Langmuir 1988,4, 215-217
215
due to the H20 deformation vibration.32@ The other two large peaks at 2568 and 1970 cm-' are characteristic of the present sample and were not distinguished on CaF,. When the pretreatment temperature is raised, all these peaks are weakened, especially in the intensity of the band a t 3000-3400 cm-'; in other words, the higher the pretreatment temperature, the smaller the amount of surface rehydration, corresponding to the decrease in H20 content. In addition, it is interesting to conclude that the molecular H 2 0 is strongly adsorbed on the surface as is seen from the peak a t 1656 cm-' and that the characteristic peaks a t 2568 and 1970 cm-l prove also the presence of the molecularly and strongly adsorbed H20.34 Table I shows the relation between the amounts of physisorbed and chemisorbed H 2 0 molecules, the latter being defmed as the amount remaining on the surface after the evacuation at Torr for 4 h a t room temperature. V, is the monolayer capacity of physisorbed H20, which is calculated from the second adsorption isotherm. The amount of chemisorbed H20, v h , was obtained as follows.
The chemisorbed H 2 0 denoted as C in Figure 4 was not reproduced, suggesting the chemisorbed H20 to be in inner layers of the crystal, while that denoted as A and B was reproduced by exposing the sample to saturated H 2 0 vapor, which indicates that these two kinds of adsorbed H 2 0 exist just on the surface. Thus, v h was estimated by subtracting the H 2 0 content of the peak C from the total amount of chemisorbed H 2 0 (Figure 4). The ratio V,/ Vh is very large, ranging from 1.32 to 2.35 on the surface on which the 2D condensation of H20 occurs. On the surface of metal oxides, a similar ratio was calculated, but v h was taken as the number of surface hydroxyls that were formed by dissociative adsorption of H,O; the ratio was found to approximately 0.5 or 1.0; the latter being often concerned with the 2D condensation of H20.2J2 In the case of CaF2,1ethe existence of the molecularly adsorbed H 2 0 was not concluded, because the characteristic IR peaks near 2568 and 1970 cm-l were inexplicit. However, the surface properties of CaF2 are found to be similar to those of SrF,, as discussed above. Therefore, it is reasonable to conclude that for the most part the adsorbed H 2 0 can be chemisorbed molecularly also on CaF,. If we take this standpoint, the ratio V,/ v h will be very large also on CaF,, since the original value should be doubled.18 Such a large ratio suggests that the 2D condensation of H20 occurs on a surface on which adsorbed H 2 0 molecules exist together with other kinds of surface species. Further development of the surface model remains to be done.
(32) Hair, M. L. Infrared Spectroscopy in Surface Cherniatry;Marcel Dekker: New York, 1967. (33) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco, 1960. (34) Kuroda, Y.; Morimoto, T. Langrnuir, in press.
Acknowledgment. The present work was partly supported by a Grant-in-Aid for Scientific Research, NO. 5747007, from the Ministry of Education, Science, and Culture of the Japanese Government. Registry No. SrF2,7783-48-4; Ar, 7440-37-1; HzO, 7732-18-5.
Table I. Relation between Amounts of Physisorbed and Chemisorbed HzO on SrFz
pretreatment temn
oc
25 150 500 600
VP
molecules nm-2 10.63 10.60 10.22 10.06
vh,
lvh
molecules nm-2 H,&H,O 8.06 6.07 4.35 3.37
1.32 1.75 2.35 2.99
Anesthetic Effect and a Lipid Bilayer Transition Involving Periodic Curvature K. Larsson Chemical Center, University of Lund, Box 124, S-221 00 Lund, Sweden Received February 5, 1987. In Final Form: August 18, 1987 The presence of extremely low concentrations of chloroform, halothane, or ethyl ether induces a phase transition from a planar bilayer structure (La) to a cubic phase in membrane model systems. Phosphatidylcholine lipids in the form of aqueous dispersions of the Laphase (liposomal dispersions)were examined, and the new phases formed were characterized by X-ray diffraction. It is proposed that the effect of inhalation anesthetics is due to a corresponding phase transition in the axon membrane. It is assumed that the cubic phase consists of a lipid bilayer forming an infinite periodic minimal surface separating two water channel systems and that there also exists a two-dimensionalanalogue to this type of structure. The two-dimensionalperiodic minimal surface structure of the lipid bilayer can be expected to block the sodium channels due to conformational effects induced. Increased disorder of the hydrocarbon chains of the bilayer toward the methyl end group region by an anesthetic agent is discussed as the driving force of the phase transition. The pressure antagonism against the anesthetic effect, the effect of different hydrocarbon chain composition of the lipid, and the relative potency of chloroform and ethyl ether can be explained according to the proposed mechanism of the anesthetic effect.
