Langmuir 1993,9, 1179-1183
1179
Head Group-Water Interactions in Lipid Bilayers: A Comparison between DMPC- and DLPE-Based Lipid Bilayers K. V. Damodaran and Kenneth M. Merz, Jr.* 152 Davey Laboratory, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received December 15,1992. In Final Form: March 3,1993
Molecular dynamics simulations on dimyristoylphosphatidylcholine (DMPCI-basedlipid bilayers are reported. The resulting trajectory is compared to a previous one for dilauroylphosphatidylethanolamine (DLPE)-basedlipid bilayers. The major difference between these two systemsappearsto be the formation of a clathrate shell at the surface of the DMPC-based lipid bilayer. This arises due to the presence of the hydrophobic -N(CH&+ group in DMPC which is replaced by the hydrophilic-NH3+ group in DLPE. Implications of this observation on the hydration force problem are discussed. Introduction The repulsiveforceexperienced between bilayer surfaces separated by solvent is known as the solvation pressure or the hydration pressure when the solvent is water.' Experimentally it is known that the hydration repulsion decays exponentiallywith the bilayer separation and that the equilibrium separation of the bilayers, in a multilamellar system, depends on the extent of lipid hydration.' The origin of this repulsive force has been the subject of intense debate, with several models having been proposed to explain the experimental observations. For example, McIntosh and co-workers2~3 have shown that the solvation pressure depends on the packing density of the solvent molecules and the surface dipole potential, which has contributions from both the head groups and the oriented solvent molecules in the interface region. They argue that the interbilayersolvent molecules are ordered by the dipole potential which gives rise to the observed hydration pressure. However, early molecular dynamics (MD) simulationsby Kjellander and Marcelja*and more recently by Berkowitz et a L 5 p 6 did not show an exponential decay of the orientational polarization. Israelachviliand Wennerstr6m7t8have discussed the repulsive forces between bilayers in terms of entropic forces arising from the confinementof thermally excitedundulations of the bilayer surfaces into a smaller region as the two membranes approach one another. In their model they suggest that "genuine hydration effectsplay a minor r0le".~9~ However, it is clear from the large body of experimental data that the solventorderingnear the head group is very important in the hydration repulsion of the bilayers.1 ~~~~~
(1) Rand, R. P.; Parsegian, V. A. In The Structure of Biological Membranes; Yeagle, P., Ed.; CRC Press: Boca Raton, FL, 1992. (2) McIntosh, T. J.; Magid, A. D.; Simon, S. A. Range of the Solvation Pressure between Lipid Membranes: Dependence on the Packing Density of Solvent Molecules. Biochemistry 1989,2%, 7904-7912. (3) Simon, S. A.; McIntosh, T. J. Magnitude of the Solvation Pressure Depends on Dipole Potential. h o c . Natl. Acad. Sci. U.S.A. 1989,86, 9263-9267. (4) Kjellander,R.;Marcelja,S. Polariaation of Water between Molecular Surfaces: a Molecular Dynamics Study. Chem. Scr. 1985,25, 73-80. (5) Berkowitz, M. L.; Raghaven, K. Computer Simulation of a Water/ Membrance Interface. Langmuir 1991, 7,1042-1044.
(@Raghavan, K.; Reddy, M. R.; Berkowitz, M. L. A Molecular Dynamics Study of the Structure and Dynamics of Water between DilawoylphosphatidylethanolamineBilayers. Langmuir 1992, 8, 233-
240.
(7) Israelachvili, J. N.; Wennerstrom, H. Hydration or Steric Forces between Amphiphilic Surfaces? Langmuir 1990,6,873-876. (8) Israelachvili, J. N.; Wennerstrom, H. Entropic Forces between Amphiphilic Surfaces in Liquids. J. Phys. Chem. 1992,96,520-531.
