Langmuir 1995,11, 13-15
13
Divalent Cation-Cyclodextrin Interactions at the Air-Water Interface. A Three-Stage Process M. Eddaoudi,? A. Baszkin,$ H. Parrot-Lopez,? M. M. Boissonnade,$and A. W. Coleman*?+ URA 1843, CNRS, Laboratorie de Chimie Organique, Centre &Etudes Pharmaceutique, Universitt Paris Sud, 92290 Chatenay-Malabry, France, and URA 1218, CNRS, Physico Chimie des Surfaces, Centre &Etudes Pharmaceutique, Universitt Paris Sud, 92290 Chatenay-Malabry, France Received April 11, 1994. In Final Form: July 19, 1994@ The interaction,at the air-water interface,between heptakis-6-O-(dimethyl-tert-butylsilyl)-/3-cyclodextrin and calcium chloride has been studied using Langmuir isotherms and surface potential measurements. The results show that at CaClz concentrations below 5 x M there is coordination of the Ca2+cations with bridging of cyclodextrin units leadin to a more compact and stable monolayer and a decrease in apparent molecular area from 278 to 257 lf2. For Ca2+concentrations between 5 x and 5 x M there is further coordination at the macrocyclic rim with inclusion of the anion accompanied by monolayer expansion and a decrease in monolayer stability. For CaClz concentrations above 5 x 10-l M there is inclusion of the cation with expulsion of anions and water from the macrocyclic cavity as shown by large changes in the surface potential and by the lack of change in apparent molecular area. Calcium is a large divalent cation (0.99 A radius) of extreme biological imp0rtance.l While present in extracellular fluids in relatively high concentration ( M) within the cytoplasm, its concentration is highly controlled (lo-’ M). Obviously it is of major importance in biomineralization,2 but Ca2+ is also involved in cell-cell recognition processes as a part of lectidsaccharide int e r a c t i o n ~or ~ saccharidelsaccharide interactions4 and plays a role in the modification of the properties of biological membranes through interaction with the phosp h o l i p i d ~ .All ~ of these processes occur a t cell walls and show the importance of understanding the interactions of Ca2+ with interfaces in biological systems. Calcium polysaccharide interactions are widely used in the food industry for preparation of gels.6 There has recently been interest in the possibility of cyclodextrins acting as ion channels, either as transmembrane channels in vesicles7or speculatively for covalently linked tubules.8 However these propositions seem to contradict the evidence from crystallographic investigations of cyclodextrin-cation complexes in which the cation is always found exterior to the cavity but generally bound to the cycl~dextrin.~ We are interested in the construction of nanometric structures based on amphiphilic cyclodextrins1°and the possible use of these systems as “intelligent” vectors for t
URA 1843,CNRS, Laboratoire de Chimie Organique.
* URA 1218,CNRS, Physico Chimie des Surfaces.
Abstract published in Advance ACS Abstracts, November 15, 1994. (1)Fransto de Silva, J . J. R.; Clarendon, R. J. P. W. The Biological Chemistry of the Elements; Clarendon Press: Oxford, 1991;Chapter 10. (2)Mann, S. J. Chem. Soc., Dalton Trans. 1993,1. (3)Drickamer, Kurt J. B i d . Chem. 1988,263,9557-9560. (4)Stewart, R.J.;Boggs, J. M. Biochemistry 1993,32,10666-10674. ( 5 ) (a)Wilschut, J.;Scholma, J.; Eastman, S. J.;Hope, M. J.;Cullis, P. R. Biochemistry 1992,31,2629-2636. (b) Tampe, R.; Galla, H. J. Eur. J. Biochem. 1991,199,187-193. (c) Leckband, D. E.; Helm, C. A,; Israelachvili, J. Biochemistry 1993,32,1127-1140. (6)Painter, T. In The Polysaccharides;Aspinall, G .O.,Ed.; Academic Press: New York, 1983,Vol. 2,pp 195-285. (7)Pregel, M. J.; Jullien, L.; Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1992,31,1637. (8)Harada, A.; Li, J.; Kamachi, M. Nature 1993,364,516-518. (9)(a)Nicolis, I.; Charpin, P.; Villain, F.; De Rango, C.; Coleman, A. W. Acta Crystallogr. 1991,C45,1829-1833. (b) Charpin, P.; Nicolis, I.; De Rango, C.; Coleman, A. W. Acta Crystallogr. B, in press. (c) Saenger, W.; Betzel, C.; Hingerty, B.; Noltemeyer, M.; Weber, G.; Hamilton, J . A. J.Inclusion Phenom. 1983,1, 181-191. @