Introduction The effect of inhalation anesthetics is generally considered to be due to a change in the lipid bilayer of the neuronal membrane (cf. ref 1). The mechanism behind (1)Winter,
P. M.; Miller, J. N. Sci. Amer. 1985, 252, 94. 0743-7463/88/2404-0215$01.50/0
blocking of the sodium channels induced by this change in the b i d region ofthe membrane, however, is not known. Evidence is given in the present paper indicating that the action of anesthetics is due to a phase transition from a planar to a periodically curved lipid bilayer. The general structure of cubic lipid-water phases is consistent with a lipid bilayer forming an infinite periodic 0 1988 American Chemical Society
216 Langmuir, Vol. 4, No. 1, 1988
Larsson
The space group Im3m is indicated by the intensity distribution.2 When halothane was added in the same way as chloroform to the liposomal dispersion, the phase transitions were about the same as those shown in Figure t(‘0 1. The corresponding cubic phase showed a unit-cell axis of 88 A. The corresponding phase behaviors of chloroform and Yo CHCl3 ( w l w ) dispersions of DOPC and SBPC were also compared. Both lipids showed a similar approximate region of chloroform Figure 1. Effect of the addition of chloroform on an EYPC dispersion of the La phase (liposomes)in water. The concentration concentration, when the L, phase is transformed into the region is shown where a visible precipitate of the cubic phase (C) cubic phase, as was evident from formation of a visible is formed and also the approximate limit where all of the L, phase precipitate without birefringence. The unit-cell axis of the has been transformed. SBPC phase formed by chloroform and coexisting with the L, phase was 93.4 A (correspondingto space group Im3m). minimal surface with continuous water channel systems The effect of chloroform on a dispersion of DPPC was on both sides.2 The minimal surface, with zero average observed in the polarizing microscope a t 50 “C, in order curvature everywhere, is free of intersections. Thus this to be above the hydrocarbon chain melting temperature. type of structure is closely related to the lamellar liquid It was found that about 50% higher chloroform concencrystalline phase, L,, consisting of infinite lipid bilayers trations was needed in order to obtain the transition into separated by water. The polar heads of the bilayer form the cubic phase. parallel surfaces on both sides of the minimal surface. In order to compare another local anesthetic agent, the There are three fundamental types of such cubic minimal effect of ethyl ether on the transitions shown in Figure 1 surface structurs, and corresponding X-ray data have been was examined. The concentration when a visible precipobserved.2 itate of the cubic phase could be detected was shifted The possibility of the existence of a two-dimensional upwards, to a concentration of about 0.7% (w/w) of ethyl analogue to the three-dimensional minimal surface strucether. ture has recently been a n a l y ~ e d .The ~ presence of proteins If an aqueous system of membrane lipids undergoes a in membranes makes possible the formation of the same transition from a planar bilayer structure (corresponding type of periodic curvature, with zero average curvature in to the L, phase) to a cubic phase with a bilayer structure all points along the center of the bilayer. consisting of periodic minimal surface, it should be exIn a preliminary note a drastic effect by chloroform on pected that the bilayer of the corresponding membrane the thermal phase transition from the L, to the cubic phase tends to be transformed in the same way. Provided that in aqueous samples of monopalmitoyl glycerol was remembrane proteins plug the “holes” of a two-dimensional p ~ r t e d . ~The present work concerns this transition in bilayer sectioned from the cubic minimal surface curved aqueous lipid systems as models for the axon membrane. bilayer, it is in fact possible that the lipid bilayer has Experimental Section identical structures in a membrane compared to the corresponding cubic lipid-water ample.