- Email: [email protected]
The effect of amphiphilic counterions on the gel-fluid phase transition of the lipid bilayer
The Effect of Amphiphilic Counterions on the Gel-Fluid Phase Transition of the Lipid Bilayer Krystian Kubica Department of Physics and Biophysics Agricultural University Norwida
25
50 375 Wroclaw Poland
ABSTRACT Monte Carlo simulation based on the ten-state Pink’s model has been applied to study the effect of amphiphilic counterions on the gel-fluid phase transition of lipid bilayers. Based on the theoretical results obtained, one can explain the lack of measurable effect of alkylsulphonate on lipid membrane permeability. An anionic compound with long alkyl chain when applied to a membrane modified with a cationic compound of the same chain length can cancel the effect of the latter due to the formation of an ionic pair that resembles the lipid molecules that form the membrane. The results obtained were compared with experimental results and proved helpful 0 Elsevier Science Inc., 1997 in giving them a more satisfactory interpretation.
1.
INTRODUCTION
Fluidity of the lipid bilayer seems to be essential for cellular function because it can affect active transport, activity of some membrane enzymes and cell leakage. The gel-fluid transition depends primarily on the fatty acid composition of the membrane lipids. It can be controlled by biosynthesis of appropriate fatty acids or their selective uptake from the environment. For a membrane to be almost totally fluid at a physiological temperature, the membrane lipid composition must change according to temperature.
APPLIED MATHEMATICSAND COMPUTATION87:261-270(1997) 0 Elsevier Science Inc., 1997 00963003/97/$17.00 PI1 S009&300~96)00301-3 655 Avenue of the Americas, New York, NY 10010
262
K. KUBICA
Such an effect can be achieved in a number of ways [l]: - by changing the saturation of hydrocarbon chains, - by incorporating shorter-chain fatty acids into the membrane, or - by changing the number of charged amphiphilic molecules, (the result obtained in the present work). On the other hand, a number of amphiphilic compounds may modify the membrane properties by incorporating into it. We have studied experimentally a mixture of cationic and anionic amphiphilic compounds applied to modify the transport of ions across lecithin liposome membranes. The cationic compound was N-dodecyloxymethylene-N-methylporpholinium chloride (DOMM) and the anionic ones were a series of sodium alkylsuphonates (ASn) with different alkylchain length. The experimental results obtained [2, 31 impelled the author to investigate the effect of the compounds on model membrane properties using the Monte Carlo simulation. A literature study has shown that the Monte Carlo simulation based on the ten-state Pink’s model gives a good description of the gel-fluid transition and the influence of chain length of some amphiphilic compounds on membrane properties [4, 71.
2.
THE MODEL AND CALCULATION
METHOD
In the present paper has been applied the ten-state Pink’s model [8], successfully used for description of the gel-fluid transition in the lipid bilayer. According to the model, each hydrocarbon chain can assume one of ten possible configurations characterized by internal energy E,, cross-section area of the acyl chain A,, and internal degeneracy 0,. For the fluid phase, the most probable is the highly excited state with the largest internal energy and cross-section area. The opposite state, with the lowest values of the aforementioned quantities, is the all-trams configuration. The lipid membrane is modelled as two mutually noninteracting triangular lattices. To make the model membrane a closed surface, as experiments are performed on closed lipid surfaces [2], periodic conditions are imposed in both horizontal and vertical directions. The model assumes that each site ( no d)e o f a t’ rmnglu ar ltt a ice is occupied by a hydrocarbon chain that may be in any of the ten states. According to this assumption one double-chain lipid molecule occupies two nodes of the lattice, and each chain is treated separately. In addition to molecules that model double-chain lipids with 16-carbon atom chains (DPPC), single-chain molecules with electrically charged polar heads were
263
The Effect of Amphiphilic Counterions on the Gel-Fluid Phase
considered. Thus, the Hamiltonian of the model considered takes the form T
where: Li, is the occupation variable (0 or 1) of state a of the acyl-chain site i, JO is the coupling constant, E, is the internal conformation energy, A, is the cross-section area I,, is the shape dependent nematic parameter, 7r is the internal pressure, r, j is the effective radius, c= 1/4?rE, E,
The Metropolis method was used to study the lipid chain dynamics and to obtain thermodynamic averages. The studied lattice was composed of 10,000 nodes. The system was equlibrated for 100 Monte Carlo steps per site in a given temperature, then 1000 steps per site were performed during which thermodynamic quantities were calculated [9]. Next the temperature was increased by 0.5” K and the procedure was repeated until 320” K was reached. 3.
