Comparison of Enantiomeric and Racemic Monolayers of 2

Jun 1, 1995 - ... de Miguel, Marta Pérez-Morales, María T. Martín-Romero, Eulogia Muñoz, Tim H. .... Frank Hoffmann, Heinrich Hühnerfuss, and Kei...
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Langmuir 1995,11, 2206-2212

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Comparison of Enantiomeric and Racemic Monolayers of 2-Hydroxyhexadecanoic Acid by External Infrared Reflection-Absorption Spectroscopy Volker Neumann, Arne Gericke,? and Heinrich Huhnerfuss” Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 0-20146Hamburg, Germany Received February 7,1995. I n Final Form: March 27, 1 9 9 P Surface pressure/area (TVA) isotherms and infrared reflection-absorption spectra (IRRAS) of racemic and enantiomeric 2-hydroxyhexadecanoicacid (HHDA)Langmuir films showed that the presence ofbivalent cations in the aqueous subphase gives rise to considerable compression (Ca2+> Pb2+ Zn2+)and to increased chiral discrimination (Ca2+ Pb2+ Zn2+)in HHDA monolayers. Although both in the presence ofPb2+and Zn2+cations the TVA isotherms ofthe L-enantiomerexhibit the more condensed characteristics, the IRRAS measurements show that Pb2+cations induce, against expectation,heterochiral discrimination in compressed HHDA films, while the presence of Zn2+ cations leads to preferred homochiral interactions. In the latter two instances, the conformational order of the alkyl chain is nearly independent of the compressional status of the monolayer; i.e., the optimum order induced by the presence of Pb2+and Zn2+ cations is already attained at very low surface pressures around 1 mN m-l. The different influence of bivalent cations on chiral recognition is assumed to be causedby different complexes formed by the respective cation and the functional group of the film-formingsubstances. Hence, in addition to van der Waals and electrostaticinteractions as well as hydrogen-bondformation, potential complex formationbetween bivalent cations and the head group appears t o play an important role for heterochiral and homochiral discrimination in chiral Langmuir films.

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Introduction Self-assembling mechanisms as well as functionality of supramolecular systems are known to depend on the structure and the order of the respective mo1ecules.l In particular, many enzymatic reactions are closely related to membranes and their diastereomeric structure. Therefore, much effort has been devoted to gaining more insight into the order of chiral monolayers over the last two decades.2 Emphasis has been placed upon investigations of monolayers consisting of N-acyl amino acid amphiphiles3-14 or of 1,2-dihexadecanoyl-sn-glycero-3phosphocholine (often referred to as “dipalmitoylphosphatidylcholine” or DPPC),15-23 i.e., model substances that

* Author to whom all correspondence should be addressed.

’

Present address: Department of Chemistry, Rutgers University 73 Warren Street, Newark, N J 07102. Abstract published in Advance ACS Abstracts, J u n e 1, 1995. (1) Lehn, J. M. Angew. Chem. 1988, 100, 91, and literature cited therein. (2) Stewart, M.; Amett, E. M. In Topics in Stereochemistry;Eliel, E . L., Allinger, N. L., Eds.; Wiley: New York, 1982; Vol. 13, p 195. (3) Bouloussa, 0.;Dupeyrat, M. Biochim. Acta 1988, 938, 395. (4) Harvey, N. G.; Mirajovsky, D.; Rose, P. L.; Verbiar, R.; Amett, E. M. J.Am. Chem. SOC.1989,111, 1115. ( 5 ) Harvey, N. G.; Amett, E. M. Langmuir 1989, 5, 998. (6) Heath, J. G.; Amett, E. M. J . Am. Chem. SOC.1992,114,4500. (7) Harvey, N. G.; Rose, P. L.; Mijakovsky, D.; Amett, E. M. J . Am. Chem. SOC.1990, 112, 3547. (8)Stine, K. J.; Uang, J. Y.-J.; Dingman, S. D. Langmuir 1993, 9, 2112. (9) Stine, K. J.;Whitt, S. A.;Uang, J. Y.-J. Chem. Phys.Lipids 1994, @

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fulfill several important requirements for simulating membranes and interfacial processes a t membrane surfaces. For such surface-active compounds with a single chiral center, a preferential D:D or L:L interaction is denoted as “homochiral” behavior, while a preferential D:L interaction is called “heterochiral” behavior. Homochiral interactions are of particular interest, because they raise the possibility of phase separation into regions of the Land the D - e n a n t i o m e r ~ . ~ This , ~ ~so-called ~~~ “chiral discrimination” or “chiral symmetry breaking” 26 was observed within monolayers, but it may also occur in the form of m i c e l l e ~ , ’ ~where J~ circular dichroism spectra indicated the importance of organized amide-amide hydrogen-bonding. This latter aspect, i.e., the question of whether or not hydrogen-bond formation is crucial for the formation of chiral discrimination, is presently the subject of considerable scientific debate.4,9 Recently, McConnellZ3as well as Stine and co-workers8-l0showed that fluorescence microscopy is well-suited for the study of chiral discrimination in Langmuir monolayers, if chiral symmetry breaking is manifested in the shapes of micron-sized domains of the ordered phase curving in either direction or showing dendritic morphologies. However,the latter authors noted that fluorescence observations are confined to macroscopic scales and can determine neither molecular characteristics like conformational order of the hydrophobic alkyl chains nor the structure and the hydration of the head group of the film-

