Langmuir 2000, 16, 7445-7448
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Rearrangements of N-Octyl-D-gluconamide Fibers and Bilayers on Gold and Silicon Surfaces Christian Messerschmidt,† So¨nke Svenson,† Wolfgang Stocker,‡ and Ju¨rgen-Hinrich Fuhrhop†,* Freie Universita¨ t Berlin FB Biologie, Chemie, Pharmazie Institut fu¨ r Chemie/Organische Chemie, Takustr. 3, D-14195 Berlin, Germany, and Humboldt-Universita¨ t zu Berlin, Institut fu¨ r Physik, Invalidenstr. 100, D-10115 Berlin, Germany Received February 25, 2000. In Final Form: May 30, 2000 N-Octyl-D-gluconamide (1) is known to form noncovalent quadruple helices in bulk aqueous media. Upon adsorption to mica these fibers survive; on gold they rearrange to give a head-to-tail bilayer. This is the thermodynamically most stable arrangement found in 3D-crystals. Upon heating to 86 °C, a rearrangement to a tail-to-tail bilayer occurs. This cannot be detected in atomic force microscopy (AFM) height diagrams, but leads to pronounced changes of phase shifts in the tapping mode of AFM. Tapping also induces a reversal of the bilayer rearrangement.
Introduction Chiral amphiphiles containing secondary amide groups often form spherical micelles in water at temperatures above 80 °C, where hydrogen bond chains break, and then rearrange to form fibers upon cooling. For the open-chain N-octylgluconamide amphiphile quadruple, helices made of micellar cylinders with the width of a molecular bilayer were obtained and were characterized by detailed image analysis of transmission electron micrographs1,2 and solidstate NMR spectroscopy.3,4 Such fibers have very large surface energies and should rearrange quickly to more stable crystals without curvature. This process, however, only occurs quickly if the enantiomeric gluconamides are mixed (“chiral bilayer effect”). Pure enantiomers must reorient from tail-to-tail bilayers to head-to-tail monolayers in order to form crystal sheets.5,6 This process is slow in water, and therefore the fibers survive indefinitely under favorable conditions. Similar monolayer and bilayer arrangements occur much faster in achiral crystals of fatty acid esters.7 We deposited the noncovalent polymeric gluconamide fibers on solid surfaces, namely gold and mica, and followed their rearrangement by atomic force microscopy (AFM) measurements. Rapid decomposition was expected, but we could not predict whether monolayer or bilayer sheets would form. Tight crystal packing, which enforces the rearrangement from bi- to monolayers, should not play a role in a single-surface bilayer. Furthermore, polar mica and metallic gold surfaces should not attach to the same †
Frie Universita¨t Berlin. Humboldt-Universita¨t zu Berlin Institut fu¨r Physik. * To whom correspondence should be addressed.
‡
(1) Ko¨ning, J.; Bo¨ttcher, C.; Winkler, H.; Zeitler, E.; Talmon, Y.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1993, 115, 693. (2) Boettcher, C.; Stark, H.; van Heel, M. Ultramicroscopy 1996, 62, 133. (3) Svenson, S.; Kirste, B.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1994, 116, 11969. (4) Svenson, S.; Ko¨ning, J.; Fuhrhop, J.-H. J. Phys. Chem. 1994, 98, 1022. (5) Fuhrhop, J.-H.; Schnieder, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (6) Mu¨ller-Fahrnow, A.; Hilgenfeld, A.; Hesse, H.; Saenger, W.; Pfannemu¨ller, B. Carbohydr. Res. 1988, 176, 165. (7) Larsson, K. In The Lipid Handbook; Gunstone, F. D., Harwood, J. L., Padley, F. B., Eds.; Chapman and Hale: London, 1986; p 321.
