 71%) as a white solid: mp 86-88 °C (toluene/n-hexane); [R]D +39.7 (c 1.2, CHCl3); 1H NMR 3.4 (s, 3H, OCH3), 3.47 (dd, 1H, J1-2 ) 3.5 Hz, J2-3 ) 9.0 Hz, H-2), 3.55-4.08 (m, 16H, H-3, -4, -6, -6′, 3 × OC2H4), 4.15 (m, 1H, H-5), 4.2-4.35 (m, 4H, OC2H4), 4.45-4.52 (m, 2H, H-b, -b′′), 4.62 (d, 1H, Jgem ) 12 Hz, H-b′), 4.84 (s, 1H, H-1), 4.86 (d, 1H, Jgem ) 11 Hz, H-b′′′), 7.1 (s, 2H, H-1, -4, naphth), 7.23 (d, 2H, Ar), 7.3-7.37 (m, 4H, H-5, -8, naphth, Ar), 7.37-7.5 (m, 10H, Ar), 7.52-7.58 (m, 4H, Ar), 7.67 (dd, 2H, H-6, -7, naphth); 13C NMR 149.2, 149.1 (C-2, -3, naphth), 138.4, 138.1 (C-1 Ar), 140.8, 140.7 (Ar), 129.4 (C-9, -10, naphth), 128.8, 128.5, 127.3, 127.2, 127.1, 126.4, 125.7, 124.3 (Ph, C-6, -7, naphth), 108.45 (C-1, -4, naphth), 98.3 (C-1), 82.65 (C-4), 81.0 (C-2), 77.8 (C-3), 75.3, 73.8, 73.5, 71.8, 71.2, 71.0 (OC2H4), 70.4 (C-5), 69.9, 69.6, 69.45, 68.85 (OC2H4, OCH2Ph), 55.4 (OCH3). Anal. Calcd for C39H46O10: C, 69.42; H, 6.87. Found: C, 69.4; H, 6.9. EIMS+ (70 eV) calcd for C39H46O10 674.3. Found 675.4 [M + H]+ (40%), 373.3 [M - C18H25O3]l+ (100%). Synthesis of Methyl [2,3-b]{11,12-(2,3-naphtho)-1,4,7,10,13,16hexaoxacyclooctadeca-11-ene}-2,3-dideoxy-4,6-di-O-(4-phenylbenzyl)-R-D-glucopyranoside (2). A mixture of diol B (248 mg, 0.50 mmol), NBu4HSO4 (680 mg, 2 mmol), and 4-phenylbenzyl bromide47 (500 mg, ∼2 mmol) in toluene (40 mL) was vigorously stirred for 24 h at room temperature with 50% aq NaOH (50 mL). Chilled water (200 mL) was added, the organic phase isolated, and the aqueous phase extracted with CH2Cl2 (3 × 80 mL). The organic phases were combined, washed with water (3 × 20 mL), dried over MgSO4, and finally chromatographed (CH2Cl2/Et2O, 9:1) to yield the bis-ether 2 (252 mg, 61%) as a white solid: mp 152-154 °C (toluene/n-hexane); [R]D ) +34.8 (c 2, CHCl3); 1H NMR 3.41 (s, 3H, OCH3), 3.5 (dd, 1H, J1-2 ) 3.4 Hz, J2-3 ) 9.5 Hz, H-2), 3.58-4.2 (m, 17H, H-3, -4, -5, -6, -6′, 3 × OC2H4), 4.23-4.35 (m, 4H, OC2H4), 4.51 (d, 1H, Jgem ) 10.7 Hz, H-b), 4.53 (d, 1H, Jgem ) 12.3 Hz, H-b′), 4.69 (d, 1H, H-b′′), 4.87 (s, 1H, H-1), 4.9 (d, 1H, H-b′′′), 7.12 (bs, 2H, H-1, -4, naphth), 7.23 (d, 2H, Ar), 7.3-7.37 (m, 4H, H-5, -8, naphth, Ar), 7.37-7.5 (m, 10H, Ar), 7.52-7.58 (m, 4H, Ar), 7.67 (dd, 2H, H-6, -7, naphth); (47) Van der Vleugel, D. J. M.; Vliegenthart, J. F. G. Carbohydr. Res. 1982, 105, 168.

