J. Phys. Chem. B 2002, 106, 4419-4423
4419
Chiral Discrimination in 1-Stearylamine-Glycerol Monolayers D. Vollhardt* and U. Gehlert Max-Planck-Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany ReceiVed: July 3, 2001; In Final Form: October 29, 2001
The effect of the chirality on the thermodynamic behavior, the morphological features, and the 2D crystal structures of 1-stearylamine-glycerol monolayers are studied. The results are based on measurements of the surface pressure-area (π-A) isotherms, fluorescence microscopy, and synchrotron X-ray diffraction at grazing incidence (GIXD). Although the π-A isotherms of the enantiomeric forms and the racemic mixtures agree largely to each other, the filigree domain patterns show remarkable differences, obviously driven by the chirality. The domains of the R(+) enantiomer are curved clockwise, those of the S(-) enantiomer counterclockwise, whereas those of the racemic mixtures are without a specific sense of direction. Chiral discrimination effects are also found in the lattice structure. The enantiomeric monolayers have an oblique lattice where at compression the tilt direction changes continuously from angles nearly toward NN direction to angles nearly toward NNN direction. The condensed phases of the 1:1 racemic mixtures give rise to rectangular-centered lattices indicating a phase transition accompanied by the change in the tilt direction from NN at 1 mN/m to NNN at 5 mN/m. The filigree domain patterns indicate a comparatively low ordering of the alkyl chains. Correspondingly, a short-range translational order can be concluded from the low position correlation and high tilt angles of the alkyl chains at low surface pressures.
Introduction Chirality-based phenomena are of permanent interest. Chiral discrimination arises from differences in the interaction potential between molecules of the same chirality and that of opposite chirality. Some general reasons for wanting to understand the phenomenon of chiral discrimination include the chiral preference in nature realized in homochiral evolution and the design of pharmaceuticals of specific chirality.1 Chiral monolayers provide favorable systems to study chirality-dependent interactions under defined conditions. As monolayer systems can be investigated over a wide range of specific states, they have been used as model systems to study chiral discrimination by comparing the surface pressure (π) versus area per molecule (A) isotherms of enantiomeric and racemic monolayers.2-4 Comparison of enantiomeric and racemic monolayers revealed chirality-dependent differences in the π-A isotherms. Homochiral discrimination exists if the interaction between the same enantiomers is more favorable while heterochiral interaction is indicated if the interaction between the two different enantiomers is more auspicious. With the development of sensitive imaging techniques such as fluorescence and Brewster angle microscopy (BAM), more effective methods have been available to visualize the effect of chirality on the morphology of monolayers on microscopic scale. Chirality-dependent effects have been found in various amphiphilic monolayers. This concerns particularly the N-acyl amino acid amphiphiles,5-9 some phospholipids,10,11 but also aldonamides12,13 or tailored amphiphiles.14 Monoglycerols are interesting amphiphiles that make possible the comparison of the enantiomeric and racemic monolayers. Using fluorescence microscopy, BAM and GIXD chiral discrimination effects have been already found for monoglycerol ethers,15-17 esters,18 and amides.19 * To whom correspondence should be addressed.
Figure 1. Chemical structure of the enantiomeric R(+) and S(-) form of 1-stearylamine-glycerol. The chiral C atom is marked with a star (*).