RESULTS
The considered electrostatic interaction was simplified to that between neighbor molecules that have a resultant non-zero charge of their polar heads. The energy of interaction between an ion and rotating dipole was neglected. However, the hydration shell of the polar part of the lipid bilayer was taken into account via the dielectric constant E = 81. According to literature data [lo, 111, it was assumed that the first tree-chain segments are fixed along the bilayer normal. Another assumption refers to the remaining GC segments. In the literature, the angle between the CC bonds is assumed to be 120” [ll]. However, it seems better to assume 109” for the value of the angel, since this value is more in line with the sp3 hybridization. That assumption required finding the value of lateral pressure T, as well as the interaction constant JO, in order to have the
K. KUBICA
264
gel-fluid transition with 16carbon chains to occur at 314 K. The new values are: 7~ = 24 X 10V3 N/m, J, = 1.365 X 10-20 J. These values differ from literature data due to the change of the angle. Figures 1 and 2 show the dependence of specific heat C, and average area per chain A on temperature for a membrane formed of the modelling molecules DPPC. At the temperature 314 K occurs a jump in the values of C, and A. Figures 3 and 4 show analogous relationships for a membrane formed of the model molecules DPPC modified by various amounts of anion molecules having 16-carbon atom alkyl chain. The number of modifier molecules is expressed as a percent of the total discrete area C of the membrane (10,000 nodes of the network). The presence of negatively charged amphiphilic modifiers causes an increase in the gel-fluid transition temperature. Modifying the membrane with a mixture containing equal number of anionic and cationic molecules with 16carbonatoms alkyl chain does not cause any change in the plots of C, and A as functions of temperature, compared with the unmodified membrane. Assuming that the membrane permeability is proportional to cluster area 161, the depe n dence has been given of relative permeabilities of modified membranes (i.e., relative to unmodified membranes) for three selected occupancy fractions (i.e. the membrane area fraction occupied by modifier
T IKI FIG. 1. molecules.
Temperature dependence of the specific heat per lipid molecule for DPPC model
The Effect of Amphiphilic Counter-ions
245
3il
365
318
on the Gel-Fluid Phase
315
3h
3f5
3h
265
3i5
34
T IKI FIG. 2. Temperature dependence of the average cross-section area per molecule for DPPC model molecules.
FIG. 3. Temperature dependence of the specific heat per lipid molecule. Lipid membrane was modified with negatively charged long-chain (16carbon atoms) amphiphile molecules. The amount of modifier molecules is expressed by percent of the total discrete membrane area. (2n16 - 2%, 4n16 - 4%, 87~16- 8%, 16n16 - 16%)
266
K. KUBICA
35. 2
A la16
??hi6
0 4ni6
f ; 39
2
245
3h
3i5
3h
315
3h
3t5
339 335 T IN
I 349
FIG. 4. Temperature dependence of the average cross-section area per molecule. Lipid membrane is modified with a negatively charged long-chain (16carbon atoms) amphiphile molecules. The amount of modifier molecules is expressed by percent of the total discrete membrane area. (ln16 - l%, 47216 - 4%, 8n16 - S%, 16n16 - 16%)
molecules): 2%, 4% and 8%. Since the modifiers tend to form anionic-cationic pairs, the fraction is determined by the percent of ionic pairs relative to the whole membrane. An ionic pair, like the lipid molecule DPPC, occupies the nodes of the membrane network. From the relationships presented, it follows that the relative membrane permeability decreases both with increasing temperature, up to 314 K, and concentration of the amphiphilic anionic modifiers. Permeability of the membrane modified with the mixture of anionic-cationic modifiers is almost constant and close to permeability of the unmodified membrane.
4.