69 --, A1 *-. (lO)Parazak,D.P.;Uang,J.Y.-J.;Turner,B.;Stine,K. J.Langmuir

(19) Grainger, D. W.; Reichert, A,;Ringsdorf, H.; Salesse, C. FEBS 1994.10.3787. Lett. 1989, 252, 73. (11) Gericke, A.; Hiihnerfuss, H. Langmuir 1994,10, 3782. (20) Taneva, S.;Ariga, K.; Tagaki, W.; Okahata,Y.J . ColZoidInterface (12) Miyagashi, S.; Nishida, M. J . Colloid Interface Sci. 1978, 65, Sci. 1989, 131, 561. 380. (21) Mendelsohn, R. Proc. SPIE-Int. SOC.Opt. Eng. 1993, VoZ. 2089, (13) Shinitsky, M.; Haimovitz,R. J.Am. Chem.SOC.1993,115,12545. 41. (14) Eckardt,C. J.;Swanson,D.R.;Takacs,J.M.;Khan,M.A.;Gong, (22)Amett, E. M.; Gold, J. M. J . Am. Chem. SOC.1982, 104, 636. X.; Kim, J. H.; Wang, J.; Uphaus, R. A. Nature 1993, 362, 614. (23) McConnell, H. M. In Annual Reviews of Physical Chemistry; (15)Gutberlet, T.; Milde, K.; Bradaczek, H.; Haas, H.; Mohwald, H. Strauss,H. L.,Babcock, G. T., Leone, S. R., Eds.;AnnualReviews: Palo Chem. Phys. Lipids 1994, 69, 151. Alto, 1991; Vol. 42, p 171. (16) Suzuki, A.; Cadenhead, D. A.Chem. Phys. Lipids 1986,37,69. (24)Andelman,D. J . Am. Chem. SOC.1989, 111, 6536. (17) Moy, V. T.; Keller, D. J.;Gaub, H. E.; McConnell, H. M. J . Phys. (25) Andelman, D.; Orland, H. J. Am. Chem. SOC.1993,115,12322. Chem. 1986,90, 3198. (26) Selinger, J. V.; Wang, Z. G.; Brunisma, R. F.; Knobler, C. M. (18) Gaub, H. E.; McConnell, H. M. J . Phys. Chem. 1986,90,6830. Phys. Rev. Lett. 1993, 70, 1139.