Among the lipids with neutral head groups, it has been observed that lipids with phosphatidylcholine (PC) head groups tend to hydrate more and have large interlamellar solvent regions compared to lipids with phosphatidylethanolamine (PE) head gro~ps.l*~ T w o representative members of the PE and PC class (dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylethanolamine (DLPE)) have had their structure, dynamics, and phase properties investigated at various degrees of hydration. DMPC has a gel to liquid crystalline transition temperature of 297 K,1° and in the liquid crystallinephase DMPC swells upon absorbing water. Depending on the degree of hydration, the lipid bilayer has a surface area of -60-70 A2/lipid and an interlamellar distance of -27 A.ll The number of water molecules per lipid in the fully hydrated state has been reported to be between 25 and 30." DLPE has a relatively smaller head group and is solvated by a smaller number of water molecules? In hydrated DLPE the area/lipid and the interlamellar distance have been reported to be -50 A2 and -5 A, respectively? In this paper we examine the head group-water interactions in the L, phase of the DMPC lipid bilayer in water using a 200-ps molecular dynamicstrajectory and compare with those in DLPE using a similar trajectory from our earlier work.12 These two lipids are of interest from the point of view of the hydration force,since the interlamellar distances in DLPE and DMPC represent two extreme cases. Methods Model. The starting structure of DMPC was built from the crystal structure by increasing the lipid-lipid spacing proportionally so as to obtain an area per lipid of -68 &/lipid, characteristic of the hydrated Laphase, and interdigitatingand two monolayers so that a bilayer thickness of -35 A was obtained." Each monolayer had 16 lipids, and the interlamellar spacing (Le., distance between head group atoms in the two ~~
(9) McIntosh, T. J.; Simon, S. A. Area per Molecule and Distribution
of Water in Fully Hydrated Dilauroylphosphatidylethanolamine. Biochemistry 1986,25,4948. (10) Blume, A. Apparent Molar Heat Capacities of Phospholipids in AqueousDispersion. Effects of Chain Lengthand Head Groupstructure. Biochemistry 1983,22, 5436-5442. (11) Lis, L. J.; McAlister, M.; Fuller, N.; Rand, R. P. Interactions Between Neutral Phospholipid Bilayer Membranes. Biophys. J. 1982, 37,657-666. (12) Damodaran, K. V.; Merz, K. M. J.; Caber, B. P. Structure and
Dynamica of the Dilauroylphosphatidylethanolamine Lipid Bilayer. Biochemistry 1992,31, 7656.
0743-746319312409-1179$04.00/0 0 1993 American Chemical Society
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1180 Langmuir, Vol. 9,No.5,1993 monolayers) was -27 A. In our model the head groups of the two monolayers face each other at the center of the computational cell.12 With periodic boundary conditions, this simulates a multilamellar lipid bilayer with the hydrocarbons of the monolayers in contact. All methyl and methylene groups in the lipid except the methyl groups in the choline head group were represented as united atoms. The bilayer was solvated by adding 842 SPC/EL3water molecules (-26 waterdlipid) in the head group region of the computational box. Partial charges for the lipid atoms were obtained using quantum mechanical electrostatic potential (ESP) fitting on the entire DMPC m~lecule.'~ Bond, angle, and dihedral parameters were taken from the AMBER force field.15J6The van der Waals interactions were represented by the OPLS parameter set with 1-4 electrostatic and van der Waals interactions scaled by factors of 2 and 8, re~pective1y.l~ Computational Detdls. MD simulations were performed using thesameprocedureasadoptedforthesimulation ofDLPE,12 except that AMBER Version 4.018 was used instead of AMBER 3.0.19The simulation was done at a requested temperature of 315 K, which is well above the gel to liquid crystalline phase transition temperature. A temperature relaxation time constant of 0.20 ps was applied separately to the lipid and water temperatures.20 Using this scheme, we found that the solvent region maintained a slightly higher temperature (-340 K)on average than the lipid region (-315 K). The average system temperature was -327 K. The simulation was carried out using constant-volume periodic boundary conditions. An initialwpsMD simulationwas performed for equilibration. In all cases all of the atoms that comprised our simulation box were allowed to move. Subsequently, the atomic coordinates and velocitieswere collected every 20 MD steps (0.030 pa) for 140 ps. Atomic probability distributions along the bilayer normal for different regions of the lipid and solvent molecules, pair distribution functions (g(r))of the water molecules from the head group atoms, and the distribution of the alkyl chain tilt were calculated over this trajectory. The dynamics of the system was also examined using velocity autocorrelation functions and mean square displacements for the water molecules. This was also done for DLPE using our previous trajectory.l*