COMPARISON
WITH EXPERIMENT
In the previous section the results are presented of computer simulation on theoretical models of amphiphilic anionic and cationic compounds in the lipid membrane. The results obtained can be helpful for interpretation of
The Effect of Amphiphilic Counterions on the Gel-Fluid Phase
267
2P 0 2W16
’ 8lRi6
FIG. 5. Temperature dependence of the specific heat per lipid molecule. Lipid membrane is modified with an equimolar mixture of negatively and positively charged long-chain (l&carbon atoms) amphiphile molecules. The amount of modifier molecules (ionic pairs) is expressed by percent of the total discrete membrane area. (2 np16 - 2%, 8 np16 - 8%)
experimental data concerning permeability of the membrane of unilamellar lecithin liposomes in the presence of the amphiphilic modifiers cationic DOMM and anionic S,, with n = 2, 4, 6, 8, 10, 12. In general, one can say that modification of a liposome membrane with the DOMM + S,, mixture causes an increase in membrane permeability for labelled sulphate ions when the S, alkyl chains are short, while the same mixture with long S,, alkyl chains causes a decrease in the relative permeability coefficient (relative to unmodified membrane). When interpreting experimental results, it was difficult to explain the increase in permeability of the membrane modified with the cationic DOMM compound only (12 = carbon atom chain) and for the unmeasurable effect of the S, compounds. The liposomes used in the experiment were formed with egg yolk lecithin using the Singleton method [12]. It can thus be assumed that besides lecithin, it contains about 5% of other amphiphilic compounds, anions including. Negative charge of liposomes thus formed was also confirmed in electrophoretic measurements (unpublished data). Applying the theoretical results obtained in the present work to experimental data, it is apparent that modifying a membrane that contains negative anionic “impurities” with DOMM should induce an increase in
268
K. KUBICA
35 A 1~16
Q 16ap16
FIG. 6. Temperature dependence of the average cross-sectional area per molecule. Lipid membrane is modified with an equimolar mixture of negatively and positively charged long-chain (16carbon atoms) amphiphile molecules. The amount of modifier molecules (ionic pairs) is expressed by percent of the total discrete membrane area. (1~116 - I%, 16np16 - 16%)
.2-
$5
3%
3i5
3ie
31 TWI
FIG. 7. Temperature dependence of relative permeability P/P, (relative to unmodified membrane) of model membrane modified with different amounts of a negatively charged amphiphilic compound. The amount of modifier molecules is expressed by percent of the total discrete membrane area. (2n - 2%, 472 - 4%, 8n - 8%)
The Effect of Amphiphilic Counterions
on the Gel-Fluid Phase
269
i-
pm0 .B-
na
,C
FIG. 8. Temperature dependence of relative permeability P/P, (relative to unmodified membrane) of the model membrane modified with different amount OS an equimolar mixture of negatively and positively charged amphiphilic compound. The amount of modifier molecules (ionic pairs) is expressed by percent of total discrete membrane area. (2 np - 2%, 4 np - 4%, 8np - 8%)
membrane permability relative to the unmodified one (neglecting impurities). Membrane modification with anionic modifiers should induce, according to the simulation results, a decrease in membrane permeability so small that it is within the range of experimental error. Hence, such an increase was not found. Modifying the membrane with the long-chain anionic and cationic compounds did not result in marked changes in the relative membrane permeability. This observation is in line with the simulation results. Assuming that the passive membrane permeability is the result of lateral density fluctuations and that modifiers with shorter chains cause greater density fluctuations [6], one can expect that the DOMM + S,, mixtures with short alkyl chains should increase permeability of the lipid membrane. This supposition is also in accord with experimental observations. At present work is in progress on obtaining such a result from computer simulation. 5.
CONCLUDING
REMARKS
As follows from the above considerations, native amphiphilic molecules with negatively charged polar heads may markedly affect permeability of biological membranes. Modifying such membranes with long-chain cationic
270
K. KUBICA
amphiphile compounds, one can control ion transport across the membranes and, ultimately, cancel the affect of amphiphilic counterion embedded in the membrane previously. This
work was sponsored
by the Polish
Research
Committee
(KBN),
grant no 6 P203 003 07.
REFERENCES 1.
2.
3.
4.
5.
6.
7.
8.
9.
10. 11.
12.
