Incorporation of Amphiphilic Cyclodextrins into Liposomes as Artificial

Publication Date (Web): January 24, 2013 ... The cyclodextrins can act as host molecules for hydrophobic guest molecules, even when they are dispersed...
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Incorporation of Amphiphilic Cyclodextrins into Liposomes as Artificial Receptor Units Ulrike Kauscher,† Marc C. A. Stuart,§ Patrick Drücker,‡ Hans-Joachim Galla,‡ and Bart Jan Ravoo*,† †

Organic Chemistry Institute and ‡Institute of Biochemistry, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany § Biophysical Chemistry, Groningen Biomolecular Science and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands S Supporting Information *

ABSTRACT: In this article, we describe the introduction of amphiphilic β-cyclodextrins into liposomes to act as artificial receptor units. Using dynamic light scattering, dye encapsulation, and cryogenic transmission electron microscopy, we show that amphiphilic β-cyclodextrins can be mixed in any proportion with a typical mixture of phospholipids and cholesterol to provide stable, spherical, and unilamellar mixed vesicles. It is also possible to form giant unilamellar vesicles with mixtures of lipids and cyclodextrin. The permeability of the mixed vesicles increases with the percentage of cyclodextrin. The cyclodextrins can act as host molecules for hydrophobic guest molecules, even when they are dispersed at a low percentage in the vesicle membrane. It is shown that mixed vesicles can be decorated with carbohydratefunctionalized guest molecules, with photoresponsive guest molecules, and with dye-functionalized guest molecules. Taken together, it is demonstrated that the host−guest chemistry of amphiphilic cyclodextrins is fully compatible with a liposomal bilayer membrane and the advantages of each can be combined to give superior nanocontainers.

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INTRODUCTION Vesicles are a particularly interesting class of dynamic supramolecular structures that can be formed from a wide variety of amphiphilic molecules in aqueous solution.1−3 Liposomes are a subclass of vesicles composed exclusively of naturally occurring lipids (including cholesterol). Vesicles have been extensively investigated as nanocontainers for the targeted delivery of drugs.4 However, because of their similarity to biological cell membranes, vesicles have also been an extremely useful model system for investigating a broad variety of phenomena that are typical of cellular membranes.5 In recent years, we have investigated a versatile class of vesicles composed exclusively of amphiphilic cyclodextrins (CDs). CDs are cyclic oligomers of D-glucose that typically contain six, seven, or eight carbohydrate units in a macrocycle (six, α-cyclodextrin; seven, β-cyclodextrin; eight, γ-cyclodextrin).6 If the primary side of the CDs is functionalized with hydrophobic alkyl chains and the secondary side is functionalized with hydrophilic oligo(ethylene glycol) groups, then the resulting amphiphilic macrocycles form unilamellar bilayer vesicles in aqueous solution.7 In many respects, these vesicles are comparable to liposomes, but they have one outstanding feature that makes them particularly useful: CD vesicles can be modified with any desired surface functionality simply by adding a suitable guest molecule to the vesicle solution. This implies that no synthesis of functional amphiphiles is required and that functional guest molecules can be combined simply by adding a mixture of guest molecules © XXXX American Chemical Society

to the vesicles. As a result of highly specific host−guest inclusion, the guest molecules bind spontaneously in a high density to the CD vesicle surface. When appropriately functionalized guest molecules are used, it is possible to bind hydrophobic ions,8 zwitterions,9 polymers,10 ligands,11,12 carbohydrates,13−15 peptides,16 proteins,17 and DNA18 to the vesicles. As a consequence of host−guest interaction, it is possible to mediate the adhesion and aggregation of the vesicles,11,12,19,20 to change their shape,16 and to bind and release proteins21 and DNA.18 In this article, we investigate the properties of vesicles composed of a mixture of amphiphilic CD and lipids. Our investigations were guided by three questions. On one hand, it is our aim to verify whether such mixed vesicles can be prepared at all because it is well established that many lipids and also cholesterol can form strong inclusion complexes with CDs.22−27 For example, cholesterol and β-CD form a highaffinity 1:2 inclusion complex and β-CD can be used to extract cholesterol from lipid membranes.28 On the other hand, we reasoned that is not unlikely that even if mixed vesicles are formed, the cavities of the amphiphilic CDs would be mostly occupied by cholesterol and/or lipid so that the versatile host Special Issue: Interfacial Nanoarchitectonics Received: November 14, 2012 Revised: January 22, 2013

