Langmuir 2007, 23, 6991-6996
6991
Role of Dipolar Interaction in the Mesoscopic Domains of Phospholipid Monolayers: Dipalmitoylphosphatidylcholine and Dipalmitoylphosphatidylethanolamine K. Thirumoorthy and N. Nandi* Chemistry Department, Birla Institute of Technology and Science, Pilani, Rajasthan 333031, India
D. Vollhardt* Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany ReceiVed January 19, 2007. In Final Form: March 26, 2007 The role of dipolar interactions in determining the lipid domain shapes at the air-water interface with a change in the chemical structure of the head groups of lipids is theoretically studied. The phospholipids considered are dipalmitoylphosphatidylcholine (D,L-DPPC) and dipalmitoylphosphatidylethanolamine (DPPE). Despite closely similar chemical structures, the domains of the two lipids are strikingly different. The DPPC domains exhibit elongated arms, while the DPPE domains are nearly round-shaped. To compare the dipolar repulsions in the domains of the two phospholipids, different energy-minimized conformers of DPPC and DPPE are studied using the semiempirical quantum chemical method (PM3). It is found that the dipole moment of DPPC is significantly larger than that of DPPE. The in-plane and out-of-plane components of the dipole moments are calculated using grazing incidence X-ray diffraction data at different surface pressure values, as used in the experiment. The result indicates that the magnitude of the dipolar interaction is significantly larger in DPPC than that in DPPE over the surface pressure range considered. The enhanced dipolar repulsion corroborates well with the difference in the domain shapes in the two phospholipid monolayers. The larger dipolar repulsion in DPPC leads to development of elongated domain arms, while relatively less dipolar repulsion allows a closed shape of the condensed-phase DPPE domains.
I. Introduction Amphiphilic monolayers at the air-water interface exhibit a large variety of domain shapes in the condensed state.1-5 It is established that the domain morphology is essentially determined by the following factors: (1) line tension at the domain boundary, (2) dipolar repulsion between the molecules, and (3) molecular chirality. In recent years, molecular studies of dipolar repulsion in monolayers have been initiated, principally driven by the observation that a small change in the molecular charge distribution of the head group leads to a drastic variation in the aggregate domain shape, which can be explained on the basis of the dipolar structure of the relevant head groups.6-8The studies reveal the subtle role of in-plane and out-of-plane dipole moments in determining the variation in the domain shape. Such studies indicate that underlying molecular interactions can dictate the mesoscopic shape in monolayers. Importantly, the same principle of microstructural-mesostructural correlation, as observed in these biomimetic systems, might be operative in similar biological systems such as membranes. * To whom correspondence should be addressed. (N.N.) E-mail: nnandi@ bits-pilani.ac.in. Fax: 91-1596-244183. (D.V.) E-mail: vollh@ mpikg-golm.mpg.de. Fax: 49-331-567-9202. (1) Mo¨hwald, H. Annu. ReV. Phys. Chem. 1990, 41, 441. (2) McConnell, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171. (3) Nandi, N.; Vollhardt, D. Chem. ReV. 2003, 103, 4033. (4) Nandi, N.; Vollhardt, D. Thin Solid Films 2003, 433, 12. (5) Nandi, N.; Vollhardt, D. In Bottoms up nanofabrication: Supramolecules, self assemblies and organized films; Ariga, K., Ed.; American Scientific Publishers: Valencia, CA, 2007; Chapter 5, pp 1-29. (6) Nandi, N.; Vollhardt, D. J. Phys. Chem. B 2004, 108, 18793. (7) Thirumoorthy, K.; Nandi, N.; Vollhardt, D. J. Phys. Chem. B 2005, 109, 10820. (8) Thirumoorthy, K.; Nandi, N.; Vollhardt, D.; Oliveira, O. N., Jr. Langmuir 2006, 22, 5398.
The chemical structures of the two compounds are compared in Figure 1, and their BAM images of the condensed-phase domains of the corresponding enantiomeric molecules are presented in Figure 2. A comparison of the chemical structure shows that the two lipid structures are identical except the head group region. While DPPC has +N(CH3)3 present in the head group, the DPPE headgroup has +NH3 in the corresponding molecular segment. Despite this similarity, the domain shapes are different, revealing the difference in the underlying molecular interaction. Both equilibrium and nonequilibrium domain shapes of DPPC and DPPE are well investigated by combining experimental techniques such as surface pressure isotherm measurement, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) and theory, with a major focus on the effect of chirality, particularly on the shape of DPPC.9-17 Remarkable differences in the mesoscopic domain shapes can be observed by BAM (Figure 2). The equilibrium enantiomeric domain of DPPC develops elongated armed triskelion structures with curvature in the arms, whereas the enantiomeric domains of DPPE are nearly round-shaped subdivided by curved defect lines at which the orientation jumps (9) Nandi, N.; Vollhardt, D. J. Phys. Chem. B 2002, 106, 10144. (10) Vollhardt, D. In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.; Marcel Dekker: New York, in press. (11) Brezesinski, G.; Dietrich, A.; Struth, B.; Bo¨hm, C.; Bouwman, W. G.; Kjaer, K.; Mo¨hwald, H. Chem. Phys. Lipids 1995, 76, 145. (12) Weidemann, G.; Vollhardt, D. Colloids Surf., A 1995, 100, 187. (13) Weidemann, G.; Vollhardt, D. Biophys. J. 1996, 70, 2758. (14) Nandi, N.; Vollhardt, D. J. Phys. Chem. B 2002, 106, 10144. (15) Dahmen-Levison, U.; Brezesinski, G.; Mo¨hwald, H. Thin Solid Films 1998, 327, 616. (16) Brezesinski, G.; Dietrich, A.; Struth, B.; Bo¨hm, C.; Bouwman, W. G.; Kjaer, K.; Mo¨hwald, H. Chem. Phys. Lipids 1995, 76, 145. (17) Maltseva, E. Doctoral Thesis, University of Potsdam, 2005.
