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Chiral Discrimination and Pattern Formation in N-Dodecylmannonamide Monolayers at the Air-Water Interface D. Vollhardt,*,† G. Emrich,† T. Gutberlet,‡ and J.-H. Fuhrhop§ Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chaussee 5, D-12 489 Berlin, Germany, Institut fu¨ r Experimentelle Physik I, Universita¨ t Leipzig, Linne´ strasse 5, D-04 103 Leipzig, Germany, and Institut fu¨ r Organische Chemie, Freie Universita¨ t Berlin, Takustrasse 3, D-14 195 Berlin, Germany Received May 13, 1996. In Final Form: August 16, 1996X N-Dodecylaldonamide monolayers are representative two-dimensional systems for the study of chiral discrimination effects of diastereomers. Enantiomeric and racemic monolayers of N-dodecylmannonamide are investigated at the air-water interface using Brewster angle microscopy, surface pressure-area isotherms, and constant surface pressure relaxation measurements. The striking differences in the π-A isotherms demonstrate homochiral discrimination with preferred interaction between the same enantiomers. The condensed phase domains of the enantiomers are feather-like dendrites with anisotropically grown side arms, which are chirality-dependent clockwise or counterclockwise curved. The growth patterns of the racemic monolayers evolve similar side arms, but curved in both directions. This indicates spontaneous chiral seggregation. The chiral properties of the N-dodecylmannonamide monolayers are completely different than those of the heterochiral N-dodecylgluconamide monolayers. These chiral differences are correlated to the sugar head group conformations which are known from X-ray diffraction and/or cross polarization magic angle spinning 13C-NMR spectroscopy.
Introduction Langmuir monolayers at the air-water interface provide unique and simple models for studying chiralitydependent intermolecular forces in two-dimensional restricted but highly organized self-assemblies under defined conditions.1,2 In amphiphilic monolayers with a chiral center, molecular packing and orientational order of the condensed phase domains can be studied in detail. Surface pressure (π)-area (A) isotherms have been used to demonstrate chiral discrimination effects by comparing monolayers of pure enantiomers and their racemic mixtures.3-5 Depending on the interaction between the enantiomeric forms, two types of enantiomeric mixtures can be distinguished.6 Homochiral discrimination corresponds to the condition ED-D or EL-L > ED-L, whereas heterochiral discrimination is indicated for ED-D or EL-L < ED-L, where E is the effective interaction energy and the subscripts refer to the corresponding chiral forms D, L, or DL. With the recent development of polarized fluorescence microscopy7-12 and Brewster angle microscopy,13-17 effective optical methods have been available to visualize * To whom correspondence may be addressed: fax, (030) 6392 3102; e-mail,
[email protected]. † Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. ‡ Universita ¨ t Leipzig. § Freie Universita ¨ t. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Vollhardt, D. Adv. Colloid Interface Sci. 1996, 64, 143. (2) Vollhardt, D.; Emrich, G.; Melzer, V.; Weidemann, G.; Gehlert, U. In Short and Long Chains at Interfaces; Daillant, J., et al., Eds.; Editions Frontieres: Gif-sur-Yvette, 1995 pp 149. (3) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Acc.Chem. Res. 1989, 22, 131. (4) Heath, J. G.; Arnett, E. M. J. Am. Chem. Soc. 1992, 114, 4500. (5) Rose, P. L.; Harvey, N. G.; Arnett, E. Adv. Phys. Org. Chem. 1993, 28, 45. (6) Andelman, D. J. Am. Chem. Soc. 1992, 111, 6536. (7) Weis, R. M.; McConnell, H. M. Nature 1984, 310, 47. (8) Weis, R. M.; McConnell, H. M. J. Phys. Chem. 1985, 89, 4453. (9) Stine, K. J.; Uang, J. Y.-J.; Dingman, S. D. Langmuir 1993, 9, 2112. (10) Parazak, D. P.; Uang, J. Y.-J.; Turner, B.; Stine, K. J. Langmuir, 1994, 10, 3787.