D. L. Melchior, Lipid phase transitions and regulation of membrane fluidity in Prokaryotes, in Current Topics in Membranes and Transport 17, 282 (S. Razin and S. Rottem, Eds.) Academic Press (1982). J. Kuczera, A. Fogt, K. Kubica, S. Przestalski, S. Witek and J. Plucinski, Effect of a surface active anions on the activity of dedecyloxymethylenemethylmarpholinium chlorides as modifier of sulphate ion transport across lecithin liposome membrane, Studia Biophysics 133, 209-220 (1989). J. Sarapuk, H. Kleszczynska, S. Przestalski and M. Kilian, Biological activity of N-dodecyloxymethylene-N-methylmorpholinium chloride enhanced by sodium sulphonates, Tenside, Surfactantq Detergents 29, 349-351 (1992). J. H. Ipsen, K. Jorgensen and 0. G. Mouritsen, Density fluctuations in saturated phospholipid bilayers increase as the acyl-chain length decreases. Biophys. J. 58, 1099-1107 (1990). K. Jorgensen, J. H. Ipsen, 0. G. Mouritsen, D. Bennett and M. J. Zuckermann, A general model for the interaction of foreign molecules with lipid membranes: drugs and anaesthetics, BBA 1062, 227-238 (1991). K. Jorgensen, J. H. Ipsen, 0. G. Mouritsen, D. Bennett and M. J. Zuckermann, The effects of density fluctuations on the partitioning of foreign molecules into lipid bilayers: Application to anaesthetics and insecticides, BBA 1067, 241-253 (1991). 0. G. Mouritsen, A. Boothroyd, R. Harris, N. Jan, T. Lookman, L. MacDonald, D. A. Pink and M. J. Zuckermann, Computer simulation of the main gel-fluid phase transition of lipid bilayers, J. Chem Whys. 79, 2027-2041 (1983). D. A. Pink, T. J. Green and D. Chapman, Raman scattering in bilayers of Biochemistry 19, saturated phosphatidylcholines. Experiment and Theory. 349-356 (1980). 0. G. Mouritsen, K. Jorgensen, J. H. Ipsen, M. J. Zuckermann and L. CruzeiroHansson, Computer simulation of interfacial fluctuation phenomena. Physica Scripta T33, 42-51 (1990). H. Hauser, Conformation of phospholipids crystal structure of a lysophosphatidylcholine analogue. J. Mol. Biol 137, 249-264 (1980). J. H. Ipsen, 0. G. Mouritsen and M. Bloom, Relationships between lipid membrane area, hydrophobic thickness, and acyl-chain orientational order: The effects of cholesterol. Biophys. J. 57, 405-412 (1990). W. S. Singleton, M. S. Gray, M. L. Brown and I. L. White, Chromatographitally homogenous lecithin from egg phospholipidis. J. Am Oil Chem Sot. 42, 53-56 (1965).
25
50 375 Wroclaw Poland
ABSTRACT Monte Carlo simulation based on the ten-state Pink’s model has been applied to study the effect of amphiphilic counterions on the gel-fluid phase transition of lipid bilayers. Based on the theoretical results obtained, one can explain the lack of measurable effect of alkylsulphonate on lipid membrane permeability. An anionic compound with long alkyl chain when applied to a membrane modified with a cationic compound of the same chain length can cancel the effect of the latter due to the formation of an ionic pair that resembles the lipid molecules that form the membrane. The results obtained were compared with experimental results and proved helpful 0 Elsevier Science Inc., 1997 in giving them a more satisfactory interpretation.
1.
INTRODUCTION
Fluidity of the lipid bilayer seems to be essential for cellular function because it can affect active transport, activity of some membrane enzymes and cell leakage. The gel-fluid transition depends primarily on the fatty acid composition of the membrane lipids. It can be controlled by biosynthesis of appropriate fatty acids or their selective uptake from the environment. For a membrane to be almost totally fluid at a physiological temperature, the membrane lipid composition must change according to temperature.
APPLIED MATHEMATICSAND COMPUTATION87:261-270(1997) 0 Elsevier Science Inc., 1997 00963003/97/$17.00 PI1 S009&300~96)00301-3 655 Avenue of the Americas, New York, NY 10010
262
K. KUBICA
Such an effect can be achieved in a number of ways [l]: - by changing the saturation of hydrocarbon chains, - by incorporating shorter-chain fatty acids into the membrane, or - by changing the number of charged amphiphilic molecules, (the result obtained in the present work). On the other hand, a number of amphiphilic compounds may modify the membrane properties by incorporating into it. We have studied experimentally a mixture of cationic and anionic amphiphilic compounds applied to modify the transport of ions across lecithin liposome membranes. The cationic compound was N-dodecyloxymethylene-N-methylporpholinium chloride (DOMM) and the anionic ones were a series of sodium alkylsuphonates (ASn) with different alkylchain length. The experimental results obtained [2, 31 impelled the author to investigate the effect of the compounds on model membrane properties using the Monte Carlo simulation. A literature study has shown that the Monte Carlo simulation based on the ten-state Pink’s model gives a good description of the gel-fluid transition and the influence of chain length of some amphiphilic compounds on membrane properties [4, 71.