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Figure 1. Schematic representation of vesicles prepared from a mixture of amphiphilic β-CD and DOPE, DOPC, and cholesterol.

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chemistry of pure CD vesicles could be severely hampered. Finally, we were interested in the permeability of mixed vesicles. From previous work, we know that CD vesicles can encapsulate dyes, but they are rather leaky compared to typical liposomes, in particular if the amphiphilic CD is modified with n-dodecyl groups.7,8 Content leakage from CD vesicles can be suppressed if n-hexadecyl rather than n-dodecyl chains are introduced onto the CD.8 However, it should be emphasized that CD vesicles with n-hexadecyl chains have a phasetransition temperature (Tm) of 48 °C, which complicates their handling.7,8 It would be worthwhile to determine to what extent the composition of a mixed vesicle of CDs and lipids determines the permeability of the vesicles. It is widely documented in the literature that CDs can act as artificial ion channels in liposomal membranes,29,30 but it is unlikely that large, polar molecules pass through the cavity of a CD. Thus, it was our hypothesis that mixed vesicles would be less leaky than pure CD vesicles. To test our hypotheses, we performed a set of experiments on a model system of bilayer vesicles composed of amphiphilic β-CD (substituted with seven hydrophobic n-dodecyl chains on the primary side and seven hydrophilic oligo(ethylene glycol) groups on the secondary side) and a typical mixture of lipids dioleyl phosphatidyl ethanolamine (DOPE), dioleyl phosphatidyl choline (DOPC), and cholesterol. Large and giant vesicles were prepared from hydrated lipid films. The structure of these vesicles is illustrated in Figure 1. The properties of these vesicles were investigated using dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), confocal laser scanning microscopy (CLSM), and dyeencapsulation experiments. In addition, we investigated the functionalization of the mixed vesicles with a set of guest molecules available from previous work.15,18 These guest molecules have azobenzene or adamantane units that bind to the CDs on the surface of the vesicles. We demonstrate that decorating the vesicles with tailor-made guest molecules can lead to selective binding of proteins, to photoresponsive vesicle aggregation, and to selective fluorescent labeling of the vesicles.