S0743-7463(96)00471-4 CCC: $12.00
chiral discrimination effects in the morphology of amphiphilic monolayers. For different chiral monolayers, a comparison of the morphological images with the corresponding π-A isotherms has demonstrated the high sensitivity of these optical techniques to the specific twodimensional stereochemistry. Additional interest has been devoted to the study of amphiphilic monolayers with more than one chiral center. The different diastereomers of the N-dodecylaldonamides are good canditates for the investigation of appropriate monolayers. Long-chain amphiphilic N-alkylaldonamides form three-dimensional aggregates in aqueous solutions and gels, the morphology of which depend on the stereochemical configuration of the polyolic head groups.16-18 Recently we demonstrated a representative twodimensional sytem with striking chiral discrimination comparing N-dodecylgluconamide (Glu) monolayers of the pure enantiomeric forms and the racemic mixtures.19,20 The surface pressure isotherms suggested heterochiral discrimination. Chiral discrimination in the monolayer morphology was visualized and studied by Brewster angle microscopy. D- and L-N-dodecylgluconamides form identical dendritic crystals at the air-water interface. These dendrites grow anisotropically with straight main axes and numerous straight side-branches developing preferentially into one direction along the main axes. No curvature was observed. The corresponding racemate, (11) Van Esch, J. H.; Nolte, R. J. M.; Ringsdorf, H.; Wildburg, G. Langmuir, 1994, 11, 1955. (12) Rietz, R.; Brezesinski, G.; Mo¨hwald, H. Ber. Bunsenges. Phys. Chem. 1993, 97, 1394. (13) Weidemann, G.; Vollhardt, D. Colloids Surf. A 1995, 100, 187. (14) Weidemann, G.; Vollhardt, D. Thin Solid Films 1995, 264, 94. (15) Weidemann, G.; Vollhardt, D. Biophys. J. in press. (16) Fuhrhop, H.-J.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc 1987, 109, 3387. (17) Fuhrhop, J.-H.; Boettcher, C. J. Am. Chem. Soc. 1990, 112, 1768. (18) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565. (19) Vollhardt, D.; Gutberlet, T.; Emrich, G.; Fuhrhop, J.-H. Langmuir 1995, 11, 2661. (20) Emrich, G.; Vollhardt, D.; Gutberlet, T.’ Kling; B.; Fuhrhop, J.-H. Prog. Colloid Polym. Sci. 1995, 98, 266.
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namely, N-dodecyl-D,L-gluconamide, forms compact monolayers with isotropic morphology already at 4 mN m-1. In this paper, we report the chiral discrimination effect of another diastereomeric N-alkylaldonamide, namely, N-dodecylmannonamide (Man). They are studied by surface pressure (π)-area (A) isotherms, constant surface pressure relaxation, and Brewster angle microscopy. We focus on the presentation of the totally different morphological characteristics of the two diastereomer and enantiomer pairs based on mannose and glucose in the 2D condensed phase domains. The differences in the chiral behavior are discussed on the basis of the molecular configuration and its consequences on intermolecular interactions in the monolayer arrangement and thus the orientational order.
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Figure 1. Surface pressure vs area isotherms of the enantiomeric and racemic monolayers of N-dodecylmannonamide at 20 and 30 °C for a compression rate of 0.1 nm2 molecule-1 min-1. Homochiral discrimination is indicated for both temperatures.
Results
Experimental Section The π-A isotherms and the constant surface pressure relaxation measurements were recorded with a computerinterfaced film balance, equipped with a Teflon trough and temperature control unit. The morphology of the condensed phase structures of the monolayers were visualized and inspected by Brewster angle microscopy. The Brewster angle microscope BAM 1 (NFT, Go¨ttingen, Germany) was mounted at a computer-interfaced film balance with a large trough area. The experimental system was equipped with temperature control and nitrogen flushing of the cabinet to prevent contamination. The light source of the BAM was a He-Ne laser (10 mW). The BAM images were taken with a CCD camera. A video system (video recorder and monitor) was used for image formation and inspection. Distortion of BAM images resulting from the observation at the Brewster angle were corrected by digitizing video images of the CCD camera and using special image processing software. The lateral resolution of the Brester angle microscope is approximately 4 mm. Further details of the experimental setup and the Brewster angle microscope have been described elsewhere.1,21 Experiments were carried out using the enantiomeric N-dodecyl-D- or -L-mannonamide and their racemic mixtures prepared as described elsewhere.22 The N-dodecylmannonamides were used without further purification and dissolved in CHCl3/ethanol/water (7.3/2.5/0.2 (v/v/ v)). The solvents used were obtained from Merck (p.a. grade). The subphase water was Millipore filtered. Solutions (10-3 M) of the enantiomeric N-dodecylmannonamides were prepared and spread on the Langmuir trough filled with the bidistilled water by a syinge. The racemic mixture was obtained by mixing appropriate amounts of the two enantiomeric solutions. After the solvent was evaporated, the monolayer was compressed at a rate of 0.04-0.10 nm2 molecule-1 min-1. (21) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf. 1993, 76, 187 (22) Fuhrhop, J.-H.; Schnieder, J.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861.