0743-7463/95/2411-2206$09.00/00 1995 American Chemical Society

Monolayers of 2-Hydroxyhexadecanoic Acid

Langmuir, Vol. 11, No. 6, 1995 2207

forming compounds. These latter parameters are astance47-49comprise a group of unsaturated hydroxy- and sumed to be important for the macroscopic chiralityhydroxyketoalkanoic acids. In the leucocytes, prostagdependent domain structures (e.g.,direction of curvatures landins are being formed in the course of the so-called and rigidity of domains) as observed by fluorescence “arachidonic acid cascade”. Cytochrome P450 dependent microscopy, and therefore, a comprehensive study of chiral monooxygenases metabolize arachidonic acid to several effects in Langmuir monolayers has to include these products such as epoxy eicosatrienoic acids and hydroxy aspects. eicosatetraenoic acids (HETEs) including 16-, 17-, 18-, 19-, and ~ O - H E T E S .Furthermore, ~~ Yamane et al.50 A potential method well-suited for addressing issues presented evidence that rat brain exhibits a high NADPHrelated to the conformational order and organization of dependent o-hydroxylation activity and that this o-hymonolayers and biological membranes appears to be droxylation system may be the major metabolic pathway infrared (IR) s p e c t r o ~ c o p y . ~IR ~ ~spectroscopy ~ ~ ~ ~ ~ - ~ is ~ one of some polyunsaturated long-chain alkenoic acids in rat of the earliest spectroscopic techniques used for investibrain. Recently, Terech et al. showed that 12-hydroxygations of amphiphilic systems, and Fourier transform octadecanoic acid is of special interest because of the IR (FTIR) instrumentation has revitalized this experiexceptionally large variety of solvents which can be mental approach. This holds, in particular, since the gelled.51 In addition, 2(R)-hydroxyhexadecanoicacid was pioneering works by Dluhy and c o - w o r k e r ~Mendelsohn ,~~ isolated from marine algae (U. pertusa and Porphyra sp.) and c o - w ~ r k e r s , ~ and ~ , ~ Huhnerfuss ~-~~ and co-workby Kajiwara et al.,52who presented evidence that this e r ~ , ~ ~ who ~ ~were ~ -able ~ ~to ,acquire ~ ~ -spectra ~ ~ of compound may be formed enzymatically from hexademonolayers a t the airlwater interface by IR reflectioncanoic acid. absorption spectroscopy (IRRAS). The technique involves Previous investigations on hydroxyalkanoic acids monoa single external reflection from the film-covered airlwater layers by Kellner and Cadenhead were confined to interface under controlled conditions of surface tension. measurements of differences in the surface pressure (II) Thus detailed informations about the structure and the interactions of head groups such as carboxylic a c i d ~ , ~ ~ versus , ~ ~ area per molecule ( A ) isotherms and surface and a m i d e s l l ~ ~ ~ potential measurements of racemic films.53 The present carboxylic acid e ~ t e r s , 4 l -carboxylate^,^^ ~~ work is focused upon the dependence of the monolayer became accessible. Furthermore, Gericke and Huhnerfuss characteristics on the enantiomeric composition of this showed that this method is also well-suited for the class of compounds. Emphasis will be placed upon IRRAS investigation of enzymatic processes at membrane monomeasurements of the chirality-dependent conformational l a y e r ~ In . ~conclusion, ~ a powerful new method is available alkyl chain order of 2-hydroxyhexadecanoicacid and of which allows insight both into the conformational order the dependence of the molecular order on the presence of of the alkyl chain and the head group structure of the bivalent cations in the aqueous subphase including Ca2+, most important film-forming molecules. Pb2+,and Zn2+. Furthermore, the preferential occurrence In the present work, this IRRAS-methodwill be applied of homochiral and heterochiral interactions, respectively, to another group of surface-active compounds that has will be investigated as well as the potential formation of thus far largely been neglected in surface-chemical different complexes between 2-hydroxyhexadecanoic acid investigations, hydroxyalkanoic acid amphiphiles. Hyand the respective bivalent cation. droxyalkanoic acids are constituents of membranes.45In particular, some ceramides, as a part of sphingolipids, Experimental Section contain saturated as well as unsaturated D-2-hydroxy~,~-2-hydroxyhexadecanoic acid was puhhased from Lancaster and D-3-hydroxyalkanoicacids.46 Furthermore, prostag(Morecambe,GreatBritain). The enrichment ofthe L-enantiomer landins which are of considerable physiological imporwas accomplished by recrystallization of the (SI-(-)-(1-phenylethyllammonium salt of the acid from ethanol according to the (27)Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373. procedure described by Karlsson and Pa~cher.5~ The enantio(28) Gericke, A.; Huhnerfuss, H.; Michailov, A. V. Proc. SHE-Int. meric excess thus obtained was more than 96% as determined SOC. Opt. Eng. 1992, Vol. 1575, 554. by capillary gas chromatography of the methyl ester derivative (29) Gericke, A.; Huhnerfuss, H.; Michailov, A. V. Vib. Spectroscop. using a chiral stationary phase consisting of heptakis(2,6-di-O1993, 4, 335. methyl-3-0-n-penty1);O-cyclodextrin (for details, see refs 55(30) Gericke, A.; Simon-Kutscher,J.;Hiihnerfuss, H. Langmuir 1993, 9, 2119. (31)Gericke, A.; Simon-Kutscher,J.;Hiihnerfuss, H.Langmuir 1993, 9, 3115. (32) Gericke, A.; Hiihnerfuss, H. J. Phys. Chem. 1993, 97, 12899. (33)Gericke, A.; Hiihnerfuss, H. Proc. SPIE-Int.SOC. Opt. Eng. 1993, Vol. 2089, 570. (34) Flach, C.; Brauner, J. W.; Mendelsohn, R.App1. Spectrosc. 1993, 47, 982. (35) Pastrana-Rios, B.; Flach, C. R.; Brauner, J. W.; Mautone, A. J.; Mendelson, R. Biochemistry 1994, 33, 5121. (36) Flach, C. R.; Brauner, J. W.; Taylor, J. W.; Baldwin, R. C.; Mendelsohn, R. Biophys. J . 1994, 67, 402. (37) Flach, C. R.; Brauner, J. W.; Mendelsohn, R. Biophys. J . 1993, 65, 1994 (38) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlat, J. M. Appl. Spectrosc. 1993, 47, 869. (39) Blaudez, D.; Buffeteau, T.; C o n u t , J. C.; Desbat, B.; Escafre, N.; Opt. Eng. 1993, Vol. Pezolet, M.; Turlat, J. M. Proc. SPIE-Int. SOC. 2089, 414. (40) Gericke, A.; Huhnerfuss, H. Thin Solid Films 1994, 245, 74. (41) Gericke, A.; Hiihnerfuss, H. Ber. Bunsenges. Phys. Chem. 1995, 99, 641. (42) Gericke, A.; Huhnerfuss, H. Langmuir 1995, 11, 225. (43) Huhnerfuss, H.; Gericke, A,; Alpers, W.; Theis, R.; Wismann, V.; Lange, P. A. J. Geophys. Res. 1994, 99, 9835. (44) Gericke, A.; Huhnerfuss, H. Chem. Phys. Lipids 1994,74,205. L. Biochemie; Spektrum-der-wissenschaft(45) Stryer, Verlagsgesellschaft: Heidelberg, 1990, p 578. (46) Pascher, I. Biochim. Biophys. Acta 1976,455, 433.