Amphiphilic Crown Ethers in Langmuir Monolayers 13C

NMR 149.2, 149.15 (C-2, -3, naphth), 140.9, 140.8, 140.7 (Ar), 129.4 (C-9, -10, naphth), 128.8, 128.5, 127.3, 127.2, 127.1, 126.4, 125.7, 124.3 (Ph, C-6, -7, naphth), 108.45 (C-1, -4, naphth), 98.3 (C-1), 82.65 (C-4), 81.0 (C-2), 77.8 (C-3), 75.3, 73.8, 73.5, 71.8, 71.2, 71.0 (OC2H4), 70.4 (C-5), 69.9, 69.6, 69.45, 68.85 (OC2H4, OCH2Ph), 55.4 (OCH3). Anal. Calcd for C51H54O10: C, 74.07; H, 6.58. Found: C, 74.3; H, 6.3. ESMS+ (40 V) calcd for C51H54O10 826.4. Found 849.3 [M + Na]+ (85%); EIMS+ (70 eV) found 628.3 [M -OMe-C13H11]l+ (25%). Synthesis of Methyl [2,3-b]{11,12-(2,3-naphtho)-1,4,7,10,13,16hexaoxacyclooctadeca-11-ene}-2,3-dideoxy-4,6-di-O-dodecyl-R-Dglucopyranoside (3). 240 mg of 50% NaH in oil (5.0 mmol) was rapidly washed with absolute pentane and dispersed in absolute THF (50 mL). Diol B (0.990 g, 2 mmol) was added and the mixture magnetically stirred for 1 h at room temperature under argon. 1.5 mL of 1-bromododecane (6.0 mmol) and 130 mg of dried KI (0.78 mmol) were added, and the mixture was immediately heated to reflux and magnetically stirred for 20 h. After complete cooling, the solvent was evaporated under reduced pressure and the residue dissolved in CH2Cl2 (50 mL), washed with water (3 × 25 mL), dried over MgSO4, and finally chromatographed (CH2Cl2/ EtOH, 96:4) to yield the bis-ether 3 (731 mg, 44%) as a white solid: mp 71-72 °C (EtOH); [R]D +34.0 (c 2, CHCl3); 1H NMR δ 0.9 (m, 6H, 2 × CH3-C11H22), 1.25-1.4 (m, 36H, 2 × CH3C9H18-CH2), 1.55-1.65 (m, 4H, 2 × CH3-C9H18-CH2), 3.363.45 (m, 6H, H-2, OCH3, OCH2-C10H20-CH3), 3.5-3.7 (m, 19H, H-3, -4, -5, -6, -6′, 3 × OC2H4, OCH2-C10H20-CH3), 4.25-4.35 (m, 4H, OC2H4), 4.85 (s, 1H, J1-2 ) 3.6 Hz, H-1), 7.14 (bs, 2H, H-1, -4, naphth), 7.35 (dd, 2H, H-5, -8, naphth), 7.69 (dd, 2H, H-6, -7, naphth); 13C NMR 149.2 (C-2, -3, naphth), 129.4 (C-9, -10, naphth), 126.4, 124.3 (C-6, -7, naphth), 108.2 (C-1, -4, naphth), 98.0 (C-1), 82.45 (C-4), 80.6 (C-2), 77.75 (C-3), 73.2, (C-6), 73.1, 71.8, 71.4, 70.9, 70.7 (OCH2), 70.2 (C-5), 69.7, 69.5, 69.4, 69.3, 69.1 (OCH2, 55.0 (OCH3), 32.0, 30.6, 29.8, 29.7, 29.6, 29.5, 26.4, 26.3, 22.8 (CH3-C10H20-CH2O), 14.2 (CH3-C10H20-CH2O). Anal. Calcd for C49H82O10: C, 70.81; H, 9.94. Found: C, 70.5; H, 9.7. EIMS+ (70 eV) calcd for C49H82O10 830.6. Found 831.7 [M + H]+ (16%). Synthesis of Methyl [2,3-b]{11,12-(2,3-naphtho)-1,4,7,10,13,16hexaoxacyclooctadeca-11-ene}-2,3-dideoxy-4,6-di-O-hexadecyl-RD-glucopyranoside (4). 120 mg of 50% NaH in oil (2.5 mmol) was added to a solution of diol B (0.445 g, 1 mmol) in 40 mL of absolute DMF and the mixture magnetically stirred at 60 °C under argon for 30 min. 1.0 mL of 1-bromohexadecane (3.2 mmol) was added and the mixture heated and stirred for 2 h at 90 °C. After complete cooling, the solvent was evaporated under reduced pressure and the residue dissolved in CH2Cl2 (50 mL), washed with water (3 × 25 mL), dried over MgSO4, and finally chromatographed (CH2Cl2/EtOH, 97:3) to yield the bis-ether 4 (537 mg, 56%) as a white wax: mp 56-58 °C (iPrOH/n-hexane); [R]D ) +27.8 (c 2, CHCl3); 1H NMR δ 0.9 (m, 6H, 2 × CH3-C15H30), 1.25-1.4 (m, 52H, 2 × CH3-C13H26-CH2), 1.55-1.65 (m, 4H, 2 × CH3C13H26-CH2), 3.36-3.45 (m, 6H, H-2, OCH3, OCH2-C14H28CH3), 3.5-4.2 (m, 19H, H-3, -4, -5, -6, -6′, 3 × OC2H4, OCH2C14H28-CH3), 4.25-4.35 (m, 4H, OC2H4), 4.85 (s, 1H, J1-2 ) 3.6 Hz, H-1), 7.14 (bs, 2H, H-1, -4, naphth), 7.35 (dd, 2H, H-5, -8, naphth), 7.69 (dd, 2H, H-6, -7, naphth); 13C NMR 149.2 (C-2, -3, naphth), 129.4 (C-9, -10, naphth), 126.4, 124.3 (C-6, -7, naphth), 108.2 (C-1, -4, naphth), 98.0 (C-1), 82.45 (C-4), 80.6 (C-2), 77.75 (C-3), 73.2, (C-6), 73.1, 71.8, 71.4, 70.9, 70.7 (OCH2), 70.2 (C-5), 69.7, 69.5, 69.4, 69.3, 69.1 (OCH2, 55.0 (OCH3), 32.0, 30.6, 29.8, 29.7, 29.6, 29.5, 26.4, 26.3, 22.8 (CH3-C14H20-CH2O), 14.2 (CH3C14H20-CH2O). Anal. Calcd for C57H98O10: C, 72.57; H, 10.47. Found: C, 72.5; H, 10.4. EIMS+ (70 eV) calcd for C57H98O10. Found 943.7 [M + H]+ (16%). 2.2. Monolayer Experiments. Monolayer experiments were carried out with a KSV 5000 barostat (KSV, Helsinki). A Teflon trough (15 cm × 58 cm × 1 cm) with two hydrophilic Delrin barriers (symmetric compression) was used in all experiments. The system was equipped with an electrobalance and a platinum Wilhelmy plate (perimeter 39.24 mm) as a surface pressure sensor and a surface potential measuring head with a vibrating electrode. A platinum plate (4 cm diameter) immersed 4 mm below the water surface was used as a counter electrode. The apparatus was closed in a Plexiglas box, and temperature was kept constant at 18 °C. Before each use, the trough and the barriers were cleaned