Thermodynamic and morphological properties of the enantiomeric and racemic monolayers of N-dodecylmannonamide have been coupled to characterize the chiral discrimination effects of this diastereomer of N-dodecylaldonamide. 1. Surface Pressure Measurements. For the demonstration of chiral discrimination in the monolayer thermodynamics, the surface pressure (π)-area (A) curves of the enantiomeric N-dodecylmannonamides and their racemic mixtures have been recorded. Figure 1 shows the π-A curves of the monolayers of the enantiomeric N-dodecyl-D-mannonamide and the racemic N-dodecylD,L-mannonamide recorded at 20 and 30 °C at considerably high compression rates. The π-A curves of the corresponding L enantiomeric monolayers are identical within experimental limits. Between 15 and 40 °C, a significant shift of the π-A curves to lower areas per molecule is observable with increasing temperature. The π-A curves show a continuous increase in the surface pressure, even if the area per molecule, A, is in the range A e 0.2 nm2/ molecule where the cross sectional area of the molecule discounts the packing exclusive of a monomolecular layer. Up to approximately 25 °C at A ) 0.2 nm2/molecule and with π > 20 mN m-1, quite high surface pressures were obtained. In this temperature range, a decompression of the compressed monolayer and a renewed compression have reproduced the original π-A curve. This implies that a significant dissolution of monolayer material in the aqueous bulk phase can be discounted. On further increase in the temperature above 25 °C, the dissolution of the N-dodecylmannonamide monolayers increases considerably, finally leading to the total loss of the monolayer material beyond 40 °C. To reduce an increasing dissolution of monolayer material, resulting in a shift of the π-A curves to lower area per molecule, the π-A curves have been recorded at high compression rates. The π-A curves of the racemic mixture of the N-dodecylD- and -L-mannonamide monolayers show similar shifts with temperature as those of the pure enantiomers. However the comparison of the π-A curves of the enantiomeric and racemic monolayers reveals the enantiomeric monolayers are more densely packed for all temperatures investigated (see Figure 1 for T ) 20 and 30 °C). Surface pressure relaxation has been measured for both the enantiomeric and racemic monolayers. Constant surface pressure relaxation at 10 mN m-1 for a pure enantiomer and the corresponding racemic mixture are presented in Figure 2. The relaxations of both enantiomers agree with each other. The differences in the relaxation rates between the enantiomeric forms and the
Pattern Formation in Monolayers
Figure 2. Constant surface pressure relaxation at p ) 10 mN m-1 of the enantiomeric and racemic monolayers of Ndodecylmannonamide at 25 and 30 °C. The difference in the relaxation curves between the enantiomeric and the racemic monolayers is opposite to that of the diastereomeric Ndodecylgluconamide monolayer.19
racemic mixtures are also not large, but in contrast to diastereomer N-dodecylgluconamide the relaxation of the enantiomeric forms is more rapid. The relaxation increases with temperature. Up to approximately 25 °C the temperature effect is only small, above this temperature it increases significantly because of dissolution of N-dodecylmannonamide in the aqueous subphase (see Figure 1). 2. Brewster Angle Microscopy (BAM). Inspection of the monolayers by BAM has revealed even more chirality dependent effects. The condensed phase aggregates are formed already in the zero pressure region of the π-A isotherm. Figure 3 shows that the enantiomers form feather-like or palm-branch-like dendritic structures with curvatures. The curvature of the side branches is clockwise for the L-mannonamide and counterclockwise for the D-mannonamide. This difference obviously reflects the chirality of the enantiomers. At 0-5 mN m-1 the dendritic side arms grow rapidly and reach the final state within less than a minute. The growth rate depends on the supersaturation of the lowdensity continuous 2D phase. Figure 4 presents three growth steps of N-dodecyl-L-mannonamide at 0.2 mN m-1 and at 30 °C. The growth rate amounts to approximately 200 mm/s. The dendrites grow with curved main axes and numerous curved side branches. The development of the side branches concerning the fixing point designated by the arrow in the growth kinetics as presented in Figure 4 corroborates the chirality-dependent sense of the side arm curvature. The racemate exhibits a similar growth pattern, but the main axes now evolve clockwise as well as counterclockwise. Curved