drug delivery using saccharide antennae as biologically recognizable systems.l’ It is thus necessary to understand the interaction of such systems with biologically active cations, such a s Ca2+,prior to their use in vitro or in vivo. Simultaneously such studies will be important for the subject of cyclodextrin-based ion channels. We wish to describe here the interaction of Ca2+ with a model membrane of the amphiphilic cyclodextrins, heptakis-60-(tert-butyldimethylsily1)-p-cyclodextrin( 1).lz This compound was chosen for a number of reasons, firstly the alkylsilyl ether group will not interact with Ca2+,secondly the Langmuir compression isotherm shows a clearly reproducible transition from monolayer to multilayer assemblies13 which allows measurement of stability of the monolayer, and thirdly the measured mean area a t this collapse (278 Az)is considerably larger than the calculated area for p-CD (220 A?, suggesting a facile interaction of water or ions between and with the cyclodextrin molecules forming the monolayer. In Figure 2 is given the compression isotherm of 1 a t the air-water interface,14the curve shows a first collapse at A = 278 Az,n = 47 mN m-l and a second collapse of the multilayer a t A = 70 A2,ll = 67.8 mN m-l. The curve is typical for cyclodextrin systems where the monomolecular layer is stabilized by the presence of a narrow impermeable hydrophobic layer parallel to the water subphase, as observed for the per-6-bromo,15 and per-6azido derivatives.16 In parts a and b of Figure 3 are shown respectively the mean apparent molecular areas and surface pressures observed a t the first collapse versus log(CaCl2 concentration in the subphase). (10)Sommer, F.;Minh-Duc, T.; Skiba, M.; Wousseidje, D.; Coleman, A. W. Supramol. Chem. 1993,3,17. (11)Parrot-Lopez, H.; Leray, E.; Coleman, A. W. Supramol. Chem. 1993,3,37-42. (12) Fugedi, P. Carbohydr. Res. 1989,192,366-369. (13)Zhang, P.Ph.D. Thesis, U. Paris Sud, 1992. (14)Compression isotherms of 1 were obtained with tridistilled water subDhase using a Lauda film balance: 7.533 x 1014molecules 1L-l of 1 inCHCl3; 15-pL was spread on the aqueous surface and the solvent allowed to evaporate during 30 min, a compression rate 2 cm min-l was used. The isotherms are strictly reproducible with surface pressure and area differences of 0.1% for the comparison of areas and surface pressures the values used are the average of a t least four experiments. (15)Nicolis, I.; Coleman, A. W.; Charpin, P.; Villain, F.; Zhang, P.; de Rango, C. J. Am. Chem. Soc. 1993,115,11596. (16)Munoz, M.; Parrot-Lopez, H.; Kassellouri, A.; Coleman, A. W. Pol. J. Chem. 1993,67, 1981.
0743-7~63/95/2411-0013$09.00/0 0 1995 American Chemical Society
14 Langmuir, Vol. 11, No.1, 1995
Letters
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Figure 4. p l dipole moment as a function of log[Ca2+1.
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100
300
200
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(A') Figure 2. Compression isotherm for hepatkis-6-O-tert-b~tyldimethylsily1);B-cyclodextrin,the kink point used for subsequent measurements is shown by an arrow. MOLECULAR AREA
mum of 257 Az at [Ca2+l= 5 x M; this is accompanied by a n increase in collapse pressure to 48 mN m-l. In the second stage there is now a n increase in apparent molecular area to a limit value of 274 Az a t [Ca2+]1 5 x M. Pressure initially decreases to 46 mN m-l a t M before increasing to a value of 47 mN m-l [Ca2+]= a t a concentration of 5 x M. In order to obtain further information on the nature of the process, the equilibrium surface potential'* of the monolayers was measured as a function of Ca2+concentration in the aqueous phase, Figure 4. The method used gives equilibrium values of the surface potential and is particularly suited to absorption studies a t the interface.lg Using equation pcli= A AVI12n
whereA is the area per molecule and AVis the maximum surface potential (mV),we obtainpl, the projection of the total dipole moment orthogonal to the interface in mD.20 The observed dependence mirrors that of A versus log[Ca2+luntil a value of 4 x lo-' M where there is a break -4
-3
-2 -1 tog[Ca2*1
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(18)Plaisance, M.; Ter-Minassian-Saraga, L.C. R. Acad. Sci. Paris
1970,270,1269-1272.