Results
(a) Solvent Structure. In Figure 1we give the pair distribution functions (B(r))of water oxygens and hydrogens with respect to the nitrogen atom in the head group region for both DLPE and DMPC. The ammoniumgroup of DLPE hydrogen bonds to water as well as to the nonesterified oxygens on the phosphate groups of neighboring lipids,lZ while the trimethylammonium group in DMPC being hydrophobic in character does not form strong interactions with the solvent or the noneeterified oxygens of the phosphate group. The differences in the (13) Berendsen, H. J. C.; Grigera,J. R.; Straatama,T. P. The Missing Term in Effective Pair Potentiale. J. Phys. Chem. 1987,91,6289-6271. (14) Merz, K. M., Jr. Analysis of a Large Database of Electrostatic Potential Derived Atomic Point Charges. J. Comput. Chem. 1992, 13, 749-767. (15) Weiner, 5.J.; Kollman, P. A.; Case, D. A.; Singh, U. C.; Ghio, C.; Alagona, G.; Profeta, S.; Weiner, P. A New Force Field fo Molecular Mechanical Simulation of Nucleic Acids and Proteins. J. Am. Chem. SOC.1984,106,766-784. (16) Weiner, 5.J.; Kollman, P. A,; Nguyen, D. T.; Case, D. A. An All Atom Force Field for Simulations of Proteins and Nucleic Acids. J. Comput. Chem. 1986,7,230-252. (17) Jorgeneen,W. L.;Tirado-Rives,J. The OPLS Potential Functions for Proteins: Energy Minimizations for Crystals of Cyclic Peptides and Crambin. J. Am. Chem. SOC.1988,110,1657-1666. (18) Pearlman, D. A.; Case, D. A.; Caldwell, J. C.; Seibel, G. L.; Singh, U. C.;Weiner, P.; Kollman, P. A., University of California,San Francisco, 1991. (19) Singh, U.C.;Weiner,P. K.;Caldwell,J.;Kollman, P. A.,University of California, San Francisco, 1986. (20)Berendsen,H. J.C.;Potama,J. P. M.;vanGunsteren,W. F.;DiNola, A. D.; Haak,J. R. Molecular Dynamics with Coupling to an External Bath. J . Chem. Phys. 1984,81,3684-3690.
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Figure 1. Radial distribution (g(r))functions for DLPEbased (top) and DMPC-based (bottom) bilayers.
head group-solvent interactions between DLPE and DMPC are clearly seen in Figure 1. The N-H.-O(wat) hydrogen bonds orient the water molecules with the oxygens pointed toward the nitrogens in DLPE. This causes the oxygen and hydrogen pdfs to peak at -2.8 and 3.6 A, respectively. The hydrophobic trimethylammonium groups in DMPC induce formation of clathrates in the nearby waters which causes the oxygen and hydrogen pdfs to peak at the same distance (-4.6 A); since these waters are hydrogen bound mainly among themselvesand to the phosphate groups, there is no strong orientational preference toward the nitrogen, although the -N(CH&+ group is positively charged. That the orientational polarization profile alone does not bring out the role of the solvent ordering in the hydration force is clear from Figure 2 where we have shown these profiles along the bilayer normal in DMPC and DLPE. Although the water structure near the head group seem to be different,the orientational polarization profile in these two systemsdoes not show significant differences, especially the regions 15-20 and 40-45 A in the w e of DMPC and 15-20 and 26-30 A in the case of DLPE which are of interest from the point of view of solvent ordering and the hydration repulsion. It should also be noted that although the first peaks for water oxygen and hydrogen pdfs in DMPC (see Figure 1)appear at identical distances, the orientational polarizationhas a nonzerox component. This is because the clathrate shell around the -N(CH&+ group contains some molecules which are aligned along the x direction of the bilayer. This is to be contrasted with DLPE where the water moleculesare aligned directly toward the -NH3+ group through the formation of hydrogen bond contacts. The polarizationcurvefor DLPE shows spikelike features in the interface region, due to water penetration. These waters are highly ordered due
-
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Langmuir, Vol. 9, No. 5, 1993 1181