side branches and sometimes random curvature of the dendritic structures have been found. The 2D dendritic crystals are brittle and break when they touch each other upon compression. Further compression to higher surface pressures then leads to a dense layer which reflects homogeneously. After decompression the continuous solid layer disintegrates at first into platelike pieces and transforms slowly into the fluid-like state. Discussion Chiral discrimination effects have been studied as integral parts of the thermodynamic properties of the monolayers by the π-A isotherms and the relaxation measurements and of the morphological features of the pattern formation in the condensed state by BAM. N-Dodecylmannonamide monolayers are representative 2D systems with striking differences in the π-A isotherms, which demonstrate clearly homochiral discrimination. In
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the case of homochiral discrimination, the interaction between the same enantiomers is preferred, i.e., ED-D or EL-L > ED-L. Consequently for both temperatures, the racemic monolayers are more expanded than the enantiomeric monolayers as demonstrated in Figure 2. In contrast, diastereomeric N-dodecylgluconamide monolayers show heterochiral discrimination effects. In his theoretical study, Andelman6 attributed homochiral discrimination to dominant electrostatic interaction. The enantiomeric N-alkylaldonamides can form cyclic networks of hydrogen bonds of the hydroxyl groups. Here the development of hydrogen bonds stabilizes a rectangular orientation of the molecules to each other. Orientation and conformation of amphiphilic gluconamides as found in the crystal structure have been correlated with the behavior of the chiral monolayers.19 The hydrogen bonds between the amide groups and between the synoriented hydroxyl groups have been found to be the basis for the superstructures of the self-aggregated amphiphilic aldonamides in an aqueous solution.19 The homochiral discrimination effect of N-dodecylmannonamide monolayers is opposite to the recently found heterochiral behavior of the diastereomeric N-dodecylgluconamide monolayers. The only difference in the chemical structure of the two aldonamides is the chirality of C2 of the open sugar chain. The drastic differences of the two diastereomeric monolayers are obviously due to this slight configurational change. The time dependence of the constant surface pressure relaxation of the enantiomeric and racemic monolayers is not very different but indicates a faster area relaxation of the enantiomeric monolayer than for the racemic monolayer. The chiral discrimination effect of the area relaxation is likewise opposite to that of N-dodecylgluconamide.19 The relaxation mechanism seems to be complex. The big increase of material loss with temperature above 25 °C observed for enantiomeric and racemic monolayers is caused by dissolution of the monolayer material into the aqueous subphase. At T e 25 °C, dissolution into the bulk phase can be neglected. Here two main reasons are responsible for the moderate relaxation observed: (i) reduction of 2D supersaturation during the dendritic crystallization; (ii) possible epitactical overgrowth of an additional monolayer of the growth kinetics as suggested by BAM videos.23 Thus the formation of at least two-layered structures with a headto-tail layered arrangement similar to the packing in 3D crystals of amphiphilic aldonamides is probable. The diastereomeric aldonamide monolayers are ideal two-dimensional systems for studying chiral discrimination effects on the morphology at the dendritic crystallization. The comparison of the homochiral N-dodecylmannonamide monolayers with heterochiral N-dodecylgluconamide monolayers reveals striking morphological differences of the two diastereomeric forms. The condensed phase dendrites of the enantiomeric monolayers of both diastereomeric monolayers have low mechanical stability. Immediately after solidification, the brittle dendrites are frozen in their nonequilibrium state and remain metastable in this state as long as the monolayer state is not changed. Thus the low surface pressure state at the beginning of the compression is best suited to discriminate the chiral forms. The direction of the curvature of the enantiomeric N-dodecylmannonamide dendrites indicates the chirality. The chiral behavior of the diastereomeric N-dodecylgluconamide monolayers has been shown to be very different.19 Here the results of the π-A isotherms indicate heterochiral discrimination. D- and L-dodecylglucon(23) Emrich, G.; Vollhardt, D. Unpublished results.