Langmuir, Vol. 20, No. 15, 2004 6261 using cotton soaked in chloroform, gently brushed with ethanol and then with tap water, and finally rinsed with water purified by reverse osmosis (electrodeionized and osmosed water; Elix 3; Millipore, France). All solvents used for cleaning the trough and the barriers were of analytical grade. Aqueous subphases for monolayer experiments were prepared with water purified by reverse osmosis, which had a surface tension of 72.75 mN/m at 20 °C. For all compounds studied, the compression isotherm experiments were performed using 1 mM HClO4 subphases,48,49 pH 3.0, or 1 mM HClO4/3.25 × 10-2 mM L- or D-valine, alanine, tryptophane, and phenyglycine solutions, pH 3.0. All products used to prepare the subphase solutions were of analytical grade. Any residual surface-active impurities were removed before each experiment by sweeping and suction of the surface. Monolayers were spread, using calibrated solutions of crown ethers of a concentration of about 1 mg/mL, prepared with spectrophotometric grade chloroform (Aldrich, A.C.S.). The stability of the surface potential (∆V) signal was checked before each experiment, after cleaning the subphase surface. After the ∆V signal had reached the constant value, it was zeroed, and the film was spread on the subphase. After the equilibration time of 20 min, the films were compressed at the rate 2.5 mm/min (4.2-5.8 Å2 molecule-1 min-1). A PC computer and KSV software were used to control the experiments. Each compression isotherm was performed at least three times. The standard error was (0.5 Å2 with mean molecular area measurements and (2.5 mV with surface potential measurements, respectively. 2.3. BAM Experiments. The experimental setup used consisted of a computer-interfaced Langmuir balance combined with a Brewster angle microscope (BAM 2; NFT Go¨ttingen, Germany). The Langmuir balance and microscope were sheltered in a Plexiglas cabinet to avoid film perturbation and contamination. The surface pressure-area isotherms were performed using filter paper as a Wilhelmy plate. After evaporation of the spreading solvent, the monolayer was continuously compressed at the rate 12 Å2/(molecule min). The microscope was sensitive to changes of the refractive index resulting from differences in thickness, density, and molecular orientation. An analyzer in the reflected beam path detected optical anisotropy caused by different molecular orientations in the monolayer. The reflected light passed through a lens to a charge coupled device (CCD) camera, and the resulting signal was fed to a video system. The BAM images were treated with image processing software from Compic (Germany) to correct the distortion resulting from the observation at the Brewster angle.