Figure 1. AFM images of fibrous aggregates of 1 lying directly on silicon surfaces: (a) network of fibers; (b) high-resolution phase image of the quadruple helices.
functional groups of amphiphiles. Different degradation processes were expected. Finally, we wanted to determine, whether the disruptive surface forces could be overcome by addition of detergents, which are known to impede crystallization.8 Experimental Section AFM measurements were performed using a Digital Instruments Nanoscope IIIa (Santa Barbara, CA) in tapping mode. All images were taken under ambient conditions. Height and phase images were recorded simultaneously. Silicon cantilevers (Digital Instruments) with a spring constant of 45-60 N/m and a resonance frequency in the range of 270-350 kHz were used. The scanning rate was usually 1.5 Hz, except for the image in Figure 1b, for which only 0.5 Hz was used. The tapping mode phase offset was always zeroed before scanning at a distance of 5 nm from the sample. Tapping conditions can be characterized by the rSp value:9 rSp ) ASp/A0. ASp is the amplitude in contact with the sample, and A0 is the free amplitude, which was set between 40 and 50 nm corresponding to about 2-3 V (sensitivity values were readjusted before scanning by taking the slope of the force-distance curve, and usually amounted to 0.07 V/nm). Light tapping corresponds to an rSp value of higher than 0.9; hard tapping conditions have a value of lower than 0.8. To ensure light tapping conditions, the maximum setpoint at which stable imaging was possible was chosen. To avoid instabilities in phase imaging, the drive frequency was not set at the resonance frequency of the cantilever, but slightly (0.1 kHz) lower. Under (8) Fuhrhop, J.-H.; Svenson, S.; Bo¨ttcher, C.; Ro¨ssler, E.; Vieth, H.M. J. Am. Chem. Soc. 1990, 112, 4307. (9) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, 385.
10.1021/la0002684 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/18/2000
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these conditions, reproducible results for phase imaging were obtained. The absolute values for phase shifts depended on the individual tip used, but differences between the two bilayer arrangements reported here ranged around 10 ( 3°. The relative humidity during measurement was 30-40%, monitored by a standard humidity measuring device. Samples were prepared on silicon wafers (Aurel), and were cleaned by treatment with a mixture of hydrogen peroxide (30%), ammonia, and water (1:1:10) at 80 °C for 30 min. Ten microliters of a gel of N-octylD-gluconamide (1) (prepared by heating 1 wt % 1 in water to above 80 °C and slow cooling thereafter) was spread on the wafer and blotted off. Immediate blotting leads to a higher population of fibers, and longer immersion times lead to more bilayers and crystals. Gold surfaces were prepared according to the procedure of Wagner et al.10
Results and Discussion Noncovalent fibers tend to flatten and to form planar bilayers or crystallites on planar surfaces. An early AFM investigation of gluconamide fibers on mica yielded only aggregates lying on top of bilayers.11,12 We report here on a procedure to retain fiber networks directly on the surface by adding sodium dodecyl sulfate (SDS) to the aqueous gel at a molar ratio of 1:5. This detergent has been shown to increase the lifetime of fibers in the gel8 by dissolving emerging crystallites in micelles. The same process also slows the formation of planar layers on solid surfaces. We also found that after application of the fibrous gel to the mica surface before blotting, the fast preparation was essential to retain the fibers on the surface. Blotting off the gel immediately after contact with the surface by pressing a filter paper on the surface was found to be the most simple and efficient procedure. If one waits for 30 s, rearrangement to planar bilayers is already complete. Figure 1a shows a network of fibers on silicon obtained by the quick procedure. These networks predominantly contain the quadruple helices, which have been characterized by transmission electron microscopy (TEM). On silicon or mica no bilayers underneath the fibers could be found, either by close-up images (Figure 1b) or by scratching experiments. On gold, however, under all conditions employed only a bilayer of 1 was found. Figure 2 shows fibers lying on top of