t
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c
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45 -4
-3
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Figure 3. (a)Apparent molecular area of ButMezSi-,9-CD.(b) Surfacepressure at the first kink point in Figure 2 as a function of log[Ca2+l.
Both plots show that interaction of Ca2+ions with the amphiphilic molecule 1 is a distinct multistage process, showing adsorption at different available 5ites.l' In the first stage there is a decrease in the apparent molecular area, with increasing Ca2+concentration, giving a mini(17)Birdi, K. s.Lipidand Biopolymer Monolayers at Liquid interface; Plenum Press: New York, 1989.
(19)Experimental surface potential measurements: Amphiphilic cyclodextrinswere spread from a solution of pure chloroform by means of a micropipet (Microman Gilson 25 pL) at the aqueous subphase surface. Aliquot8 of cyclodextrin were thus successivelydeposited for the surface potential measurements at constant area (11.7cm2). The measurements were performed a t 23 & 0.5"Cin thermostated enclosed chambers, to reduce airborne contamination. Prior to the surface potential measurements of deposited monolayers the water interface was cleaned by suction through a narrow pipet. The surface potential of spread cyclodextrin monolayers was measured as previously described.26 The Keithley Instruments Electrometer (Model 610C)was connected to two identical 241Amair ionizing emitting electrodes suspended at about 2-3 mm above the reference (left cell) and the measuring cell (right cell) as previously described. The two cells are connected by a liquid bridge for electrical continuity. The measured difference in surface potential (AV) is that between the surface potential of the measuring cell (VM)and the monolayer free reference cell (VR). In the presence of a cyclodextrinmonolayer,the surface potential of the system is given by the following equation: AV = V(M) - V(R). The potential jump at the water-air interface, AV, induced by formation of a monolayer, is defined by the Helmholz equation as AV = (EO)-lnpl, where n = 1/A is the number of molecules,A the mean area per molecule, and EO the permitivity in the vacuum. The quantity p~ is the effective dipole moment in the direction perpendicular to the surface. The formation of a monolayer brings about a change in surface potential which is proportional to the change of the vertical component of the dipole density of the spread molecule with respect to the pure water surface. In the case of cyclodextrinmonolayers the local contributions ofthe hydrophobic and hydrophilic parts of the CDs will both influence onto the total surface potential. The sensitivities of measurements were 1 mV in the 0-100 mV range and 10 mV in the 100-400 mV range. It was considered that equilibrium was established when the value of AVdid not change after 30 mn. All reported surface potential values are mean values of at least three measurements. The standard deviation of the mean never exceeded f 5 mV. (20)Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966.