3. Results and Discussion 3.1. Properties of the Langmuir Films Formed with the Crown Ethers. Structures of the chiral, optically pure crown ethers used in this study are shown in Figure 2. These amphiphilic molecules form stable Langmuir films at the air/water interface. The behavior of crown ethers in the films was studied using surface pressure-molecular area (Π-A) isotherms and surface potential-molecular area (∆V-A) isotherms and, in some cases, Brewster angle microscopy. The four crown ethers differ in their interfacial behavior, as indicated by the characteristic parameters of the compression isotherms of the films spread on the acidified water subphase (Figures 4-7 and Table 1). The water subphase used in all experiments was acidified to pH 3.0 with HClO4. This pH was chosen to allow protonating the amino groups (pKa above 9) of the amino acids48,49 used in the complexation experiments. As indicated by the Πcoll values of the Π-A compression isotherms, the films formed with compounds 3 and 4, bearing alkyl chains, are more stable than the films formed with compounds 1 and 2, bearing aromatic moieties. While the Πcoll values of the films formed with compounds 1 and 2 are very close, the (48) Shinbo, T.; Yamaguchi, T.; Nishimura, K.; Sugiura, M. J. Chromatogr. 1987, 405, 145. (49) Peacock, S. C.; Cram, D. J. J. Chem. Soc., Chem. Commun. 1976, 282.

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Figure 2. Molecular structures of the crown ethers studied.

Figure 3. Morphology of the films formed with the crown ethers 1 (a-c) and 3 (a′-c′) on the water subphase, pH 3.0. Brewster angle microscopy images were taken at the following surface pressures: (a, a′) around 0.5 mN/m; (b, b′) around 10 mN/m; (c, c′) at the collapse of the two films, that is, at 16.6 and at 26.5 mN/m for crown ethers 1 and 3, respectively. The bright horizontal strip seen in part c′ may be due to the formation of a giant fold.53 Scale: the length of the images is 560 µm; that is, 1 cm equals 109.4 µm.