Monoglycerol amines are another interesting type where the chiral R- and S-forms are accessible by a usual chemical preparation procedure. Therefore, the present work focuses on the effect of the chirality on the thermodynamic behavior, the morphological features, and the 2D crystal structures of these monolayers. The experiments were performed with monolayers of the racemic R(+) and S(-) forms of 1-stearylamine-glycerol; the chiral nature of them is shown in Figure 1. The CIP system20 was used to characterize the configuration of the chiral molecules. The -NH- group is admittedly less hydrophil than the corresponding -O-, -CO-O-, or -CO-NH- groups of the other monoglycerol types with the consequence of lower contrast between the monolayer phases. To enhance the contrast, we used an amine with a rather long alkyl chain (C18H37-) and fluorescence microscopy for imaging. As amines are able to form hydrogen bonds, chiral discrimination should be expected. Recent studies have shown the dominant effect of the amide group on the monolayer properties. The amide-amide hydrogen bonding is responsible not only for the high crystallinity of the monolayers14,19 but also for the chiral recognition in Nacylamino acid monolayers.21 Finally, the chirality-dependent monolayer features can be compared for the four monoglycerol
10.1021/jp0125294 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/05/2002
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types with chemically different linking groups between glycerol and alkyl chain. Experimental Section Synthesis of Amphiphile. The R(+) and S(-) enantiomers of 1-stearylamine-glycerol (3-octadecylamino-1,2-propandiol) were synthesized with a purity of >99% according to the following procedure.22 Equimolar amounts of octadecylamine and R(+)- or R(-)-glycidol were dissolved in dry methanol. The mixture was refluxed for 30 min, and then the solvent was removed. For purification, the crude product was recrystallized six times from n-hexane, and a white powder with a purity of >99% was obtained. The chemical purity was confirmed by elemental analysis and HPLC. Experimental Procedures The surface pressure-area (π-A) isotherms were measured using a computer-interfaced film balance with a Wilhelmy type pressure measuring system. The contrast of the very filigree nonequilibrium structures was not high enough to be visualized well by Brewster angle microscopy. Imaging of the monolayer morphology was performed with a fluorescence microscope (Zeiss, Axiotron). The spread monoglycerol solution contained 1.5 mol-% L-R-phosphatidylcholin-β-(NBD-aminohexanoyl)γ-palmitoyl (Sigma, Munich) as fluorescence dye. Excitation of the fluorescence probe was achieved using a high-pressure mercury lamp. Discrimination of excitation and emission was regulated by using conventional dichroic mirrors and a standard blue filter. Details of the method have been described elsewhere.23 The GIXD experiments were performed using the liquidsurface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany. The beam was made monochromatic by a beryllium crystal (002). The monochromatic synchrotron beam was adjusted to strike the water surface with a grazing incidence angle RI ) 0.85Rc, where Rc ∼ 0.14° is the critical angle for total reflection. A linear position-sensitive detector (PSD) (OED-100-M, Braun, Garching, Germany) was used to monitor the diffracted intensity as a function of the vertical scattering angle Rf. A Soller collimator in front of the PSD provides the resolution for the horizontal scattering angle 2θxy which was approximately 0.01 Å-1. The wave vectors ki and kf of the incident and diffracted photons have the same absolute value because of the quasi-elastic nature of the scattering. The scattering vector Q ) kf - ki can be written in terms of an in-plane component Qxy
Qxy ≈ (4π/λ)sin θxy and an out-of-plane component Qz
Qz ≈ (2π/λ)sin Rf where λ is the X-ray wavelength. The diffracted intensities were corrected for polarization, effective area, and Lorentz factor. The intensities were least-squares fitted to model peaks as Lorentzian in the in-plane direction and Gaussian in the outof-plane direction. The lattice parameters can be obtained from the peak positions. The lattice repeat spacing dhk, the polar tilt angle t of the long molecule axis, and the tilt azimuth ψxy were calculated from the positions of the Qxy and Qz maxima24,25 according to
dhk ) 2π/Qhkxy
Figure 2. π-A isotherm of the enantiomeric and racemic 1-stearylamine glycerol monolayers at 35 °C.
Qhkz ) Qhkxycosψhk tan t where h,k denotes the order of reflection. The lattice parameters a,b, and γ were calculated from the lattice spacing dhk and from these the unit cell area Axy was calculated:
Axy ) ab sin γ. The cross section per alkyl chain A0 is related to the unit cell area Axy (area per molecule parallel to the interface) and the tilt angle t
A0 ) Axy cos t Assuming an exponential decay of positional correlations with increasing separation within the layer, the full width at halfmaximum of a Bragg peak ∆xy corrected for resolution effects of the detector is taken to determine the coherence length ξ ) 2π/x(∆2xy - ∆2res), where the second term under the root represents the resolution of the Soller collimator.26 The monolayer material (1-stearylamine-glycerol) was dissolved in a 9:1 (v:v) mixture of n-heptane (for spectroscopy, Merck) and ethanol (p.A., Merck) for spreading on the aqueous subphase. The fluorescence microscopy studies were performed with a compression rate of 0.04 nm2 molecule-1 min-1. The subphase water used for the experiments was purified by a Millipore desktop unit (Millipore, Eschborn). Results and Discussion 1-Stearylamine glycerol monolayers undergo a main-phase transition of first order from a fluidlike (gaseous) state to a condensed phase at higher temperatures of the accessible range. Figure 2 shows the π-A isotherm for 35 °C which have the pronounced nonhorizontal “plateau” region characteristic for the two-phase coexistence region. The π-A isotherms for the racemic and enantiomeric monolayers are identical so that chirality-dependent differences in the thermodynamic behavior do not occur. The exact coordinates of the main-phase transition point (Ac ) 0.53 nm2/molecule, πc ) 4.9 mN/m) cannot be taken directly from the isotherm, but they result as the intersection point between the two different slopes. Formation and growth of condensed-phase patterns can be expected at A e Ac. Fractal-like domain textures can be seen by Brewster angle microscopy, but the characterization of the morphology is difficult as the filigree textures are at the resolution limit. Better information can be obtained by fluorescence microscopy. Using this method, it is seen that at low compression rates the 1-stearylamine glycerol monolayers develop fractal-like filigree
Chiral Discrimination
J. Phys. Chem. B, Vol. 106, No. 17, 2002 4421 TABLE 1: Data of the Lattice Structure of S(-) 1-Stearylamine Monoglycerol Monolayers at 20 °Ca
Figure 3. Fluorescence micrograph of the chiral discrimination in the nonequilibrium domains of 1-stearylamine glycerol monolayers. (a) S(-)-enantiomer, (b) R(+)-enantiomer, (c) racemate. The bar represents 100 µm.