2.
THE MODEL AND CALCULATION
METHOD
In the present paper has been applied the ten-state Pink’s model [8], successfully used for description of the gel-fluid transition in the lipid bilayer. According to the model, each hydrocarbon chain can assume one of ten possible configurations characterized by internal energy E,, cross-section area of the acyl chain A,, and internal degeneracy 0,. For the fluid phase, the most probable is the highly excited state with the largest internal energy and cross-section area. The opposite state, with the lowest values of the aforementioned quantities, is the all-trams configuration. The lipid membrane is modelled as two mutually noninteracting triangular lattices. To make the model membrane a closed surface, as experiments are performed on closed lipid surfaces [2], periodic conditions are imposed in both horizontal and vertical directions. The model assumes that each site ( no d)e o f a t’ rmnglu ar ltt a ice is occupied by a hydrocarbon chain that may be in any of the ten states. According to this assumption one double-chain lipid molecule occupies two nodes of the lattice, and each chain is treated separately. In addition to molecules that model double-chain lipids with 16-carbon atom chains (DPPC), single-chain molecules with electrically charged polar heads were
263
The Effect of Amphiphilic Counterions on the Gel-Fluid Phase
considered. Thus, the Hamiltonian of the model considered takes the form T
where: Li, is the occupation variable (0 or 1) of state a of the acyl-chain site i, JO is the coupling constant, E, is the internal conformation energy, A, is the cross-section area I,, is the shape dependent nematic parameter, 7r is the internal pressure, r, j is the effective radius, c= 1/4?rE, E,
The Metropolis method was used to study the lipid chain dynamics and to obtain thermodynamic averages. The studied lattice was composed of 10,000 nodes. The system was equlibrated for 100 Monte Carlo steps per site in a given temperature, then 1000 steps per site were performed during which thermodynamic quantities were calculated [9]. Next the temperature was increased by 0.5” K and the procedure was repeated until 320” K was reached. 3.
RESULTS
The considered electrostatic interaction was simplified to that between neighbor molecules that have a resultant non-zero charge of their polar heads. The energy of interaction between an ion and rotating dipole was neglected. However, the hydration shell of the polar part of the lipid bilayer was taken into account via the dielectric constant E = 81. According to literature data [lo, 111, it was assumed that the first tree-chain segments are fixed along the bilayer normal. Another assumption refers to the remaining GC segments. In the literature, the angle between the CC bonds is assumed to be 120” [ll]. However, it seems better to assume 109” for the value of the angel, since this value is more in line with the sp3 hybridization. That assumption required finding the value of lateral pressure T, as well as the interaction constant JO, in order to have the
K. KUBICA
264
gel-fluid transition with 16carbon chains to occur at 314 K. The new values are: 7~ = 24 X 10V3 N/m, J, = 1.365 X 10-20 J. These values differ from literature data due to the change of the angle. Figures 1 and 2 show the dependence of specific heat C, and average area per chain A on temperature for a membrane formed of the modelling molecules DPPC. At the temperature 314 K occurs a jump in the values of C, and A. Figures 3 and 4 show analogous relationships for a membrane formed of the model molecules DPPC modified by various amounts of anion molecules having 16-carbon atom alkyl chain. The number of modifier molecules is expressed as a percent of the total discrete area C of the membrane (10,000 nodes of the network). The presence of negatively charged amphiphilic modifiers causes an increase in the gel-fluid transition temperature. Modifying the membrane with a mixture containing equal number of anionic and cationic molecules with 16carbonatoms alkyl chain does not cause any change in the plots of C, and A as functions of temperature, compared with the unmodified membrane. Assuming that the membrane permeability is proportional to cluster area 161, the depe n dence has been given of relative permeabilities of modified membranes (i.e., relative to unmodified membranes) for three selected occupancy fractions (i.e. the membrane area fraction occupied by modifier
T IKI FIG. 1. molecules.
Temperature dependence of the specific heat per lipid molecule for DPPC model
The Effect of Amphiphilic Counter-ions
245
3il
365
318
on the Gel-Fluid Phase
315
3h
3f5
3h
265
3i5
34
T IKI FIG. 2. Temperature dependence of the average cross-section area per molecule for DPPC model molecules.