a bilayer (Figure 2a). In a section analysis (Figure 2b) the typical height of a bilayer (3.5 ( 0.1 nm) was found. Figure 2c,d shows a scratching experiment on gold. The V-shaped structures are fibers lying on top of a bilayer of 1. A hole in the bilayer could be enlarged by scratching the bilayer with an AFM tip. This difference can be traced back to the large surface forces of gold manifesting themselves in gold’s larger Hamaker constant of 40.13 At this subnanometer distance, however, other interactions come into play, which cannot be quantified by simple AFM methods. The molecules will, however, certainly be more attracted by gold than by mica, leading to a rapid flattening of spherical assemblies. The effect of SDS is, as discussed earlier,12 simply related to the dissolution of microcrystallites in water. If such crystallites are present as nuclei for the formation of planar, crystalline layers, the formation of surface layers is obviously also retarded. Rapid drying by blotting then makes the fibers survive on solids of low attractive force. (10) Wagner, P.; Hegner, M.; Gu¨ntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867. (11) Tuzov, I.; Cra¨mer, K.; Pfannemu¨ller, B.; Magonov, S. N.; Whangbo, M.-H. New J. Chem. 1996, 20, 23. (12) Tuzov, I.; Cra¨mer, K.; Pfannemu¨ller, B.; Magonov, S. N.; Whangbo, M.-H. New J. Chem. 1996, 20, 37. (13) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1994.
Figure 2. AFM images of fibrous aggregates of 1 lying on top of a bilayer of 1 on gold. (a) Overview; (b) section analysis of holes in the bilayer; (c,d) scratching experiment in which the bilayer between V-shaped fibers is partly removed by tapping at higher forces: (c) after 1 scan; (d) after 15 scans at high force; the hole in the middle of the image becomes larger.
The story does not, however, end with the formation of a planar surface layer. One may, at first, expect formation of a monolayer with the gluconamide headgroups on the solid surface and alkyl chains up. Such a monolayer has never been detected by AFM. One rather finds a headto-tail bilayer. This is the same arrangement as in 3Dcrystals and there is no obvious reason why there are no multilayers or crystallites present. On gold, we then discovered a spontaneous rearrangement of most of the tail-to-tail fiber to a head-to-tail bilayer arrangement during disruption of the fibers. Upon heating to 86 °C the inverse rearrangement was demonstrated by changes of X-ray diffraction patterns of stable 3D-crystals4 when they were converted to soft bilayer crystals with an additional gauche-bend in the gluconamide headgroup. The bilayer is considerably thinner than two monolayers. This transition can be monitored by DSC (Figure 3). In the surface bilayer this rearrangement cannot be detected by simple techniques such as differential scanning calorimetry (DSC), contact angle, or X-ray diffraction, because both types of bilayers are in equilibrium and may exist side-by-side. The lateral resolution of scanning force microscopy is needed. Height measurements, however, do not help, because the height does not change (Figure 4a). The corresponding phase image (Figure 4b), however, clearly shows two domains showing different phase shifts. According to the theory of phase shifts9 stiffer materials show higher phase shifts under the light tapping conditions employed here. This assumption has been verified on polymer compounds8 and mixed Langmuir-Blodgett (LB)-monolayers.14 The amorphous crystals in the upper right corner of Figure 4 show the highest phase shift in the image and are thought to be bulk crystallites. In the bilayer regions, domains with the lower phase shift dominate, which means thermodynamically stable headto-tail bilayers are present (Figure 4c). The domain with the higher phase shift should represent the micellar tailto-tail arrangement (Figure 4d). The two other possibilities (Figure 4e,f) are unlikely because the subphase is polar. (14) Messerschmidt, C.; Schulz, A.; Rabe, J. P.; Simon, A.; Marti, O.; Fuhrhop, J.-H. Langmuir 2000, 16, 5790.
Rearrangements of N-Octyl-D-gluconamide Fibers
Figure 3. Differential scanning calorimetry experiments of monolayer head-to-tail crystals N-octyl-D-gluconamide (1) and of a racemic D,L-bilayer crystallite of 1. The peak of 86 °C corresponds to a monolayer f bilayer rearrangement in the solid state.