nonequilibrium domains at the beginning of the plateau region. Chiral discrimination effects are observed in the morphological shape of the condensed-phase patterns formed in the two-phase coexistence region at A e Ac. Figure 3 shows fluorescence microscopic micrographs of the condensed-phase domains of the enantiomers (Figure 3a and b) and the racemates (Figure 3c). Dependent on the enantiomeric form, the filigree domains are curved in opposite directions, namely, clockwise for the R(+) enantiomer (Figure 3b) and counterclockwise for the S(-) enantiomer (Figure 3a). The racemic mixtures give rise to fractal-like domains without a specific sense of direction. Similar chiral discrimination effects have been observed in other nonequilibrium domains of phospholipid monolayers.10,11 The spiral growth of condensed-phase domains was theoretically considered as a result of modulated distributions of tilt angle and free charges in chiral-polarized monolayers.27 On the basis of this qualitative approach, it was concluded that the spiral distributions of small perturbations should be the nuclei for the condensed phase. However, the approach cannot explain details of chiral monolayer behavior. The experimental fact that spiral shapes grow from initially compact domains in the similar monogycerol ether monolayers does not support this model.28,29 Recent theoretical calculations can help to understand curvature and handedness of chiral domains formed by amphiphilic monolayers.30,31 In these papers, an effective pair potential description of the groups attached to the chiral center of amino acid amphiphiles has been performed. The results suggest that the observed curvature of the domain shape is due to the twist at the interface between the condensed and fluid phase of the monolayer where the inter-residue hydrogen-bond cycle is not as regular as within the condensed phase.30,31 The theoretically predicted handedness of the amino acid amphiphiles agreed fairly well with handedness experimentally observed by Brewster angle microscopy.31 GIXD experiments were performed to study whether chiral discrimination between the enantiomeric and racemic forms also occurs in the lattice structure of 1-stearylamine monoglycerol monolayers. The contour plots of the corrected diffraction intensities as a function of the in-plane (Qxy) and out-of-plane (Qz) components of the scattering vectors of the monolayer of the S(-)enantiomers at 20 °C and at 1mN/m, 10 mN/m, and 35 mN/m are presented in Figure 4. Similar results were obtained by the monolayers of the R(+) form. Figure 4 shows three diffraction peaks for π ) 1 mN/m and π ) 10 mN/m. Up to a surface pressure of 30 mN/m, three diffraction maxima clearly separable can be observed. This indicates an oblique lattice. The data of the in-plane and outof-plane scattering vector components are listed in Table 1 which
π [mN/m]
a [Å]
b [Å]
γ [°]
1 10 25 30 35 35
5.22 4.98 4.93 4.91 4.89 4.88
5.57 5.35 5.19 5.05 5.00 8.73
104.6 112.7 117.4 118.1 119.2 90.0
t Axy A0 [°] [Å2] [Å2] 46 36 28 24 19 19
28.1 24.5 22.7 21.9 21.3 21.3
td
ψa [°]
19.7 60.2 19.9 74.7 20.0 85.1 20.0 85.0 20.0 NNN 90 20.0 NNN 90
d 0.292 0.162 0.078 0.049 0.031 0.031
a a, b, γ are lattice constants, t is polar tilt angle, Axy is molecular area, A0 is cross-section area of alkyl chain, td is tilt direction, ψa is the angle between azimuthal tilt direction and a-axis, d is distortion. The lattice parameters based on a rectangular lattice structure at 35 mN/m are presented in italics.