MATERIALS AND METHODS

Materials. Chemicals were used as received from Acros Organics (Schwerte, Germany) or Sigma-Aldrich Chemie (Taufkirchen, Germany) without further purification. The synthesis and analysis of amphiphilic β-CD and guests 1 and 2 were performed as described in the literature.8,20,13 The synthesis of guest 3 is described in the Supporting Information. Preparation of Vesicles. Unilamellar vesicles with an average diameter of 80−100 nm were obtained by the hydration of a lipid film and extrusion with a Liposofast manual extruder through a polycarbonate membrane with a pore size of 100 nm. The lipid film was obtained by evaporating a chloroform solution containing amphiphilic β-CD mixed with the desired amount of lipid mixture (DOPE/DOPC/cholesterol 2:1:1). Giant unilamellar vesicles (GUVs) were formed using the electroformation method.31 Amphiphilic β-CD was combined with the desired amounts of a lipid mixture to make a 1 mM solution of amphiphiles in chloroform. Sixteen microliters of the chloroform solution was spread on the electrode slides coated with ITO, and chloroform was evaporated in a vacuum oven at 50 °C. The film was hydrated with buffer solution (1 mM HEPES, 100 or 300 mM sucrose, pH 7.5). An alternating electric field was applied at 1 V and 10 Hz at 50 °C for 1 h. The GUVs were investigated with a Leica DMRE with a TCS SL scanning unit (Leica Microsystems Heidelberg GmbH, Mannheim, Germany). Buffer solution with 300 mM sucrose was used to improve the visualization of the GUVs by CLSM. Dynamic Light Scattering. Size distribution measurements were carried out at room temperature by using Malvern instrumentation. The concentration of amphiphiles was 0.2 mM. Cryogenic Transmission Electron Microscopy. Electron microscopy was performed according to standard procedures. In brief, a few microliters of a vesicle solution (1 mM) was placed on a glow-discharged holey-carbon-coated grid (Quantifiol 3.5/1) and vitrified in ethane using a vitrobot (FEI, Eindhoven, The Netherlands). Vitrified grids were observed in a Philips CM120 electron microscope operating at 120 keV using a Gatan cryostage (model 626). Images were recorded on a slow-scan CCD camera (Gatan) under low-dose conditions. Dye Encapsulation. A 20 mM solution of sulforhodamine B was prepared in buffer solution (20 mM HEPES, 90 mM NaCl, pH 7.2). Vesicles were obtained by the hydration of a lipid film with this buffer solution and sonification at 40 °C for 20 min. The concentration of amphiphiles was 1 mM. The solution of encapsulated guest molecules (500 μL) was loaded onto a Sephadex G-50 size-exclusion column with buffer solution as eluent. A fraction of around 2 mL was collected B

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and filled up to 5 mL to end up with an amphiphile concentration of around 0.1 mM. The fluorescence spectrum of the fraction was measured using a JASCO FP6500 Spectrofluorometer at an excitation wavelength of 490 nm. A reference sample was prepared by the addition of 0.1% Triton X-100. Optical Density Measurements. Aggregation measurements were performed in 1 mL small-volume disposable PMMA cuvettes at 400 nm (600 nm in the case of guest 2) by using a Uvikon 923 double-beam photospectrometer. Agglutination measurements were recorded at 23 °C in HEPES buffer (20 mM, pH 7.45) containing 1 mM CaCl2 and 1 mM MnCl2. Reagents were added in the following order: to a 1 mL vesicle solution (0.2 mM amphiphile concentration) the required amount of guest solution is added after 1 min. In the experiments with guest 1, 10 μL of a ConA stock solution (10 mg/mL in buffer-solution) is added after 2 min. In the experiments with guest 2, photoisomerization was achieved by irradiation at 350 nm (to obtain cis from trans), followed by irradiation at 455 nm (to obtain trans from cis).

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RESULTS AND DISCUSSION The vesicles that were used in the following experiments were obtained via the formation of a lipid film containing different ratios of amphiphilic β-CD and a mixture of lipids. The lipid mixture consisted of 50 mol % DOPC, 25 mol % DOPE, and 25 mol % cholesterol and remained constant in all experiments. The mixture of lipids was selected to provide a fluid membrane at room temperature that should function as a soft matrix for amphiphilic β-CD. The percentage of CD in the vesicles indicated in the following text refers to the molar ratio of amphiphilic β-CD and total lipid (DOPC plus DOPE plus cholesterol) so that, for example ,“50 % CD” implies a molar ratio of CD/DOPC/DOPE/cholesterol of 50/25/12.5/12.5. In the first set of experiments, unilamellar vesicles were prepared in HEPES buffer by hydration of the lipid film, followed by extrusion of the resulting dispersion. The vesicle solutions were diluted to a total amphiphile concentration of 0.2 mM. DLS results show that the vesicles prepared by extrusion through a 0.1 μm polycarbonate membrane have an average diameter of 80−100 nm, irrespective of the composition of the vesicles (Figure 2). Upon addition of 100 μL Triton X100 (0.1 wt %) to 1 mL of vesicle solution, the vesicles are instantaneously solubilized and micelles can be predominantly observed by DLS. We note that a small fraction of vesicles persists under these conditions. To obtain further information about the structure of the vesicles, cryo-TEM was performed. In this case, the mixed vesicles contained 50% amphiphilic β-CD, and the total amphiphile concentration was 1 mM. As depicted in Figure 3A, the formation of unilamellar vesicles is evident. These vesicles are smooth and spherical, their diameter is ca. 100 nm (consistent with DLS), and the thickness of the bilayer is about 5 nm (consistent with the length of the amphiphiles). From the cryo-TEM images, we did not find evidence of defects or phase separation in the membrane. Also, we did not detect any bilayer fragments or micellar aggregates. In summary, cryo-TEM indicates that homogeneous, intact unilamellar bilayer vesicles are formed. In comparison, unilamellar vesicles composed exclusively of amphiphilic β-CD present a somewhat irregular surface (Figure 3B). Further evidence of the structure of the mixed vesicles was obtained by the encapsulation of fluorescent dye sulforhodamine B in the aqueous core of the vesicles. The vesicles were prepared in a solution of sulforhodamine B at a self-quenching concentration (20 mM) and were separated from the free dye