FIG. 3. Temperature dependence of the specific heat per lipid molecule. Lipid membrane was modified with negatively charged long-chain (16carbon atoms) amphiphile molecules. The amount of modifier molecules is expressed by percent of the total discrete membrane area. (2n16 - 2%, 4n16 - 4%, 87~16- 8%, 16n16 - 16%)
266
K. KUBICA
35. 2
A la16
??hi6
0 4ni6
f ; 39
2
245
3h
3i5
3h
315
3h
3t5
339 335 T IN
I 349
FIG. 4. Temperature dependence of the average cross-section area per molecule. Lipid membrane is modified with a negatively charged long-chain (16carbon atoms) amphiphile molecules. The amount of modifier molecules is expressed by percent of the total discrete membrane area. (ln16 - l%, 47216 - 4%, 8n16 - S%, 16n16 - 16%)
molecules): 2%, 4% and 8%. Since the modifiers tend to form anionic-cationic pairs, the fraction is determined by the percent of ionic pairs relative to the whole membrane. An ionic pair, like the lipid molecule DPPC, occupies the nodes of the membrane network. From the relationships presented, it follows that the relative membrane permeability decreases both with increasing temperature, up to 314 K, and concentration of the amphiphilic anionic modifiers. Permeability of the membrane modified with the mixture of anionic-cationic modifiers is almost constant and close to permeability of the unmodified membrane.
4.
COMPARISON
WITH EXPERIMENT
In the previous section the results are presented of computer simulation on theoretical models of amphiphilic anionic and cationic compounds in the lipid membrane. The results obtained can be helpful for interpretation of
The Effect of Amphiphilic Counterions on the Gel-Fluid Phase
267
2P 0 2W16
’ 8lRi6
FIG. 5. Temperature dependence of the specific heat per lipid molecule. Lipid membrane is modified with an equimolar mixture of negatively and positively charged long-chain (l&carbon atoms) amphiphile molecules. The amount of modifier molecules (ionic pairs) is expressed by percent of the total discrete membrane area. (2 np16 - 2%, 8 np16 - 8%)
experimental data concerning permeability of the membrane of unilamellar lecithin liposomes in the presence of the amphiphilic modifiers cationic DOMM and anionic S,, with n = 2, 4, 6, 8, 10, 12. In general, one can say that modification of a liposome membrane with the DOMM + S,, mixture causes an increase in membrane permeability for labelled sulphate ions when the S, alkyl chains are short, while the same mixture with long S,, alkyl chains causes a decrease in the relative permeability coefficient (relative to unmodified membrane). When interpreting experimental results, it was difficult to explain the increase in permeability of the membrane modified with the cationic DOMM compound only (12 = carbon atom chain) and for the unmeasurable effect of the S, compounds. The liposomes used in the experiment were formed with egg yolk lecithin using the Singleton method [12]. It can thus be assumed that besides lecithin, it contains about 5% of other amphiphilic compounds, anions including. Negative charge of liposomes thus formed was also confirmed in electrophoretic measurements (unpublished data). Applying the theoretical results obtained in the present work to experimental data, it is apparent that modifying a membrane that contains negative anionic “impurities” with DOMM should induce an increase in
268
K. KUBICA
35 A 1~16
Q 16ap16
FIG. 6. Temperature dependence of the average cross-sectional area per molecule. Lipid membrane is modified with an equimolar mixture of negatively and positively charged long-chain (16carbon atoms) amphiphile molecules. The amount of modifier molecules (ionic pairs) is expressed by percent of the total discrete membrane area. (1~116 - I%, 16np16 - 16%)
.2-
$5
3%
3i5
3ie
31 TWI
FIG. 7. Temperature dependence of relative permeability P/P, (relative to unmodified membrane) of model membrane modified with different amounts of a negatively charged amphiphilic compound. The amount of modifier molecules is expressed by percent of the total discrete membrane area. (2n - 2%, 472 - 4%, 8n - 8%)
The Effect of Amphiphilic Counterions
on the Gel-Fluid Phase
269
i-
pm0 .B-
na
,C
FIG. 8. Temperature dependence of relative permeability P/P, (relative to unmodified membrane) of the model membrane modified with different amount OS an equimolar mixture of negatively and positively charged amphiphilic compound. The amount of modifier molecules (ionic pairs) is expressed by percent of total discrete membrane area. (2 np - 2%, 4 np - 4%, 8np - 8%)
membrane permability relative to the unmodified one (neglecting impurities). Membrane modification with anionic modifiers should induce, according to the simulation results, a decrease in membrane permeability so small that it is within the range of experimental error. Hence, such an increase was not found. Modifying the membrane with the long-chain anionic and cationic compounds did not result in marked changes in the relative membrane permeability. This observation is in line with the simulation results. Assuming that the passive membrane permeability is the result of lateral density fluctuations and that modifiers with shorter chains cause greater density fluctuations [6], one can expect that the DOMM + S,, mixtures with short alkyl chains should increase permeability of the lipid membrane. This supposition is also in accord with experimental observations. At present work is in progress on obtaining such a result from computer simulation. 5.