film formed with compound 4 collapses at a higher surface pressure than the one formed with derivative 3. The lower values of the molecular area at the collapse point for derivative 2 compared to derivative 1 and for derivative 4 compared to derivative 3 indicate that the hydrophobic tailgroups in molecules 2 and 4 are oriented more perpendicularly to the water surface, compared to those in 1 and 2, respectively. Taken together, these observations show that the intermolecular interactions between the hydrophobic aliphatic moieties are stronger compared to those between the aromatic moieties and, as expected, stronger with increasing chain length. The proposed

orientation of molecules 1-4 at the air/water interface is presented in Chart 1. The surface potential (∆V) of a monolayer spread at the air/water interface can be interpreted using the Helmholtz equation: ∆V ) µ⊥/0A, where µ⊥ is the effective molecular dipole moment at the interface, 0 is the vacuum dielectric permittivity, and A is the area per molecule. According to the Vogel-Mo¨bius two-capacitor model,50,51 the effective molecular dipole moment at the interface is composed of two contributions: µ⊥ ) µR + µw, where µw represents the (50) Vogel, V.; Mobius, D. Thin Solid Films 1988, 159, 73. (51) Vogel, V.; Mobius, D. J. Colloid Interface Sci. 1988, 126, 408.

Amphiphilic Crown Ethers in Langmuir Monolayers

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Figure 4. Π-A and ∆V-A isotherms (lower and upper sets of curves, respectively) of the films formed with crown ethers 1-4 on (s) a 1 mM HClO4 solution, (‚‚‚) an L-valine solution, and (- ‚ -) a D-valine solution. All subphases were acidified to pH 3.0.

Figure 5. Π-A and ∆V-A isotherms (lower and upper sets of curves, respectively) of the films formed with crown ethers 1-4 on (s) a 1 mM HClO4 solution, (‚‚‚) an L-alanine solution, and (- ‚ -) a D-alanine solution. All subphases were acidified to pH 3.0.

contribution of the tailgroup and µR represents the contribution of the headgroup, including all effects arising from hydration of the headgroup, reorientation of the water molecules near the monolayer interface, and the diffuse double layer. In the present study, the surface potential values obtained at the collapse of the film are compared.

It can be assumed that the ∆V values obtained with different crown ethers result from the variations of the tailgroup contribution, µw, with the polar headgroup contribution, µR, remaining constant with all four crown ethers. The values of µ⊥ (Table 1) increase in the order 1 > 2 > 3, which confirms the interpretations based on the

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Figure 6. Π-A and ∆V-A isotherms (lower and upper sets of curves, respectively) of the films formed with crown ethers 1-4 on (s) a 1 mM HClO4 solution, (‚‚‚) an L-tryptophane solution, and (- ‚ -) a D-tryptophane solution. All subphases were acidified to pH 3.0.

Figure 7. Π-A and ∆V-A isotherms (lower and upper sets of curves, respectively) of the films formed with crown ethers 1-4 on (s) a 1 mM HClO4 solution, (‚‚‚) an L-phenylglycine solution, and (- ‚ -) a D-phenylglycine solution. All subphases were acidified to pH 3.0.

Π-A compression isotherm results. Indeed, the increasing µ⊥ indicates a more vertical orientation of the tailgroups. The lower than expected µ⊥ value obtained with the crown ether 4 is likely due to trace impurities. It has to be noticed,

however, that the purity of the crown ether 4 is higher than 99%, as checked chromatographically. Brewster angle microscopy images of the films formed with crown ether 1, bearing benzyl groups, and with its


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Table 1. Characteristic Parameters of Compression Isotherms valine crown ether 1

crown ether 2

crown ether 3

crown ether 4

Acoll (Å2) A0 (Å2) A10 (Å2) Πcoll (mN/m) Cs-1 ∆V (V) m⊥ (D) Acoll (Å2) A0 (Å2) A10 (Å2) Πcoll (mN/m) Cs-1 ∆V (V) m⊥ (D) Acoll (Å2) A0 (Å2) A10 (Å2) Πcoll (mN/m) Cs-1 ∆V (V) m⊥ (D) Acoll (Å2) A0 (Å2) A10 (Å2) Πcoll (mN/m) Cs-1 ∆V (V) m⊥ (D)

alanine

tryptophane

phenylglycine

water

L

D

L

D

L

D

L

D

91.8 129.0 106.3 16.6 58.1 0.151 0.368 86.5 101.3 90.9 16.8 109.8 0.351 0.806 97.1 131.1 119.0 26.5 102.0 0.331 0.853 81.8 132.5 120.4 33.3 87.7 0.246 0.534