57).

The spreading solvent chloroform of licrosolv grade (Merck, Darmstadt, Germany) was used as received. The purity of the solvent was checked by capillary gas chromatography using a flame-ionizationdetector. The water was deionized and purified (conductivity< 0.05~s) by a SeralpurPro 9OC apparatus (Seral, Ransbach, Germany). The water quality was checked by fluidfluid extraction with n-hexane followed by capillary gas chromatographic analysis. The pH values of the subphases were (47) Bartmann, W. Angew. Chem. 1975,87,143. (48) Noyori, R.; Suzuki, M. Angew. Chem. 1984, 96, 854. (49) Walter, W.Lehrbuch der Organischen Chemie, 22th ed.; S.Hirzel Verlag: Stuttgart, 1991, p 284, and literature cited therein. ( 5 0 ) Yamane, M.; Abe, A.; Nakajima, M. J.Chromatogr. B 1994,662, 91. (51)Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Langmuir 1994, 10, 3406. (52)Ka~iwara,T.; Kashibe, M.; Matsui, K.; Hatanaka, A. Phytochemistry 1991, 30, 193. (53) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452. (54) Karlsson, K.-A,; Pascher, I. Chem. Phys. Lipids 1974, 12, 65. (55) Konig, W. A. Nachr. Chem. Tech. Lab. 1989,37,471. (56) Schurig,V.;Nowotny,H.-P.Angew. Chem. 1990,102,969;Angew. Chem. Int. Ed. Engl. 1990,29, 939. (57) Huhnerfuss, H. GIT Fachz. Lab. 1992,36,489.

Neumann et al.

2208 Langmuir, Vol. 11, No. 6, 1995 adjusted by adding the appropriatevolumes of 1M HC1solutions (analytical-reagentgrade; Merck, Darmstadt,Germany)to the pure water (pH-meter 761, Knick, Berlin, Germany). The pH value of the pure water was in the range of 5.7-6.0 and is referred to as “pH6”throughout the present paper. PbClz (299%)(Aldrich, Steinheim,Germany),ZnCl2 (?99%), and CaClz (99%) (Merck, Darmstadt, Germany) were used as received. The surfacepressure/area(WA)isothermswere recorded with the help of a Lauda FW-2-Langmuir trough (Lauda, Germany) that was temperature controlled in the limits of &O.l K. The compression velocity was about 0.05 nm2 molecule-1 min-l. The externalreflection-absorptionspectroscopywas performed on a Bruker IFS 66 (Karlsmhe,Germany)spectrometerequipped with a MCT detector and using a modified external reflection attachment of SPECAC (Orpington, Great Britain) which included a miniaturizedLangmuir-troughto permit thermostatic measurements and an appropriate match for water vapor compensation by carefully controlling the humidity in the sample chamber of the spectrometer. The angle of incidence of the IR beam was set to 30°, and unpolarized radiation was used. Furthermore,the Blackman-Harris apodization function with a resolution of 8 cm-I and a zero filling factor of 2 was used, and the spectra were taken by coadding 1500 scans (corresponding with a data collection time of about 3 min). The reflectionabsorption is defined as -log(R/R,), where R and R, are the reflectivities of the film-covered and the pure water surfaces, respectively. For an extensive description of the method the reader should refer to refs 29-32. The method and the evaluation of the data is based upon the theory of IRRAS for low-absorbing substrate^.^^ The peak positions were determinedby the “center of gravity except for the peak positions of the methylene scissoringvibrationsand the carbonyl and carboxylate stretching vibrations. In the latter instance, strong band overlappings were encountered.