Letters point with a rapid decrease in pl from 3540 mD a t 4 x 10-I M to 2680 mD at 5 x 10-1M and subsequently there exists a decrease in p l for higher Ca2+concentrations. This third event will be treated separately. We assume that the observed effects are due to the complexation of Ca2+and C1- with a cyclodextrin monolayer. In the first stage of complexation Ca2+behaves as a bridging ligand, as has previously been observed for interaction with phospholipids,21 as well as in the solid state for /3-CD C a c l ~ .If~1is considered to pack at the air-water interface as a series of cylinders, the Ca2+ion forms bridges between molecules of 1, in the interstitial zones. These sites must for a purely aqueous subphase be occupied by water molecules, holding the array together by hydrogen bondin to hydroxyl groups a t a typical distance of 2.7-2.9 f 2 2 Now Ca2+will bind the oxygen donor atoms of cyclodextrin at 2.3-2.6 A1giving rise to a diminution in the intermolecular distance and a contraction in the apparent molecular area. This is in fact observed with the effective molecular diameter of 1 decreasing from 18.8A (278 k) on a pure water subphase to 18.1A (257 A2) on a 5 x lop3M Ca2+subphase. This formation of a stabilizing network of bridging Ca2+ coordinated cations is supported by the collapse pressure (n)increasing to 48 mN m-l a t 5 x M implying an increased stability of the monolayer. We have previously observed that explusion of the water molecules from the interstitial sites in cyclodextrin monolayers leads to a tighter packing of the layers and to a decrease in pLe23 Figure 4 shows similar behavior. Such complexation will involve expulsion from the monolayer of a relatively small fraction of the total number of absorbed water molecules and will have in consequence a small effect on PL
Thus the first event in the complexation of Ca2+by 1 at the air-water interface involves occupation of external interstitial sites. While in conflict with the idea of cyclodextrins as molecular tunnels, such coordination may allow cyclodextrins to transport cations across membranes via external coordination. The second event is characterized, for concentrations of M, by a n increase in the CaCl2 greater than 5 x molecular area, a decrease in the layer stability, and changes in pl, consistent with a Langmuir absorption isotherm, for take-up of an anionic ligand. Two processes may be occurring: first more calcium ions occupyhydroxyl groups leading t o cation-cation remulsive effects and monolayer expansion; second chloride anions start to occupy the CD cavity. Inclusion or absorption of C1- is less strong than the interaction of Ca2+with the hydroxyl groups and thus there is sequential ion absorption: first Ca2+in event 1 and now C1- uptake. (21)Liao, M. J.; Prestegard, J. H. Biochim. Biophys. Acta 1981,645, 149. (22)Jeffrey, G. A.;Saenger, W. Hydrogen Bonding in Biological Structures; Springer-Verlag: Berlin, 1991;Chapter 21.
Langmuir, Vol. 11, No. 1, 1995 15 340 r
m I
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,
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.
,
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.
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The existence of a third event in Ca2+-1 interactions is seen clearly in the plot of p l versus log[Ca2+l(Figure 4) and also may be deduced from the collapse pressure (Figure 3b). A plot of AV, against log[Ca2+lis shown in Figure 5. The slopes of plots are positive for systems in which the overall charge of the monolayer is negative and negative for positively charged monolayers.24 The slope changes from +6.5 a t [Ca2+l= 4 x lo-' to -50.0 a t [Ca2+l = 5 x 10-1 M. This is accompanied with sharp decrease in pl and a slight increase in II,but no change in area occurs, suggesting that the event occurs within the cyclodextrin cavity. As noted before, a large contribution to the observedpl arises from associated water molecules. We assume now that in a third event, Ca2+ becomes coordinated within the cyclodextrin cavity expelling the included water and probably forming ion pairs with the included chloride ions. This will change the overall charge of the monolayer by complexation of included C1- or by expulsion of a water molecule present within the cavity OH- anions, as in the KOH c ~ m p l e x . ~ This suggests that amphiphilic cyclodextrins may serve as cation channels at elevated ion concentrations. The apparently abrupt change in coordination as a function of ion concentration suggests that their cavities may act as gated ion channels. In conclusion we have shown that the interaction of amphiphilic cyclodextrins with Ca2+is a complex threestage process. Initially Ca2+is coordinated in interstitial sites, a t higher concentrations this network of bridged molecules breaks down with Ca2+ coordinated on individual cyclodextrin molecules externally, and finally a t high concentration 2 5 x 10-1 M cations may be coordinated within the molecular cavity. LA940317A (23) Tchoreloff, P. C.; Boissonnade, M. M.; Coleman, A.W.; Baszkin, A. Submitted for publication in Langmuir. (24)Seimiya, T.; &to, T.; Miyasaka, H.; Ohbu, K.; Shirakawa, T.; Iwahashi, M. Chem. Phys. Lipids 1987,43, 161-177. (25)Baszkin, A.;Deyme, M.; Couvreur, P.; Albrecht, G . J. J.Bioact. Compat. Polym. 1989,4, 110.