Figure 4. Contour plots of the corrected diffraction intensities as a function of the in-plane (Qxy) and out-of-plane (Qz) components of the scattering vectors of the S(-) enantiomeric monolayer at 20 °C; above: π ) 1 mN/m, middle: π ) 10 mN/m, below: π ) 35 mN/m.
gives the unit cell parameters a, b, γ, the in-plane lattice area Axy, the chain tilt angle t, the azimuthal tilt angle ψa, the tilt direction td, and the lattice distortion for different surface pressures d. The cross-section area of the alkyl chains is typical for the free-rotator phase of n-alkanes. With increasing surface pressure, the angle between the azimuthal tilt direction and the a-axis approaches to a value of 90° that is reached at π ) 35 mN/m. Consequently, at π g 35 mN/m the alkyl chains are tilted toward next-nearest neighbors (NNN) in agreement with the fact that the contour plots show only two diffraction maxima (see Figure 4, π ) 35 mN/m). Figure 5 shows the angle between the azimuthal tilt direction and the a-axis (ψa) in dependence on the surface pressure. At compression of the monolayer, the tilt direction changes continuously from angles close to a nearest-
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Figure 5. Angle between the azimuthal tilt direction and the a-axis (ψa) in dependence on the surface pressure of the monolayer of S(-) enantiomeric 1-stearylamine glycerol at 20 °C.
Figure 7. Contour plots of the corrected diffraction intensities as a function of the in-plane (Qxy) and out-of-plane (Qz) components of the scattering vectors of the 1:1 racemic 1-stearylamine glycerol monolayer at 20 °C; above: π ) 1 mN/m, below: π ) 10 mN/m.
Figure 6. Intensity as function of the vertical component of the wave vector Qz at 35 mN/m (for Qxy between 1.44 and 1.45 Å-1 and between 1.50 and 1.51 Å-1). The extended full width at half-maximum of the diffraction peak suggests two reflexes at Qz1 ) 0.29 Å-1 and Qz2 ) 0.50 Å-1.
neighbor (NN) direction at π ) 1 mN/m to that of NNN direction at π ) 35 mN/m. As seen in Figure 4, the diffraction maxima are close together. At high surface pressures (π ) 35 mN/m), the diffraction peaks overlap each other by the high full width at half-maximum of the Bragg peak ∆xy. The overlap can be better seen if the Bragg rod intensities are plotted in dependence on the vertical component of the scattering vector. Figure 6 shows the extended peak at π ) 35 mN/m that can be spitted in two reflexes (Qz1 ) 0.29 Å-1, Qz2 ) 0.50 Å-1). According to the overlap of the reflexes, the position correlations obtained from the full width at half-maximum represent only approximate values. The correlation lengths ξ are relatively small: ξ < 100 Å perpendicular to the tilt direction of the chains, ξ < 35 Å parallel to the tilt direction of the chains. Corresponding GIXD measurements of 1:1 racemic 1-stearyloctadecylamine monoglycerol monolayers reveal that chiral discrimination exists also in the lattice structure. Racemic monolayers produce only two diffraction peaks at all surface pressures investigated. Consequently, in the studied surface pressure region of π e 35 mN/m, the 1:1 racemic monolayers have rectangular-centered lattice. Two characteristic contour plots of the corrected diffraction intensities obtained at 1 mN/m and 10 mN/m are shown in Figure 7. In the first case, the maximum of the degenerate diffraction peak is at Qz ) 0 so that the alkyl chains are tilted toward NN direction. At π ) 10 mN/m, the two reflexes are at Qz > 0 indicating chain tilt in NNN direction. The structure data calculated for the different surface pressures are listed in Table 2. It is seen that a phase transition accompanied by a change in the tilt direction from NN to NNN takes place between 1mN/m and 5 mN/m. This phase transition corresponds to the L2d/Ov transition at fatty acids.32 Comparison of the characteristic lattice
TABLE 2: Data of the Lattice Structure of 1:1 Racemic 1-Stearylamine Monoglycerol Monolayers at 20 °C a π [mN/m]
a [Å]
b [Å]
t [°]
Axy [Å2]
A0 [Å2]
td
d
1 5 10 35
6.36 4.83 5.00 4.91
8.49 10.21 10.00 8.58
43 36 35 18
27.0 24.7 25.0 21.0
19.9 19.9 19.9 19.9
NN NNN NNN NNN
0.256 0.196 0.143 0.0996
a a and b are lattice constants, t is polar tilt angle, Axy is molecular area, A0 is cross-section area of alkyl chain, td is tilt direction, and d is distortion.