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Figure 3. Chiral discrimination in the morphology of the condensed phase structures of N-dodecylmannonamide monolayers (30 °C; 0.5 mN m-1). The enantiomers evolve feather-like dendrites with side arms developed preferentially into one direction along the main axis: (a) counterclockwise for the L-form, and (b) clockwise for the D-form. (c) The side arms of the racemic monolayers are curved in both directions. This indicates spontaneous chiral seggregation.
Figure 4. Evolution of a growth pattern of a N-dodecyl-L-mannonamide monolayer. The front propagation of curved dendrites reveals the chiral character and the sense of the curvatures: counterclockwise for the L-form. The arrow indicates a reference point.
amides form identical dendritic crystals. Their dendrites grew anisotropically with straight main axis and numerous straight side branches with a preference for one direction along the main axes. No curvature is observed. Several groups have reported curvatures in the 2D condensed phase morphology associated with the presence of a single chiral center. Curvatures in the domain shape were found for the enentiomeric monolayers of the amphiphiles of N-acylvaline10 and imidazole.11 The morphological features of the condensed phase domains of phospholipid monolayers also reflect the chirality by curvatures, both of the defect lines inside of the DMPE domains and of the outer shape of the DPPC domains.13-15 The condensed phases of the phospholipid monolayers show long range orientational order. Main features of the enantiomeric phospholipid domains are (i) a continuous change of the molecular orientation and (ii) defect lines at which the orientation jumps due to the minimization of the elastic strain energy. Theoretical considerations have shown that dendritic growth is a general feature of anisotropic systems.24-26 Linear dendritic structures with symmetric side branches can be theoretically described.24 Modifications of the growth conditions have led to other regimes. For example, tip splitting has been regarded to be caused by noise amplification from the tip region during the tip growth.25 However a theoretical description of chirality-dependent anisotropic growth shapes with side branches developing preferentially into one direction along the main axes does not exist. (24) Nittman, J.; Stanley, H. E. Nature 1986, 321, 663. (25) Langer, J. S. Science 1989, 243, 1150. (26) Ben-Jacob, E.; Garik, P. Nature 1984, 310, 47.
The observed morphological differences of the diastereomeric monolayers of the two alkylaldonamides cannot be understood alone from first principles of theoretical models. There exist obviously correlations between the growth patterns of the dendritic structures and the structural features of the sugar head groups. Predominant structural features of the condensed phase of these diastereomeric molecules are the different conformations of glucon- and mannonamides, which have been determined in microcrystals by X-ray crystallography or solid state cross polarization magic angle spinning (CP/ MAS) NMR spectroscopy.27 The crystal structure analysis of N-alkyl-D-gluconamide has revealed a hydrogen bonding network between neighboring molecules and linear hydrogen bond chains between the amide function of the amphiphile.28-30 A similar hydrogen bonding system has also been found in the crystals of L-mannon acid hydrazide.30 In both cases, the conformation of the open chain sugar is found in the all-trans conformation. According to the crystal structure, in enantiomeric gluconamide monolayers, the interaction between the hydroxyl group at C2 and the amide carbonyl oxygen fixes a linear conformer of the open chain sugar. This interaction is prevented in mannonamide monolayers because of the different configuration at C2 (Figure 5). (27) Svenson, S.; Kirste, B.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1994, 116, 11969. (28) Zabel, V.; Mu¨ller-Fahrnow, A.; Hilgenfeld, R.; Saenger, B.; Pfannemu¨ller, B.; Enkelmann, V.; Welte, W. Chem. Phys. Lipids 1986, 39, 313. (29) Andre´, C.; Luger, P.; Gutberlet, T.; Vollhardt, D.; Fuhrhop J.-H. Carbohydr. Res. 1995, 272, 129. (30) Andre´, C.; Luger, P.; Rosengarten, B.; Fuhrhop, J.-H. Acta Crystallogr., Sect. B: Struct. Sci. 1993, 49, 375.