Results and Discussion The sensitivity of surface pressure/area (WA)isotherms to the specificstereochemistry of film-formingamphiphiles has been demonstrated by several authors. In particular, a chiral discrimination can be found by comparing isotherms of pure enantiomeric compounds with their enantiomer m i x t u r e ~ . ~ ~Thus ~ ~ ~far, - ~it~ isJ ~generally assumed that the enantiomeric monolayer will exhibit a more condensed isotherm than the racemic one, if the D:D or L:L interactions are preferable over the D:L interactions (“homochiral discrimination”); however, if the opposite is true, then the enantiomeric monolayer is expected to exhibit a more expanded isotherm than the racemic monolayer (“heterochiral dis~rimination”).~ In this work, W A isotherms will be compared with IR reflection absorption spectra, in order to investigate whether or not this assumption is also valid for hydroxyalkanoic acid monolayers on a subphase containing bivalent cations. 1. Surface PressureIArea Isotherms. For comparison, the WAisotherm of hexadecanoic acid on a pure water subphase (pH 2; 294 K) is shown in Figure 1. While the isotherm of this saturated unsubstituted long-chain carboxylic acid exhibits the typical characteristics of a condensed monolayer, the isotherm of the 2-hydroxy derivative (Figure 1) reflects a n expanded monolayer: upon compression, after the gas (G)Aiquid-expanded(LE) coexistenceregime (TI 0) a LE phase is attained, followed by a LEAiquid-condensed(LC)coexistenceregime (plateau regime), and finally the LC phase is reached. Basically, the isotherms of the L-enantiomer and the racemate (DL) show the same characteristics, where the curve of the L-enantiomer is slightly shifted to smaller areas/molecule and the collapse pressure of the L-enantiomer is lower. Variation of the subphase temperature between 288 and

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( 5 8 ) Cameron, D. G.; Kauppinen, J. K.; Moffatt, D. J.; Douglas, J.; Mantsch, H. H. Appl. Spectrosc. 1982, 36, 245.

mN/m

1

nm2holecule

Figure 1. nlA isotherms of DL-2-hydroxyhexadecanoicacid (DL), L-2-hydroxyhexadecanoic acid (L),and hexadecanoic acid (dashed curve); compression rate, 0.05 nm2molecule-I min-l; subphase temperature, 294 K, spreading solvent, CHC13; subphase, water, pH 2.

308 K (not shown) did not give rise to any significant changes of the isotherm characteristics of the L- and the DL-2-hydroxyhexadecanoicacid Le., no clear tendency toward homochiral or heterochiral discrimination can be inferred from the WA isotherms determined on a pure water subphase. It is well-known that the presence of bivalent cations in the subphase leads to a cation-dependent condensation of saturated unsubstituted alkanoic acid monolayer^.^^ With regard to the chiral2-hydroxy derivatives investigated herein, a more sophisticated situation is encountered. In the presence of Ca2+in the subphase (Figure 2a), the condensation effect induced by the cation is very pronounced and comparable both for the enantiomeric (L) and the racemic (DL) 2-hydroxyhexadecanoic acid monolayer, while in the presence of subphases containing Pb2+ and Zn2+,respectively, different characteristics of the nlA isotherms are found for the enantiomeric and the racemic monolayers (Figure 2b,c): a t pH 6 the WA isotherms of the enantiomeric 2-hydroxyhexadecanoic acid are more condensed and shifted to lower areas/molecule than the curves of the racemic monolayers for both a subphase containing Pb2+(Figure 2b) and Zn2+(Figure 2c). I t is interesting to note that in contrast to the present results, octadecanoic monolayers were condensed significantly more by Pb2+ions than by Ca2+ions.40 The significantly more condensed WA isotherm of the enantiomeric 2-hydroxyhexadecanoicacid monolayer, with respect to the isotherm of the racemic monolayer both on a n aqueous PbClz and ZnClz subphase, raises the question whether this effect reflects preferential homochiral interactions as usually assumed. This question will be further investigated below using IR reflection-absorption spectroscopy. 2. IR Reflection-Absorption Spectroscopy. As already stressed in the introduction, IR reflection-absorption spectroscopy (IRRAS) allows deeper insight into the characteristics of enantiomeric and racemic monolayers on a molecular scale including the conformational order of the alkyl chain as well as hydration and complex formation of the carboxylic head group.32,40-42 The infrared reflection-absorption spectra in the wavenumber ranges between 3000 and 2600 cm-l and between 2000 and 1400 cm-’, as determined on a pure water subphase in the LE/LC coexistence regime, are shown for the racemic and the enantiomeric 2-hydroxyhexadecanoic acid monolayers in parts a and b, respectively, of Figure 3. The bands a t 2920 and 2852 cm-l represent the antisymmetric and symmetric methylene stretching vi-

Monolayers of 2-HydroxyhexadecanoicAcid

Langmuir, Vol. 11, No. 6, 1995 2209

niS/m

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h

b)

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Figure 3. IR reflection-absorptionspectra for a monolayer at the aidwater interface in the wavenumber ranges 3000-2600 and 2000- 1400 cm-l; subphase temperature, 294 K, spreading solvent, CHC13; subphase, water, pH 2. (a) DL-2-Hydroxyhexadecanoic acid, 0.270 nm2 molecule-l; (b) L-2-hydroxyhexadecanoic acid, 0.259 nm2 molecule-'.