data of the racemic and enantiomeric forms (see Tables 1 and 2) shows, besides some agreement in the data, remarkable differences caused by chirality. The lattice type undergoes striking chiral discrimination that is apparent in the transition of the oblique lattice of the enantiomeric forms into the rectangular lattice of the racemic mixture. Accordingly, the lattice structure data (a, b, γ, td, and d) are completely different. As to expect, the cross-sectional area A0 of the alkyl chains of the enantiomeric and racemic forms agree with each other. The A0 value of approximately 20 Å2 corresponds to the free-rotator phase of n-alkanes. Both forms have also nearly the same polar tilt angle so that they have approximately the same molecular area. This is demonstrated in Figure 7 that shows the change of the tilt angle of the enantiomeric and racemic form in dependence of the surface pressure at 20 °C. Both forms agree in the strong dependence of the tilt angle on the surface pressure resulting finally in a complete erection of the alkyl chains at π ≈ 45 mN/m. The perpendicular orientation of the alkyl chains is corroborated by the disappearance of the optical contrast at the BAM studies as well as the kink in the π-A isotherm. The chiral discrimination in the lattice structure between the enantiomeric and racemic monolayers has been discussed on the assumption that different interactions of the molecules compensate each other.26 For the case that the racemic mixture experiences a discontinuous transition between NN and NNN tilted states which corresponds to the L2d/Ov transition in fatty acid monolayers, the interactions controlling the tilt azimuth
Chiral Discrimination
J. Phys. Chem. B, Vol. 106, No. 17, 2002 4423 been found in the condensed phases of the monoglycerol ethers that also give rise to oblique lattices in the enantiomeric monolayers and rectangular lattices, dependent on temperature an abrupt NN-NNN transition at medium surface pressure between 15 and 20 mN/m, in the racemic mixtures. Acknowledgment. The authors thank Dr. R. Wagner for the preparation of the enantiomeric substances and are grateful to Drs. G. Brezesinski and K. Kjaer for help with setting up the X-ray experiments. References and Notes
Figure 8. Tilt angle of alkyl chains of 1-stearylamine glycerol monolayers in dependence of surface pressure at 20 °C. b racemate, O enantiomer, 2 obtained by BAM.
are compensated. Consequently, the monolayer structure of the enantiomer should differ from that of the racemate. The tilt azimuth of the enantiomer varies continuously in the surface pressure region near the transition point in the racemate and adopts structures close to those of the racemate more far from the transition. Conclusions The absence of differences in the π-A isotherms of the enantiomeric forms and the racemic mixtures indicates that at most, small energetic effects occur at the complete mixing of the two enantiomers. The characteristic curvatures of the filigree domain patterns are obviously driven by the chirality of the molecules concerned. The curvature of the R(+) enantiomeric domains is clockwise whereas that of the S(-) enantiomeric domains is counterclockwise. Chiral discrimination of different kind but having the same sense of curvature has been found in the domain morphology of the monoglycerol ethers.28,29 Here, spiral shapes grow from initially compact domains at the end of the plateau region of the π-A isotherm. In enantiomeric monolayers, they are chirality-dependent curved in the same sense, but the racemic monolayers develop spirals curved in both directions. The lattice structure of the 1-stearylamine monoglycerol monolayers is also affected by the chirality of the molecules. The enantiomeric monolayers have an oblique lattice where at compression the tilt direction changes continuously from angles nearly toward NN direction to angles nearly toward NNN direction. In opposite, the condensed phases of the racemic mixtures give rise to rectangular-centered lattices. Here, a phase transition occurs that is accompanied by a change in the tilt direction from NN at 1 mN/m to NNN at 5 mN/m. Both the enaniomeric monolayers and their racemic mixtures have highly tilted molecules at low surface pressures. In agreement with the low position correlation, it allows the conclusion of low ordering of the alkyl chains in the filigree domain structure. Similar chiral discrimination effects on the lattice structure have
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