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to hydrogen bonding. Similarly, in DMPC, there are a few water moleculespresent in the 50-60-Aregionprobably hydrogen bound to the carbonyls, giving rise to the large intensity in the orientational polarization in this region. The orientational polarization profile for DMPC is smoother compared to DLPE, probably due to extensive water penetration into the interface region. The profiles for DLPE given by Berkowitz et do not show any spikelike features in the interface region, as in our profile. This is to expected, since their simulationswere performed at the gel density at which water penetration is less. The probability distribution functions for various regions along the bilayer normal have been calculated and are shown in Figure 3. Applying periodic boundary conditions on the alkyl chain distribution shows that the two monolayers have some degree of interdigitation. The peak-to-peak distances for the head groups (RNH3+and RN(CH&+ groups) in DLPE and DMPC are -13 and -22A,reapectively. Thesedishcesrepresentan increase and a decrease in the interlamellar spacing, respectively, from the starting distances used in the initial configurations, but are in reasonable agreementwith those observed experimentally using electron density maps for DLPEbased (13.1 A) and DMPC-based (25.4A) liquid crystalline bilayers.21 On the basis of the peak-to-peak distances, we can also estimate the bilayer thickness of DLPE and DMPC to be 36.6 and 40.2A, respectively. McIntosh21 reports this distance to be 33.0 f 0.6 A for DLPE, which indicates that our simulated bilayer thickness is -3 A greater than the experimental value for the liquid crystalline phase. For DMPC this distance is reported to be al.596
(21) McIntoeh,T.J. InMolecularDescriptionofBiological Membranes by Computer Aided Conformational Analysis; Brasseur, R., Ed.;CRC Press: Boca Raton, FL, 1990; pp 247-265.
37.8 f 0.8 A which is smaller than the value we obtain from our simulation (40.2 A).21 The atomic probability distributions in DLPE also show the ammonium groups projecting deep into the solvent space, probably forming hydrogen bonds with the phosphate groups of the apposing monolayer. This, and the corresponding distributions for DMPC, clearly indicates the rough nature of the bilayer solvent interface. Penetration of water into the head group-solvent interface region up to the carbonyl oxygen8 is also evident from the figure. There is more water penetration in the DMPC-based bilayer, due to the larger area per lipid and lipid-lipid spacings. Finally, we find the distribution profiles are not symmetric, probably due to insufficient averaging. (b) Head Group Dynamics. The velocity autocorrelation functions (VAF) of DMPC and DLPE head groups clearly illustrate the head group dynamics (see Figures 4 and 5). The DLPE head group encounters frequent collisions in ita motions, presumably due to the hydrogen bonds to the surroundingsolvent. The VAF' does not decay to zero even at long times. In the case of DMPC, the VAF for the head group motion decays to zero within -2 ps, indicating a much smoother motion. The normalized spectral density for the two systems shown in the figure insets also illustrates this clearly. In the case of DLPE, the head groups have to break hydrogen bonds to undergo rotational motions, which may be responsible for the observed shape of the power spectrum, while in DMPC, the head groups have a weaker interaction which explains the smoother profile for the power spectrum. This is consistent with a clathrate-like structure. Furthermore, the larger head group mass may also be a contributing factor to the intensity in the low-frequency region. (c) Water Dynamics. We have analyzed the mobility of water by classifyingthem as bulk and bound, depending on their proximity to the lipid head group atoms. We considered any water within 6 A from any head group atom as a bound water and any water molecules farther away from all head group atoms as bulk water. We chose 6 A as the cutoff since the first minimum in the g(r) for DMPC is at approximately this value (see Figure 1). In Figure 6 we give the probability distribution for bound and bulk water molecules in the DMPC case. The bound waters are involved in clathrate-like structure while the bulk water molecules are involved in interaction with the clathrate and with themselves. Note that these two regions overlap quite substantially, which again indicates the roughness of the bilayer surface. For example, we find that several of the -N(CH&+ groups protrude out into the aqueous region during the course of the simulation. This effectively pushes the clathrate ("bound") region further out into the aqueous region. This cause8 the penetration of the bound and bulk regions. We have also calculated the mean square displacement and velocity autocorrelation functions for the bound and bulk waters separately. The bulk/bound status of the water moleculeswas updated every 1 ps. On average, there were -2250-300 bulk water molecules (out of the total of 842 waters). The bound water molecules, as expected, show a lower diffusion coefficient and higher intensities in the high-frequency region of the spectral density than does the bulk water molecules.12 Conclusions
The head group-water interactionshave been compared in the DLPE and DMPC lipid-water systems. The main difference between the DMPC- and DLPE-based lipid
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1182 Langmuir, Vol. 9,No.5, 1993 50.
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