I

00

0.I

0.3

0.3

0.5

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C)

2924 -

c

29221 L

b D

-

2

Y

O I0

0.1

0.2 nmVmoleculr 0.3

0.J

5

Figure 2. n/A isotherms of DL-2-hydroxyhexadecanoicacid (DL)and L-2-hydroxyhexadecanoicacid (L); compression rate, 0.05 nm2 molecule-l min-l; subphase temperature, 294 K spreading solvent, CHCls: (a)subphase, aqueous solution of 2 mmol CaCl2 L-l, pH 3.5; (b) subphase, aqueous solution of 1 mmol PbClz L-l, pH 6; (c) subphase, 1mmol ZnCl2 L-l, pH 6.

brations, respectively, while the bands around 1735- 1739 and 1720 cm-l are due to the stretching vibrations of a n unprotonated and monoprotonated fatty acid carbonyl group, r e ~ p e c t i v e l y .The ~ ~ band around 1465 cm-l indicates the methylene scissoring mode 6(CH2). A detailed analysis of the wavenumbers of the antisymmetric methylene stretching vibration (va(CH2))with respect to the available aredmolecule supplies insight into the conformational order of the alkyl chains of L- and DL-2-hydroxyhexadecanoicacid monolayers for different compression states (Figure 4). It is well-documented that the wavenumbers of the va(CH2)vibration are conformation-sensitive and that they can be empirically correlated

taFfB:Oo

2918:

29168

0

' P

E 2920-

1

I

I

,

I

I

I

,

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5

,

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Figure 4. Wavenumbers of the antisymmetric methylene stretching vibration vs aredmolecule for DL-2-hydroxyhexadecanoic acid (0)and L-2-hydroxyhexadecanoicacid ( 0 ) ;subphase temperature,294 K spreadingsolvent,CHCls; subphase, pure water, pH 6.

with the order (i.e., with the translgauche ratio) of the hydrocarbon chain as follows (although also the subcell structure may influence the peak position59): Lower wavenumbers are characteristic of highly ordered conformations with preferential all-trans characteristics, while the number of gauche conformers increases with increasing wavenumbers and width of the band. The increased wavenumber of the methylene stretching vibration for a gauche rotamer is caused by a coupling between the carbon atoms and the methylene hydrogen, which due to interconversion around the C-C bond is positioned in the plane defined by the carbon atoms, resulting in a n increased force constant for that C-H bond.41p59-61In contrast, for an all-trans conformation all (59) Mac Phail, R. A,; Strauss, H. L.; Snyder,R. G.; Elliger, C. A. J. Phys. Chem. 1984,88,334.

Neumann et al.

2210 Langmuir, Vol. 11, No. 6, 1995

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I

3000

b)

)

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I

I

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2000 IS00 1600 wavenumber [cm-'1

1400

1200

1000

0.15

0.20

0.30 0.35 area/molecule Inmzl

0.25

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Figure 6. Wavenumbers of the antisymmetric methylene stretching vibration vs aredmolecule for DL-2-hydroxyhexadecanoic acid (0)and L-2-hydroxyhexadecanoicacid ( 0 ) ;subphase temperature,294 K spreadingsolvent,CHCl3;subphase, aqueous solution of 1mmol PbCl2 L-l, pH 6.