Figure 2. Size distribution of 0.2 mM mixed vesicle solutions in water with varying percentages of amphiphilic β-CD and lipids before and after lysis of the vesicles by the addition of 0.1% Triton X100.

Figure 3. Cryo-TEM images of vesicles consisting of (A) a 1:1 mixture of amphiphilic β-CD and lipids and (B) 100% amphiphilic β-CD (no lipids).

by gel filtration on a column of Sephadex G50. After gel filtration, the vesicles were diluted to ca. 0.1 mM total amphiphile concentration, and the sulforhodamine B fluorescence intensity was measured over time and finally after the addition of Triton X100. When only a modest percentage of CD was used, the encapsulation was successful and the difference in the fluorescence signal between the sample with and without Triton X100 was significant (Figure 4). Vesicles containing 10 mol % CD retain most of the dye for more than 1 week. Vesicles containing 30 mol % CD can encapsulate a significant amount of dye, but it leaks out over a period of C

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suppressed if n-hexadecyl rather than n-dodecyl chains are introduced onto the CD.8 On the basis of these observations, we propose that the leakage of the dye from the CD vesicles does not occur via the cavities of the CD molecules (channel mechanism)29,30 but is more likely a consequence of the increased permeation through a thinner (n-dodecyl) membrane compared to that through a thicker (n-hexadecyl) membrane. Having established that mixed and nonleaky unilamellar bilayer vesicles can be prepared from a mixture of amphiphilic cyclodextrins and lipids, we investigated the host−guest interaction occurring on the surface of the mixed vesicles. To this end, we selected a number of functional guest molecules from our previous work. These guest molecules and their mode of operation are illustrated in Figure 6. Guest 1 is a conjugate of adamantane and mannose. We have shown that this compound can bind to CD vesicles via host−guest inclusion of the adamantane unit while it binds to the lectin Concanavalin A (Con A) through the mannose unit.15 Because the lectin is tetravalent, the addition of ConA to CD vesicles decorated with guest 1 leads to the agglutination of the vesicles.15 Guest 2 is a water-soluble dimer of azobenzene. If the azobenzenes are in the trans state, then guest 2 acts as a noncovalent cross-linker that causes the aggregation of the CD vesicles.19 If it is photoisomerized to the cis state, the azobenzenes are released from the CD vesicle surface and the vesicles redisperse.19 Finally, guest 3 is a conjugate of adamantane and rhodamine (details in Supporting Information). We expect that this compound will bind to the CDs on the vesicle surface and hence function as a noncovalent fluorescent marker for the vesicles. The host−guest interaction of the mixed vesicles and guests 1−3 was investigated by using optical density measurements, DLS, and CLSM . In accordance with our previous findings on CD vesicles, the addition of guest 1 followed by lectin Con A also leads to the agglutination of the mixed vesicles. Agglutination was monitored by optical density measurements, and the experiments were repeated several times with vesicles containing different percentages of amphiphilic β-CD (Figure 7). The concentration of guest 1 added in each experiment was varied with the percentage of CD so that it would be sufficient to fill all CD cavities on the outer membrane surface (i.e., the guest concentration was always half the CD concentration). The total amphiphile concentration and the lectin concentration were kept constant in all experiments. It can be seen from Figure 7 that agglutination increases with an increase in CD content of the mixed vesicles. It is reasonable to assume that with a higher CD content the surface density of mannose also increases, which leads to faster and more extensive agglutination. These observations are entirely consistent with our previous observations on carbohydrate-decorated CD vesicles.13−15 Even vesicles containing only 10% amphiphilic β-CD display a remarkable rate and extent of agglutination. It should also be emphasized that no agglutination whatsoever is detected if the vesicles do not contain any CD (i.e., pure DOPC/DOPE/ cholesterol liposomes). These experiments provide compelling evidence that agglutination is strictly a result of the specific host−guest interaction of mannose-functionalized guest 1 on the CD vesicle surface, so it must be concluded that the CD cavities are available for host−guest interaction even in a membrane composed mostly of lipids and cholesterol. Also, any nonspecific interaction can be ruled out because no agglutination is observed in the absence of CD.