102.1 138.3 116.2 16.2 62.8 0.284 0.769 90.1 106.3 96.0 15.8 105.2 0.436 1.042 99.5 136.2 124.3 27.0 101.0 0.395 1.043 80.9 138.9 125.0 35.2 84.7 0.363 0.779

103.8 139.0 116.7 15.4 61.3 0.364 1.003 86.6 102.5 92.7 15.6 101.0 0.303 0.696 94.3 131.2 119.0 28.1 102.0 0.354 0.886 82.2 139.1 125.1 34.6 85.4 0.378 0.824

103.8 138.9 115.8 15.7 61.7 0.266 0.733 90.9 107.5 96.8 15.4 101.0 0.359 0.866 94.8 125.6 114.8 25.7 104.1 0.306 0.770 86.3 136.4 123.6 32.5 87.7 0.331 0.758

97.7 134.1 111.9 16.5 62.8 0.359 0.931 91.4 108.3 95.8 15.0 97.0 0.373 0.905 97.8 131.6 119.4 26.0 101.0 0.479 1.243 84.7 139.2 125.5 33.4 85.4 0.341 0.766

103.4 136.1 114.1 13.6 62.8 0.223 0.612 84.1 98.8 88.6 15.3 104.1 0.394 0.669 97.6 133.0 121.4 26.6 101.0 0.210 0.544 80.1 135.9 123.0 34.9 85.4 0.293 0.623

109.7 144.0 120.3 14.6 61.7 0.255 0.742 86.2 102.9 91.6 15.8 96.1 0.309 0.924 90.6 125.0 113.4 27.2 99.0 0.369 0.887 80.5 136.9 125.2 36.0 88.5 0.171 0.365

94.8 126.8 106.5 15.5 62.5 0.395 0.994 85.6 101.3 90.4 15.8 102.0 0.300 0.895 93.1 129.1 117.4 27.5 100.0 0.347 0.857 78.9 134.4 121.9 35.5 86.9 0.328 0.687

105.4 139.9 117.1 15.0 61.7 0.308 0.861 85.8 101.1 91.6 15.9 99.0 0.404 0.703 94.1 127.4 116.2 26.9 104.1 0.321 0.802 82.1 136.2 124.4 34.9 89.2 0.285 0.621

Chart 1. Orientation of Crown Ether Derivatives 1-4 at the Air/Water Interfacea

a

The conformations of the molecules were modeled using the molecular modeling program Chem 3D.

counterpart 3, bearing alkyl chains, show differences in the evolution of the film morphologies (Figure 3). Bright areas surrounding darker patches observed at the onset of the surface pressure upon compression in the case ofcompound 1 indicate a gas-liquid-expanded phase transition (Figure 3a). The bright zones are attributed to the denser liquid-expanded phase, compared to the darker gas phase.52 In the case of crown ether 3 a uniform monolayer was formed, with some rare aggregates appearing as concentric circles (Figure 3a′). At the surface pressure of approximately 10 mN/m (Figure 3b′) two phases indicating a liquid-expanded-liquid-condensed phase transition are clearly visible with compound 3, in contrast with the case of crown ether 1, where small domains can be seen (Figure 3b). The images taken at the collapse of both films (Figure 3c and c′) suggest that the mechanism of the collapse may be different in the two cases, namely formation of multilayer islands with film 1 and formation of giant folds with film 3.53 (52) Zaitsev, S. Y.; Baryshnikova, E. A.; Sergeeva, T. A.; Gromov, S. P.; Fedorova, O. A.; Yescheulova, O. V.; Alfimov, M. V.; Hacke, S.; Zeiss, W.; Mobius, D. Colloids Surf. 2000, 171, 283.

3.2. Recognition of the Amino Acids Dissolved in the Subphase by the Crown Ether Films. The monolayers of crown ethers 1-4 were spread on the water subphases, pH 3.0, containing very low concentrations (3.25 × 10-5 M) of the L- or D-enantiomers of valine, alanine, tryptophane, and phenylglycine. The adsorption of the amino acids to the monolayer and the subsequent formation of the crown ether-amino acid complexes may modify the properties of the monolayers, compared to the situation when pure water is used as subphase. Moreover, the chiral recognition of amino acids by the optically pure crown ethers may take place, due to the formation of diastereomeric complexes. The changes in the film properties can be evaluated by comparing the characteristic parameters of the surface pressure and surface potential compression isotherms. While all the characteristic parameters of the monolayers are given in Table 1, the following discussion is based essentially on the changes of the molecular area values measured at the arbitrarily chosen surface pressure 10 mN/m and on the changes of the surface potential values (53) Ybert, C.; Lu, W.; Moeller, G.; Knobler, C. M. J. Phys. Chem. B 2002, 106, 2004.