Pattern Formation in Monolayers
b
Figure 5. Proposed orientation and conformation of the head groups of aggregated N-dodecylmannonamide monolayers: (a) hydrogen bond cycles are present in pure enantiomers, but not in racemates. The hydrogen bonds are shown by dotted lines. (b) as found in 3D crystals of D-gluconamide, the chiral head group has a linear conformation, while in 3D crystallites of D-mannonamide a gauche bent at C4 is present. The all-tans configured alkyl chains R are omitted for clarity.
The corresponding hydrogen bonding system cannot be formed. This arrangement then probably leads to the observed macroscopic shape of the 2D crystals. Further conformational differences in the head group packing of both diastereomers is indicated by solid state CP/MAS-NMR spectroscopy of N-octyl-D-mannonamide and N-octyl-D-gluconamide.27 The head group of gluconamide exhibits an elongated all-trans-conformation whereas a gauche bent is observed at C4 of the mannonamide head group (Figure 5b). It is interesting to note that Perlstein31,32 proposed a computational method for the prediction of the structure (31) Perlstein, J. J. Am. Chem. Soc. 1994, 116, 455. (32) Perlstein, J. J. Am. Chem. Soc. 1994, 116, 11420.
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of one-dimensional screw, glide, and inversion molecular aggregates that has implications for the packing of molecules in monolayers and crystals. He confirmed that the complex ordering of molecules in three dimensions can be broken down into substructures, each of which is in a local energy minimum. The appearance of dendritic domains with clockwise and counterclockwise curvatures suggests that spontaneous chiral segregation occurs at solidification of the racemic monolayer mixture. A similar behavior was found in racemic monolayers of N-acylvaline amphiphiles with sickle-like arms clockwise and counterclockwise curved.10 The formation of curved domain shapes has been explained by differences in the line tension of the two sides of a domain or its segments due to a regular orientation of an enantiomer throughout the domain.7,8 The occurrence of both chiral structures within the racemic N-dodecylmannonamide monolayer thus indicates a chiral phase separation. The conformation of mannonamide in racemic microcrystals is not yet known. One may predict from the observed condensed phase structures of racemic D,L-Nalkylgluconamide monolayers that the conformation of the enantiomers is generally preserved. It is however obvious that the small change in the configuration at the carbon atom of the polyolic chain nearest to the amide linkage produces a striking change in the chiral discrimination of these diastereomers. This is more complex than known from the amphiphiles with a single chiral center. Conclusion In summary, monolayers of the diastereomeric Ndodecylaldonamides are ideal two-dimensional systems for studying chiral discrimination effects. N-Dodecylmannonamide monolayers show homochiral discrimination, the morphological characterization of which is based on the combination of Brewster angle microscopy, π-A isotherms, and constant surface pressure relaxation measurements. Brewster angle microscopy is thus an effective tool for visualizing chiral discrimination effects in the dendritic crystallization of the N-dodecylmannonamide monolayers. The enantiomers form feather-like two-dimensional crystals with chirality-dependent curvatures of the main axes and the anisotroppically grown side arms. The similar growth patterns of the racemic monolayers evolve side arms in both directions of the main axes from which spontaneous chiral seggregation can be inferred. The chiral monolayer properties of the two diastereomers N-dodecylmannonamide and N-dodecylgluconamide are very different. This can be understood by the predominant structural features and thus different conformations of the sugar head groups due to the different configuration at C2. The variety of inter- and intramolecular hydrogen bonds determinative for the conformational and aggregational freedom of the diastereomeric N-alkylaldonamides are the main factors in the supramolecular aggregation of these polycentered amphiphiles both in the condensed phases of the monolayers at the air-water interface and in the three-dimensional aggregates in aqueous solution. Acknowledgment. We thank Dr. B. Kling for the preparation of the enantiomeric N-dodecylmannonamides. Financial assistance from the Deutsche Forschungsgemeinschaft (SFB 312 “Vectorial Membrane Processes”) and the Fonds der Chemischen Industrie is gratefully acknowledged. LA960471S