Figure 4. Encapsulation of sulforhodamine in vesicles with varying percentages of amphiphilic β-CD. All experiments were performed at room temperature. (A) Fluorescence spectra taken after different times and after the addition of 0.1% Triton X100 of vesicles containing 10 mol % amphiphilic β-CD. (B) Normalized fluorescence intensity I(t)/ Imax plotted with time for vesicles containing different percentages of amphiphilic β-CD.

about 1 week. Vesicles with 50 mol % CD can encapsulate some dye, but it quickly leaks out within a few minutes after gel filtration. The same observation was made for vesicles composed of only CD.8 We conclude that mixed vesicles can encapsulate and retain cargo in their aqueous interior provided the CD content is limited to around 10 mol %. Using the Lambert−Beer equation, we calculated how much sulforhodamine was entrapped in the vesicles. The obtained data are plotted in Figure 5. The less CD used for the vesicles, the more sulforhodamine entrapped. Thus, the leakage of dye depends on the concentration of amphiphilic cyclodextrin in the vesicles. We also note that the leakage from CD vesicles can be

Figure 5. Concentration of encapsulated sulforhodamine per amphiphile for different percentages of amphiphilic β-cyclodextrin in vesicles. All experiments were performed at room temperature. D

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Figure 6. Molecular structure and mode of operation of functional guest molecules 1−3.

Figure 7. Optical density measurements of the agglutination of mixed vesicles with varying percentages of amphiphilic β-CD in the presence of monovalent guest 1 and lectin ConA. The surface coverage of guest 1 is gradually reduced with the concentration of CD on the vesicle surface. Concentrations: total amphiphile = 0.20 mM, amphiphilic βCD = 0−0.14 mM, guest 1 = 0−0.07 mM, and ConA = 0.1 mg/mL.

Further strong evidence for the thesis that the CD cavities on the surface of the mixed vesicles are available for host−guest complexation is provided by experiments with photoswitchable guest 2. Guest 2 has two azobenzene functions that are able to form inclusion complexes with CD cavities, provided that azobenzene is in the trans state. Indeed, aggregation is observed when guest 2 is added to vesicles composed of 50 mol % CD and a 50 mol % lipid mixture because of the noncovalent crosslinking of the vesicles (Figure 8). Subsequent irradiation with UV light at a wavelength of 350 nm for 30 min leads to the photoisomerization of the azobenzenes to the cis state and the release of guest 2 from the vesicle surface. As a consequence, the vesicles are redispersed. Irradiation with visible light (455 nm) for 30 min leads to the photoisomerization of azobenzene to the trans state and reaggregation of the vesicles. The photoresponsive aggregation−dispersion can be cycled many times. These results once more confirm that mixed vesicles containing amphiphilic CD are available for host−guest chemistry, much like vesicles composed only of amphiphilic CD.19 Finally, we also investigated the formation of giant unilamellar vesicles (GUVs) and their interaction with fluorescent guest 3. GUVs were prepared by electroformation in 1 mM HEPES buffer containing 100 mM sucrose. A potential of 1 V and a frequency of 10 Hz were applied for 1 h at 50 °C. Vesicles were prepared with different percentages of