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at the collapse of the films. We consider that the differentiation between two enantiomers of an amino acid or between pure water and an enantiomeric amino acid can be evidenced when the shift between the corresponding Π-A isotherms is equal to or higher than 3 Å2. Valine. The surface pressure Π-A and surface potential ∆V-A isotherms obtained with crown ether 1-4 monolayers on the subphases containing the L- or D-valine are shown in Figure 4. As can be observed with all four crown ethers, the Π-A isotherms of the films spread on water differ either from one or from both isotherms corresponding to the films spread on the subphases containing the valine enantiomers. The difference of the molecular area between the Π-A isotherms of crown ether 1 corresponding to the pure water and the Π-A isotherms corresponding to the valine enantiomers, measured at the surface pressure 10 mN/ m, is around 10 Å2, indicating formation of complexes between the crown ether and valine. A similar situation is observed with crown ether 4, but the difference between the corresponding molecular areas is around 5 Å2. The Π-A isotherms of crown ethers 1 and 4 corresponding to the subphases containing the L- or D-valine are superposable, which indicates that no chiral discrimination takes place in these systems. However, in the case of crown ethers 2 and 3, the isotherms corresponding to the L- or D-valine are shifted, respectively, by 3.5 and 5.4 Å2 at Π ) 10 mN/m, indicating that the diastereomeric interactions make the differentiation of the two enantiomers possible. While no difference is observed with crown ethers 2 and 3 between pure water and the subphase containing D-valine, the Π-A isotherms of the L-valine are shifted to higher areas. The profiles of the ∆V-A isotherms and the ∆V values (Table 1) obtained with the four crown ethers confirm the tendency in molecular recognition observed with the Π-A isotherms. However, the significant difference of µ⊥ values obtained with the valine enantiomers in the case of the film formed with crown ether 1 indicates that the enantiorecognition which was not detected using surface pressure measurements can be evidenced using surface potential measurements. The values of µ⊥ increase for the films formed on the subphases containing the valine enantiomers, compared to the case of pure water. The only exception to this tendency is the film formed with crown ether 2 on the subphase containing D-valine. The most significant increase of the µ⊥ values with subphases containing D- or L-valine is observed with crown ether 1, indicating a more perpendicular orientation of the tailgroup relative to the water surface. This effect may be explained by an easier formation of complexes in the case of the crown ethers with a higher conformational flexibility or by the fact that the formation of the complexes has a more pronounced impact on the tailgroup organization in the films of a higher compressibility, that is, having lower compressibility modulus (Cs-1) values (see Table 1). Alanine. Contrary to the case of valine, the alanine enantiomers are differentiated by crown ether 1 but not by 2, as judged by the Π-A isotherms (Figure 5). Crown ether 3 discriminates between the alanine enantiomers similarly to the case for valine, but the isotherm corresponding to the L enantiomer is shifted to lower molecular areas, compared to those for pure water. The films formed with crown ether 1 on the water and D- and L-alanine subphases have distinct properties, as shown by the shift of the corresponding Π-A isotherms and by their characteristic parameters (Table 1). The ∆V-A isotherms obtained with alanine confirm the molecular recognition observations based on the Π-A

Badis et al.