3a,b), are absent. Instead, the antisymmetric carboxylate stretching vibrations in the region 1546- 1562 cm-I are observed. The detailed analysis of the wavenumbers of the antisymmetric methylene stretching vibration (va(CH2)) 2 with respect to the available aredmolecule (Figure 6) Y reveals two notable results: firstly, the conformational - I 4 order of the alkyl chain is nearly independent of the 3000 2800 2000 I800 1600 1400 I200 IO00 compressional status of the monolayer; i.e., the optimum wavenumher Icm-ll order induced by the presence of Pb2+cations is already Figure 5. IR reflection-absorption spectra for a monolayer at attained at very low surface pressures around 1mN m-l, the airlwater interface in the wavenumber ranges 3000-2600 presumably as a result of the formation of highly and 2000- 1000 cm-'; subphase temperature, 294 K spreading condensed domains. The same conclusions were drawn solvent, CHC13; subphase, aqueous solution of 1mmol PbCl2 L-l, pH 6. (a) DL-2-Hydroxyhexadecanoic acid, 0.205 nm2 by Gericke and H u h n e r f u ~ sfor ~ ~octadecanoic acid monomolecule-l; (b) L-2-hydroxyhexadecanoicacid, 0.216 nm2 layers on an aqueous subphase containing Pb2+ ions. molecule-'. Secondly, if it is tentatively assumed that the preferential interaction is closely related to a higher conformational methylene hydrogens are out of the plane. As a conseorder, the results summarized in Figure 6 imply highly quence, the results summarized in Figure 4 imply that significant heterochiral interactions of the system Pb2+/ compression of both L- and DL-2-hydroxyhexadecanoicacid 2-hydroxyhexadecanoic acid. This result is insofar nomonolayers gives rise to increasing conformational alkyl table, as it contrasts with the usual assumption that the chain order, i.e., a decrease in gauche conformers. The more condensed ITA isotherm of the L-enantiomer monomain transition from higher to lower wavenumbers is layer (Figure 2b) reflects homochiral interactions. observed in the LELC transition regime, while in the LC The IR reflection-absorption spectra determined in the regime the order is only slightly increased. Within the presence of an aqueous subphase containing Zn2+ ions error of the method, no significant differences between are shown in Figure 7a for a DL-2-hydroxyhexadecanoic the racemic and the enantiomeric monolayer can be acid monolayer and in Figure 7b for the monolayer of the inferred from these wavenumber values determined for L-enantiomer (experimental conditions in both cases are a pure water subphase, i.e., no preferential homo- or 1mmol ZnClz L-I, pH 6, 0.235-0.236 nm2/molecule, 294 heterochiral discrimination is evident. K). The wavenumbers of the antisymmetric and the The IR reflection-absorption spectra determined in the symmetric methylene stretching vibrations, 2916-2918 presence of a n aqueous subphase containing Pb2+ ions and ca. 2850 cm-l, respectively, are also lower than those are shown in Figure 5a for a DL-2-hydroxyhexadecanoic obtained for a pure water subphase, as already discussed acid monolayer and in Figure 5b for the monolayer of the for a Pb2+-containingaqueous subphase. However, the L-enantiomer (experimental conditions in both cases are detailed analysis of the wavenumbers of the antisymmetric 1mmol PbC12 L-l, pH 6,294 K, L-enantiomer 0.216 nm2 methylene stretching vibration (va(CH2))with respect to molecule-l, racemate 0.205 nm2molecule-I). In this case, the available aredmolecule (Figure 8)reveals the notable the wavenumbers of the antisymmetric and the symmetric phenomenon that in the presence of Zn2+ the L-2methylene stretching vibrations are lower than those hydroxyhexadecanoic acid monolayer exhibits a higher obtained for a pure water subphase, Le., around 2917conformational alkyl chain order than the racemic mono2918 and 2849-2850 cm-l, respectively. These values layer. This implies that a n aqueous subphase containing are consistent with those reported by Gericke and HuhZn2+cations gives rise to preferential homochiral interacnerfuss40 for a compressed octadecanoic acid monolayer tions in compressed 2-hydroxyhexadecanoic acid monospread a t a n aqueous subphase containing 1mM PbC12. layers, while Pb2+ions largely induce heterochiral interFurthermore, the bands representing the stretching actions. Furthermore, it is worth mentioning that the vibrations of a n unprotonated and monoprotonated fatty optimum conformational order induced by Zn2+cations is acid carbonyl group, which for a pure water subphase a t also already encountered a t low surface pressures, as pH 2 was found between 1720 and 1739 cm-l (see Figure already discussed for Pb2+-containingaqueous subphases. An explanation for the condensation effect of bivalent (60) McKean, D. C.; Biedermann, S.; Burger, H. Spectrochim. Acta cations as well as the different influence of Pb2+and Zn2+ Part A 1974,33, 845. on chiral discrimination processes a t higher surface (61) Snyder,R.G.;Aljibury,A.L.;Strauss,H.L.;Casal,H.L.;Gough, K. M.; Murphy, W. F. J. Chem. Phys. 1984,81,5352. pressures can be inferred from considerations about the

Langmuir, Vol.11, No.6, 1995 2211

Monolayers of 2-HydroxyhexadecanoicAcid

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Figure 7. IR reflection-absorptionspectra for a monolayer at the aidwater interface in the wavenumber ranges 3000-2600 and 2000- 1000cm-'; subphase temperature, 294 K, spreading solvent, CHC1,; subphase, aqueous solution of 1 mmol ZnCl2 L-l, pH 6. (a) DL-2-Hydroxyhexadecanoicacid, 0.236 nm2 molecule-l; (b) L-2-hydroxyhexadecanoic acid, 0.235 nm2 molecule-'.

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Figure 8. Wavenumbers of the antisymmetric methylene stretching vibration vs aredmolecule for DL-2-hydroxyhexadecanoic acid (0) and L-2-hydroxyhexadecanoicacid (0);subphase temperature,294 K, spreadingsolvent,CHC13;subphase, aqueous solution of 1mmol ZnClz L-l, pH 6.