Figure 8. Photoresponsive aggregation and dispersion of mixed vesicles containing 50 mol % amphiphilic β-CD. (Top) Optical density measurements at a wavelength of 600 nm. (Bottom) Maximum of photoresponsive aggregation and dispersion for several cycles for the following concentrations: total amphiphile = 0.20 mM, amphiphilic βCD = 0.10 mM, and guest 2 = 0.025 mM.

amphiphilic β-CD and investigated by optical microscopy (Figure 9). The formation of large numbers of GUVs was easily attained for mixtures with 10, 30, and 50 mol % CD. For higher percentages of CD (70 or 100 mol %), the formation of GUVs was obviously less efficient because the number of formed GUVs decreases and dense aggregates of amphiphiles are observed across the slides. We attribute this to the nonionic nature of the CDs, which therefore respond less to the alternating electric field compared to zwitterionic phospholipids DOPC and DOPE. Nevertheless, GUVs could be observed for each mixture of components that was investigated. The GUVs are spherical and homogeneous, indicating the complete mixing of the amphiphiles in the bilayer membrane. E

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a negative control with rhodamine lacking the adamantane unit (Figure S1 in the Supporting Information) confirms that in this case rhodamine remains in solution and does not bind to the membrane. The images in Figure 10 also very clearly highlight the uniform distribution of rhodamine fluorescence on the GUV surface, irrespective of the vesicle composition. Because guest 3 can bind only the amphiphilic CDs on the vesicle surface, the uniform fluorescence intensity is direct evidence for the homogeneous mixing of the CDs and the lipids in the GUVs. Thus, these experiments rule out that microdomain formation or even phase separation of CDs and lipids occurs in the mixed bilayer membranes, irrespective of the CD content.

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CONCLUSIONS We have presented a novel type of vesicle that has densely packed lipid bilayer membranes with embedded CD host molecules that can bind guest molecules through the formation of inclusion complexes. Our results show that amphiphilic CDs can be incorporated into liposomes at any desired percentage without affecting the structure and stability of the vesicles. Large and giant unilamellar vesicles can be prepared using conventional methods. The permeability of the liposomes is strongly increased when the percentage of CD is increased. Nevertheless, a dye remains encapsulated in the mixed vesicles for many days if the CD content is limited to 10%. Using a set of three different guest molecules, it was demonstrated that the CDs on the surface of the mixed vesicles are available for host− guest chemistry. This is a remarkable finding because intuitively one would expect that both lipids and cholesterol will compete for inclusion in the CDs. We speculate that the reason that the CDs are still available for complexation resides in the fact that they are amphiphilic and embedded in the membrane so that the thermodynamic driving force for the inclusion of lipids and cholesterol is very small (comparable to complexation in an organic rather than an aqueous solution). It could be argued that the combination of a lipid mixture with Tm below room temperature and a CD with n-dodecyl chains enables the best of both: host−guest chemistry due to the CD and low permeability due to the lipids. Taken together, these results indicate that the host−guest chemistry of CDs can also operate effectively in considerably more complex environments than purely CD-based self-assembled or polymer systems, which opens up promising opportunities for the design of nanovectors for targeted drug delivery. In particular, we envisage that such nanovectors can be obtained by attaching a (combination of) targeting units to the outside of the vesicle via host−guest interaction with the CDs and encapsulating a drug in the vesicle interior.