^ The driving force Samples of L-a-dioleoylphosphatidylcholine(99% pure) (DOPC), L-a-dipalmitoylphosphatidylcholine (99% pure) (DPPC), behind transitions from the planar bilayer of an L, phase and egg yolk phosphatidylcholine (99% pure) (EYPC)from Sigma to a minimal surface curved bilayer of a cubic phase is a were used. A sample of pure soybean phosphatidylcholine(SBPC) change in the average molecular shape! according to the obtained from Lucas Meyer (Hamburg,West Germany), which general relations between molecular wedge shape and has been well ~haracterized,~ was also used. Lidocain hydroformation of aqueous phases demonstrated by Israelachvili chloride (Xylocain)was obtained from Astra, Sbdertdje. et al.7 Samples containing 5% (w/w) of lipid in double-distilled water The molecular cross sectional area in the center of the were shaken until a homogeneous dispersion of the L, phase curved bilayer of a cubic minimal surface structure is al(liposomes)was obtained. One gram of this dispersion was used, and chloroform, halothane (2-bromo-2-chloro-l,l,l-trifluoro- ways larger than that of the parallel surface, where the polar groups are located! The effect of chloroform or ethyl ethane), or ethyl ether (pro analysi) was added by an Agla microsyringe. The anesthetic agent was added stepwise, and the ether on the transition L, cubic phase is fully consistent limit for formation of a visible precipitate was recorded. The phase with an increased molecular wedge shape.’ The situation behavior was examined in the polarizing microscope, and the will be the same in the two-dimensional analogue^,^ corprecipitate formed by the addition of the anesthetic agent was responding to the proposed planar curved membrane analyzed in an X-ray low-angle camera of the Guinier type. b i l a ~ e r .As ~ the average direction of the disordered hydrocarbon chains always should tend to be perpendicular Results and Discussion to the lipid bilayer, there will be a force on membraneThe effect of the addition of chloroform on a lipsomal spanning proteins, “plugging”the curved bilayer, to adopt dispersion of EYPC is shown in Figure 1. A very small a similar conformation. Channel proteins would be able amount results in visible formation of a cubic phase as a to be accommodated into the bilayer during the transition precipitate. Surface changes with loss of birefringence of from a planar to a curved bilayer without strain if they the liposomes can be observed in the polarizing microscope. adopt a changed conformation as indicated in Figure 2. The exact limit, however, when chloroform solubilized in Such a conformational change can explain the blocking of the planar bilayers of the L, phase results in the formation sodium channels. of the cubic phase, is difficult to determine experimentally. A phase-transition mechanism of the inhalation anX-ray diffraction analysis of the cubic phase which can esthetic effect means that a critical concentration is needed coexist with the L, phase shows a unit-cell axis of 89.9 A. for the blocking of the nerve signal propagation, and when
1I
.,-3lSPERS13h
i h WATER
I
I
-
(2) Larsson, K. J. Colloid Interface Sci. 1986, 113, 299. (3) Hyde, S. T.; Andersson, S.; Larsson, K. 2.Kristallogr. 1986, 174,
no”
LJI.
(4) Larsson, K. Acta Chern. Scand. 1986, A60, 313. (5) BergenstAhl, B.; Fontell, K. Prop. Colloid Polym. Sci. 1983,68, 48.
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(6) Hyde, S.T.; Andersson, S.;Ericsson,B.; Larsson, K. 2.Kristallogr. 1984, 168, 213.
( 7 ) Israelachvili, J . N.; Mitchell, D. J.; Ninham, B. W . J. Chern. Soc., Faraday Trans. 2 1976, 72, 1525.