Figure 4. AFM images of a bilayer of 1. (a) Height image; (b) phase image revealing different domains; (c) crystalline headto-tail arrangement (smaller phase shift); (d) micellar tail-totail arrangement (larger phase shift); (e,f) hypothetical arrangements, which should not occur on polar surfaces. White regions correspond to the alkyl chains.
The assumption that the micellar arrangement shows higher phase shift than the “crystalline” double layer might seem surprising, but in the latter case the alkyl chains point outward, and they should be easier to penetrate than the hydrogen bond stabilized gluconamide headgroups. To prove the given assignment, the sample was heated to 120 °C and slowly cooled to room temperature. Figure 5a shows a phase image as obtained immediately after cooling. The domain with the higher phase shift dominates. This change in phase shifts can be explained by the rearrangement depicted in Figure 4c,d, but not by Figure 4e,f because in the latter case the phase shift should become smaller. After repeated scanning in the tapping mode for 2 h, islands of the primary head-to-tail domain reappeared (Figure 5b). The sample was then left standing
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Figure 5. AFM phase images of a double layer of 1 previously heated to 120 °C. (a) After 1 scan; (b) after 15 scans; (c) after 2 days without scanning; (d) height histogram, revealing a height difference of 3.7 nm corresponding to a double layer (the lower peak corresponds to the silicon surface, the higher peak to the double layer).
for 2 days. Figure 5c shows that the domain with the higher phase shift still dominates, but on the edges of the bilayer the size of the domains with the lower phase shift had increased. The upper layer had rotated to attain the thermodynamically more stable head-to-tail arrangement. At the edges, it is not necessary for the hydrophilic headgroups to pass the hydrophobic region. The rearrangement of the bilayer therefore starts on the edges and slowly continues to the inside. Tapping, however, also induces singularities within the bilayer resulting in the observed islands. A possible mechanism for the island formation might be that the tip penetrates into the bilayer and induces adsorbed molecules to flip over. The height histogram in Figure 5d shows the typical height of a bilayer of 1 of 3.7 ( 0.1 nm. This height is unaffected by repeated scanning. To estimate the reliability of the phase shift measurements, we briefly discuss Figure 5c as an example. Except for a small area on the left-hand side, all of the area is covered by a bilayer of 3.7 nm thickness. The dark area corresponds to the low phase shift as induced by the soft alkyl chains on the surface. The brighter areas, on the other hand, are hard gluconamide areas. Going from one domain to the other resulted in identical phase shifts (soft: 5° ( 3°, hard: 15° ( 3°) for either of the domains and identical phase shift differences (10° ( 3°). Other AFM images, e.g., Figure 5a,b, yield different absolute values of phase shifts, but the same trends remain detectable. Summary and Conclusion Amphiphiles with chiral or only slightly hydrophilic headgroups, e.g., gluconamides or methyl esters, often crystallize in head-to-tail fashion. Supramolecular assembly in water, on the other hand, is always induced by the hydrophobic effect and leads to the tail-to-tail arrangement. Upon decomposition of such assemblies on solid surfaces, three types of bilayers may thus result: soft (alkyl chains out) or hard (headgroups out) head-totail, or soft head-to-head or hard tail-to-tail bilayers (see Figure 4c-f). If the subphase is polar only structures in Figure 4c,d occur.
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The neutral carbohydrate surface already provides strong differences in stiffness presumably depending on the different geometry of hydrogen bond chains. One may predict that charged surfaces, e.g., carboxylates, should produce even larger differences, especially if pH-changes lead to reversible formation and disruption of hydrogen bond chains.
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Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (SFB 312 “Vectorial Membrane Processes” and SFB 348 “Mesoscopic Systems”), the Fonds der Deutschen Chemischen Industrie, and the FNK of the Free University is gratefully acknowledged. LA0002684