CONCLUDING
REMARKS
As follows from the above considerations, native amphiphilic molecules with negatively charged polar heads may markedly affect permeability of biological membranes. Modifying such membranes with long-chain cationic
270
K. KUBICA
amphiphile compounds, one can control ion transport across the membranes and, ultimately, cancel the affect of amphiphilic counterion embedded in the membrane previously. This
work was sponsored
by the Polish
Research
Committee
(KBN),
grant no 6 P203 003 07.
REFERENCES 1.
2.
3.
4.
5.
6.
7.
8.
9.
10. 11.
12.
D. L. Melchior, Lipid phase transitions and regulation of membrane fluidity in Prokaryotes, in Current Topics in Membranes and Transport 17, 282 (S. Razin and S. Rottem, Eds.) Academic Press (1982). J. Kuczera, A. Fogt, K. Kubica, S. Przestalski, S. Witek and J. Plucinski, Effect of a surface active anions on the activity of dedecyloxymethylenemethylmarpholinium chlorides as modifier of sulphate ion transport across lecithin liposome membrane, Studia Biophysics 133, 209-220 (1989). J. Sarapuk, H. Kleszczynska, S. Przestalski and M. Kilian, Biological activity of N-dodecyloxymethylene-N-methylmorpholinium chloride enhanced by sodium sulphonates, Tenside, Surfactantq Detergents 29, 349-351 (1992). J. H. Ipsen, K. Jorgensen and 0. G. Mouritsen, Density fluctuations in saturated phospholipid bilayers increase as the acyl-chain length decreases. Biophys. J. 58, 1099-1107 (1990). K. Jorgensen, J. H. Ipsen, 0. G. Mouritsen, D. Bennett and M. J. Zuckermann, A general model for the interaction of foreign molecules with lipid membranes: drugs and anaesthetics, BBA 1062, 227-238 (1991). K. Jorgensen, J. H. Ipsen, 0. G. Mouritsen, D. Bennett and M. J. Zuckermann, The effects of density fluctuations on the partitioning of foreign molecules into lipid bilayers: Application to anaesthetics and insecticides, BBA 1067, 241-253 (1991). 0. G. Mouritsen, A. Boothroyd, R. Harris, N. Jan, T. Lookman, L. MacDonald, D. A. Pink and M. J. Zuckermann, Computer simulation of the main gel-fluid phase transition of lipid bilayers, J. Chem Whys. 79, 2027-2041 (1983). D. A. Pink, T. J. Green and D. Chapman, Raman scattering in bilayers of Biochemistry 19, saturated phosphatidylcholines. Experiment and Theory. 349-356 (1980). 0. G. Mouritsen, K. Jorgensen, J. H. Ipsen, M. J. Zuckermann and L. CruzeiroHansson, Computer simulation of interfacial fluctuation phenomena. Physica Scripta T33, 42-51 (1990). H. Hauser, Conformation of phospholipids crystal structure of a lysophosphatidylcholine analogue. J. Mol. Biol 137, 249-264 (1980). J. H. Ipsen, 0. G. Mouritsen and M. Bloom, Relationships between lipid membrane area, hydrophobic thickness, and acyl-chain orientational order: The effects of cholesterol. Biophys. J. 57, 405-412 (1990). W. S. Singleton, M. S. Gray, M. L. Brown and I. L. White, Chromatographitally homogenous lecithin from egg phospholipidis. J. Am Oil Chem Sot. 42, 53-56 (1965).