isotherms. Indeed, the profiles of the ∆V-A isotherms and the ∆V values (Table 1) indicate the same tendency that was observed with the surface pressure measurements. The increase of the µ⊥ values is observed in all cases, except for the film formed withcrown ether 3. As was the case with valine, the highest impact on the film organization is observed in the case of crown ether 1. Tryptophane. The Π-A isotherms of the films formed with crown ethers 1-4 on the subphases containing Land D-tryptophane (Figure 6) show that the chiral recognition is most pronounced for crown ethers 1 and 3, as the shift of the Π-A isotherms measured at Π ) 10 mN/m for the subphases containing the enantiomers is, respectively, 6.2 and 8.0 Å2. In the case of crown ether 1, both enantiomers are distinguished from water. For crown ether 3, the Π-A isotherm corresponding to the Lenantiomer is almost superposable with that corresponding to pure water, while the isotherm corresponding to the D-enantiomer is shifted to lower molecular areas. The results obtained for the tryptophane with the ∆V-A isotherms confirm the observations based on the Π-A isotherms. However, in the case of crown ethers 2 and 4 the differentiation between the enantiomers, as well as the differentiation of the enantiomers from water, is clearly observed, as indicated both by the profiles of the ∆V-A isotherms and by the ∆V values (Table 1). Phenylglycine. The Π-A compression isotherms of the films formed with crown ethers 1-4 (Figure 7) on the subphases containing L- and D-phenylglycine show that a well pronounced chiral recognition is achieved with crown ether 1, which discriminates between the Denantiomer and water but not the L-enantiomer and water. The shift between the isotherms corresponding to the enantiomers measured at Π ) 10 mN/m is 10.6 Å2. An opposite situation is observed with crown ether 4, which differentiates between the two enantiomers and the water subphase but does not differentiate in any significant way between the two enantiomers. In the case of crown ethers 2 and 3, neither chiral recognition nor the differentiation between the enantiomers and the water subphase is observed. The results obtained with the ∆V-A isotherms in the case of phenylglycine indicate clearly that both enantiomers interact with the films formed with crown ethers 1 and 2, bearing aromatic tailgroups, and that they are differentiated by these compounds. The results obtained with the crown ethers bearing aliphatic tailgroups do not indicate chiral recognition, as was the case with the surface pressure measurements. 4. Conclusion Chiral discrimination of enantiomeric amino acids, namely, alanine, valine, tryptophane, and phenylglycine, by amphiphilic crown ethers was demonstrated using a Langmuir technique. The most efficient chiral selectors, based on the Π-A isotherm results, are crown ethers 1 and 3, which recognize the enantiomers of three of the four amino acids used in this study. Following in order of selectivity is crown ether 2, discriminating between the enantiomers of two amino acids. No chiral discrimination could be observed with crown ether 4. The ∆V-A measurements show that the results obtained with the aliphatic amino acids and the tryptophane differ from those obtained with the phenylglycine. Indeed, in the case of valine and alanine, the same tendency for molecular differentiation is observed with the four crown ethers when surface potential and surface pressure isotherms are compared. With tryptophane, the ∆V-A and Π-A results


converge clearly in the case of the short chain crown ethers 1 and 3. However, the differentiation of enantiomers, which is not observed with crown ethers 2 and 4 using Π-A isotherms, is clearly observed using ∆V-A measurements. In the case of phenylglycine, the ∆V-A measurements indicate interactions with the crown ethers bearing the aromatic but not the aliphatic moieties. We propose that the latter effect is due to the proximity of the phenylalanine and the crown ether aromatic rings in the complexes formed. No clear-cut relation could be established between the structure of the other amino acids and the crown ethers, aliphatic versus aromatic, and their interactions. The fact that the effects observed are more pronounced with crown ethers 1 and 3 may indicate that the formation of complexes is easier with the less rigid films, formed with molecules having a higher conformational flexibility. This interpretation is supported by X-ray

Langmuir, Vol. 20, No. 15, 2004 6267

and NMR results linking the complexation efficiency to the crown ether conformational liberty.54,55 The method described in this paper will be used for screening of the most promising chiral selectors for preparative separations. Acknowledgment. Discussions with Dr. Gerald Brezesinski, Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, Golm/Potsdam, Germany, and his help with BAM experiments are greatly appreciated. We thank Jeff Rice and Dr. C. Kowal for revising the English in the manuscript. LA049596K (54) Courtois, A.; El Masdouri, L.; Ge´hin, D.; Gross, B. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1986, C42, 850. (55) Sasaki, D. Y.; Waggoner, T. A.; Last, J. A.; Alam, T. M. Langmuir 2002, 18, 3714.

Enantiomeric Recognition of Amino Acids by Amphiphilic Crown

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