complex formation between the carboxylate group and the cation: this complex formation and its consequences for the compression of the film-forming substances were already extensively discussed by Gericke and Huhnerfuss40 for achiral alkanoic acids. With regard to the different influence of bivalent cations on chiral recognition due to different complexes, the present data set allows only limited insight, because a n unequivocal assignment of the carboxylate stretching vibrations to specific complex formations requires the presence of both the antisymmetric and the symmetric bands. Unfortunately, in the present spectra only the antisymmetric carboxylate vibrations are strong enough, thus allowing only qualitative considerations: in the presence of a n aqueous Ca2+ subphase (not shown), the band was found around 1580 cm-', while for a Pb2+subphase the band was split (1558 and 1546 cm-') for the racemic 2-hydroxyhexadecanoic

acid film (Figure 5a) and a sharp peak was encountered at 1562 cm-' for the enantiomeric film (Figure 5b). In the case of a n aqueous Zn2+subphase, the bands were found at 1597 (racemic monolayer; Figure 7a) and 1579 cm-l (enantiomeric monolayer; Figure 7b), respectively. These values have to be compared with the respective band positions of the antisymmetric carboxylate vibration reported by Gericke and H i i h n e r f u s ~for ~ ~octadecanoic acid monolayers in the presence of Ca2+-(1577,1565, and 1542 cm-l) or Pb2+-containingaqueous subphases (1542, 1523, and 1512 cm-l). On the basis of their results the authors postulated preferential ionic interactions between Ca2+and the carboxylate group, while the corresponding Pb2+complex was assumed to show largely covalent-type interactions. These conclusions are in line with results reported by Yazdanian et a1.,62who performed surface potential measurements of stearic and arachidic acid monolayers a t the air-water interface in the presence of Mg2+,Ca2+,Co2+,Cd2+,and Pb2+cations, respectively, in the aqueous subphase. In general, the band positions determined in the present work for the hydroxyalkanoic acids are shifted to higher wavenumbers. This effect is in part due to the inductive effect of the 2-hydroxy however, in addition, it may also be indicative of a monodentate-type bonding or of a reduced covalent character of the complex formed between the bivalent cation and the carboxylate group. A decision about the relative contributions ofthese three effects to the different complex formation of Pb2+ and Zn2+ cations and the consequences for chiral recognition requires additional spectral information, in particular, sufficiently strong bands of the symmetric carboxylate vibration, which are not accessible under the present experimental conditions. In summary, the results of this work complement the present discussion on chiral discrimination of racemic monolayers by an additional aspect: Andelman predicted preferred heterochiral interactions for pure van der Waals interactions and homochiral behavior for electrostatic ones.24 Stine and co-workers*-l0 and Gericke and Hiihnerfussll investigated monolayers consisting of chiral substances that may interact by hydrogen-bridge formation, and they raised the question whether this effect may be crucial for chiral discrimination in monolayers. It has been shown in this work that complex formation between bivalent cations and the functional groups of the amphiphile may give rise to increased chiral discrimination. Furthermore, this latter effect may induce both homochiral and heterochiral discrimination, depending on the bivalent cation and the respective complex characteristics.

Conclusions Investigation of surface pressure/area (nlA)isotherms and infrared reflection-absorption spectra of racemic and enantiomeric 2-hydroxyhexadecanoicacid Langmuir films on a pure water subphase and on a n aqueous subphase containing Ca2+, Pb2+, and Zn2+ cations, respectively, allows the following conclusions. (1)Bivalent cations in the aqueous subphase give rise to considerable condensation and increased chiral discrimination in HHDA monolayers, where the condensation effect induced by Ca2+is stronger than the effects of Pb2+ and Zn2+, while the chiral discrimination both in the presence of Pb2+and Zn2+cations is stronger than in Ca2+containing subphases. (62)Yazdanian, M.; Yu, H.; Zografi, G. Langmuir 1990, 6,1093. (63) Margareta, A,; Mateescu, GH. D. Infrared Spectroscopy-Applications in Organic Chemistry; Wiley Interscience: New York,1966; p 388.

Neumann et al.

2212 Langmuir, Vol. 11, No. 6, 1995 (2)In the presence ofPb2+and Zn2+cations, respectively, the conformational order of the alkyl chain is nearly independent of the compressional status of the monolayer; i.e., the optimum order induced by these cations is already attained a t very low surface pressures around 1mN m-l. (3) Pb2+cations induce heterochiral discrimination in compressed HHDA films, while the presence of Zn2+ cations leads to preferred homochiral interactions. This result is insofar notable, as it contrasts with the usual assumption that the more condensed nlA isotherm of the L-enantiomer monolayer as encountered both in the presence of Pb2+and Zn2+cations always reflects homochiral interactions. (4) The different influence of bivalent cations on chiral recognition is assumed to be caused by different complexes

formed by the respective cation and the functional group of the film-forming substances. (5) Consideration of potential complex formation in addition to van der Waals and electrostatic interactions as well as hydrogen-bond formation is strongly recommended when investigating heterochiral and homochiral discrimination in chiral Langmuir films.

Acknowledgment. This work was supported by the European Community as part of the “Human Capital and Mobility Programme-Dynamic Network”, contract No. ERBCHRXCT930332. Furthermore, A.G. wishes to acknowledge a fellowship support by the Deutsche Forschungsgemeinschaft, Germany. LA950090Z