Langmuir 1988,4, 217-224
217
to chloroform can be understood in terms of effects on the molecular wedge shape. Ethyl ether has a lower anesthetic potency than chloroform, and the affinity for the hydrocarbon core in relation to the aqueous medium is also lower. The pressure reversion of the anesthetic effect is in fact what should take place if the transition corresponds to L, cubic phase. This transition is a consequence of increased disorder, and in well-defined binary systems it is obtained by an increase in temperature and/or increased water content.6 An increase of pressure will act in the opposite direction. It is believed that local anesthetics act specifically on the sodium channel activation gates. In order to find out whether they have additional effect on the lipid bilayer, the addition of lidocain hydrochloride has been examined. The L, phase of EYPC, SBPC, and DOPC solubilizes a limited amount, resulting in an additional swelling of the L, phase. The formation of this L, phase starts a t about 0.15% (w/w) of lidocain hydrochloride (with 5% lipid in the dispersion), and a t coexistence the d-spacing of the lidocain-containing L, phase is 63.2 A compared to 53.5 %, for the “pure” DOPC L, phase. The corresponding values in the case of SBPC are 64.2 and 55.5 A, respectively.6 It is interesting to note that two lamellar phases with different d-spacings coexist. No cubic phase induced by the presence of lidocain hydrochloride in the bilayer was observed.
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Figure 2. Schematic illustration of the proposed transition mechanism of the axon lipid bilayer induced by inhalation anesthetics. The region near a sodium channel is shown (with two gates indicated). The transition from a planar into a periodic minimal surface curved bilayer will induce a conformational change cylindrical conical and therefore tend to close the channel at one end.
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the concentration then goes below this limit (for example, by lateral diffusion within the membrane) the signal propagation is switched on. The phase changes of a phosphatidylcholine with unsaturated chains (DOPC) compared to one with saturated ones (DPPC) are consistent with the phase-transition model based on changes in molecular wedge shape. The chains diverge more toward the center of the bilayer in the case of DOPC compared to DPPC; thus less of the disordering anesthetic agent is needed in order to obtain the transition. Also, the relative effect of ethyl ether compared
Acknowledgment. This work was supported by a grant from the Swedish Natural Science Research Council. Registry No. CHC13,67-66-3; halothane, 151-67-7;ethyl ether, 60-29-7.
Spectroscopic Determination of the Effective Dielectric Constant of Micelle-Water Interfaces between 15 and 85 “C Gregory G. Warr and D. Fennel1 Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received March 25, 1987. I n Final Form: September 2, 1987 The effective dielectric constant of the interfacial region of several ionic and nonionic micelles has been determined over the range 15-85 O C by using two lipophilic, solvatochromic dye probes. Results are measurably different from the results of bulk solvents investigated and are discussed in terms of the model of Mukerjee and Buff for dipoles at interfaces. This model is examined in detail, resulting in improvements to its predictive power. Specific chemical and solubilizate location effects in sohatochromic dye studies of interfacial dielectric constants are also examined in terms of this model. The consequences of the dielectric behavior for head-group interactions within micelles are discussed.
Introduction This work is concerned with micelle-water interfaces, with how they differ from bulk solvents, and with how they change as a function of temperature. It is motivated by several observations: ionic micelles become smaller and more highly charged with increasing temperature and the cmc’s of almost all classes of surfactants increase significantly with temperature.lS2 Solutions of double-chained (1) Mukerjee, P.; Mysels, K. J. Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.) 1971, February. ( 2 ) Evans, D. F.; Allen, M.; Ninham, B. W.; Fouda, A. J. Solution Chern. 1984, 13, 87.
surfactants such as dialkyldimethylammonium halides upon heating transform from dispersed liquid crystalline bilayers and liposomes to vesicles and bilayer^.^^^ In nonionic, poly(oxyethy1ene) surfactants the cloud point is also a signal of changing micellar structure.”8 (3) Kuneida, H.; Shinoda, K. J. Phys. Chem. 1978, 82, 1710.
(4) Miller, D. D.; Bellare, J. R.; Evans,D. F.; Talmon, Y.; Ninham, B. W. J. Phys. Chem. 1987,91, 674. (5) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P.J . Chem. SOC.,Faraday Trans. 1 1983, 79,975. (6) Nonionic Surfactants; Schick, M. J., Ed.;E.Arnold: London, 1967. (7) Triolo, R.; Magid, L. J.; Johnson, J. S., Jr; Child, H. R. J. Phys. Chern. 1982,86, 3689.
0743-7463/S8/2404-0217~01.50/0 0 1988 American Chemical Society