Figure 9. Giant unilamellar vesicles prepared from a lipid film containing (A) 30, (B) 50, (C) 70, and (D) 100 mol % CD by electroformation in 1 mM HEPES buffer containing 100 mM sucrose.

The molecular recognition of GUVs containing 10, 30, and 50 mol % amphiphilic CD was demonstrated by the addition of fluorescent guest 3. The CLSM images in Figure 10 show GUVs with a high density of rhodamine on the vesicle membrane, indicating that guest 3 binds selectively to the vesicle surface through the host−guest interaction. Importantly,

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and analysis of guest 3 and additional CLSM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 10. Giant unilamellar vesicles prepared from a lipid film containing 10 (top), 30 (middle), and 50% CD (bottom). Fluorescent staining was obtained by the addition of guest 3. (Left) CLSM images of a reconstructed 3D image of a vesicle hemisphere from confocal image sections. (Right) CLSM single image slice through the vesicle equator.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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Competitive Ternary Supramolecular System. Chem.Eur. J. 2011, 17, 10297−10303. (21) Samanta, A.; Stuart, M. C. A.; Ravoo, B. J. Photoresponsive Capture and Release of Lectins in Multilamellar Complexes. J. Am. Chem. Soc. 2012, 134, 19909−19914. (22) Debouzy, J. C.; Fauvelle, F.; Crouzy, S.; Girault, L.; Chapron, Y.; Goschl, M.; Gadelle, A. Mechanism of α-Cyclodextrin Induced Hemolysis. 2. A Study of the Factors Controlling the Association with Serine-, Ethanolamine-, and Choline-Phospholipids. J. Pharm. Sci. 1998, 87, 59−66. (23) Leventis, R.; Silvius, J. R. Use of Cyclodextrins to Monitor Transbilayer Movement and Differential Lipid Affinities of Cholesterol. Biophys. J. 2001, 81, 2257−2267. (24) Anderson, T. G.; Tan, A.; Ganz, P.; Seelig, J. Calorimetric Measurement of Phospholipid Interaction with Methyl-β-Cyclodextrin. Biochemistry 2004, 43, 2251−2261. (25) Puskás, I.; Barcza, L.; Szente, L.; Csempesz, F. Features of the Interaction between Cyclodextrins and Colloidal Liposomes. J. Incl. Phenom. Macrocyclic Chem. 2006, 54, 89−93. (26) Puskas, I.; Csempesz, F. Influence of Cyclodextrins on the Physical Stability of DPPC-Liposomes. Colloids Surf., B 2007, 58, 218− 224. (27) Hatzi, P.; Mourtas, S.; Klepetsanis, P. G.; Antimisiaris, S. G. Integrity of Liposomes in Presence of Cyclodextrins: Effect of Liposome Type and Lipid Composition. Int. J. Pharm. 2007, 333, 167−176. (28) Zidovetzki, R.; Levitan, I. Use of Cyclodextrins to Manipulate Plasma Membrane Cholesterol Content: Evidence, Misconceptions and Control Strategies. Biochim. Biophys. Acta 2007, 1768, 1311−1324. (29) Pregel, M. J.; Jullien, L.; Lehn, J. M. Towards Artificial Ion Channels: Transport of Alkali Metal Ions across Liposomal Membranes by “Bouquet” Molecules. Angew. Chem., Int. Ed. 1992, 31, 1637−1640. (30) Jog, P. V.; Gin, M. S. A Light-Gated Synthetic Ion Channel. Org. Lett. 2008, 10, 3693−3696. (31) Angelova, M. I.; Dimitrov, D. S. Liposome Electroformation. Faraday Discuss. 1986, 81, 303−311.

ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft (SFB 858) for financial support.

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dx.doi.org/10.1021/la3045434 | Langmuir XXXX, XXX, XXX−XXX