10.1021/la070168z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/26/2007
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Figure 2. BAM images of the enantiomeric DPPC and DPPE condensed-phase domains. Table 1. Grazing Incidence X-ray Diffraction Data for DPPC16 and DPPE17 at 20 °C
Figure 1. Chemical structure of DPPC and DPPE molecules.
into regions of varying molecular projections. The curvature in the arm (as well as variation of the molecular projection) in the case of DPPC was explained in terms of the chirality of the constituent molecule,3,9 and the same reasoning can be made to understand the variation of the direction of molecular projections in DPPE domains. However, the remarkable difference observed in the shapes, namely, elongated arms in DPPC and a round shape in DPPE, indicates that the relative strengths of the dipolar interaction and line tension play an important role in determining the drastically different shapes of the two lipids of closely similar chemical structure. It is to be noted that the domain-shapedependent energy is given by a competition between the line tension and dipolar repulsion. To the best of our knowledge, no measurement of line tension in the condensed-phase domain of the lipid studied is available. The present calculation assumes that the line tensions of the two lipids do not differ drastically. The effect of chirality can be observed in the curvature of both domains, as explained in the case of DPPC domains recently.3,9 It is expected that the observed differences in the overall shape (explicitly, the elongated arms of DPPC and the nearly round shape of DPPE) are due to the difference in the polarity of the head groups. A comparison of the dipolar interaction can provide insight into the role of dipolar repulsion in determining the domain shape variation. With this objective, we calculated the average dipole moment using quantum mechanical methods and its components using experimental information about the molecular orientation and lattice structure at the interface from the GIXD data,17 as presented in Table 1. The calculation of the out-ofplane and in-plane dipole moments and the area occupied by a
surface pressure (mN/m)
tilt angle (deg)
Axy (Å2)
A0 (Å2)
15 25 35 45
DPPC 35.9 33.8 30.4 27.6
25.10 24.4 23.6 22.6
20.3 20.3 20.3 20.0
5 10 20 25 30 40
DPPE 26.4 23.9 18.0 15.3 12.3 0.0
22.3 21.9 21.1 20.8 20.4 19.9
19.9 20.0 20.1 20.1 20.0 19.9
molecule is expected to provide information about the dipolar repulsion in such systems. With this end in view, the dipole moments of different energy-minimized conformations of DPPC and DPPE are calculated. The contributions of the in-plane and out-of-plane dipole moments are analyzed using the GIXD data. Details of the theoretical calculations of dipolar interactions are presented in the next section. The results are analyzed in the subsequent section followed by concluding remarks. II. Theoretical Calculation Whereas accurate information about the tilt of the amphiphilic alkyl chain can be obtained from GIXD data, such detailed experimental information about the head groups is unavailable. Different conformations of head groups are possible with different average molecular orientations and magnitudes of dipoles. We, thus, calculate the dipole moment for different energy-minimized conformations of both lipid molecules. The calculated dipole moments
Dipolar Interaction in Phospholipid Monolayers are then used to compute the in-plane and out-of-plane (perpendicular) components according to the experimentally observed molecular tilt from the GIXD data. As the GIXD data only provide the alkyl chain orientation, the present calculation assumes a rigid geometry of the molecule. Different energy-minimized conformations for each of the lipid molecules are obtained. The orientations of the molecular dipole and the molecular tail are calculated for each molecule as the conformation and dipole moments are generated using the PM3 method. The calculated dipole moments are used to estimate the in-plane µxy () [µx2 + µy2]1/2) and out-of-plane (µz) components using the experimental tilt and orientation at the interfacial plane as follows. First, the orientation of the molecule is calculated on the basis of the generated structure and coordinates of atoms as obtained for the energy-minimized structure. The coordinates are changed so that the average direction of the alkyl tail is made to coincide with that experimentally observed for tilt and azimuthal projection of the molecule (based on GIXD data). The tilt angle as obtained from GIXD data defines the spatial position of the alkyl chain. This information is incorporated to calculate the dipole moment components. The change in dipolar orientation due to molecular reorientation (corresponding to the process of coincidence of the arbitrarily generated molecular conformation with the orientations obtained from GIXD) is calculated. The dipole moment components are then recalculated, keeping the molecular orientation identical with the GIXD data. Thus, on the basis of GIXD data at various surface pressures, the corresponding dipole moments are also calculated. The present calculations assume a rigid geometry of the molecules. The dipole moment is calculated using a standard molecular modeling technique applying the well-known quantum mechanical program package (CHEM 3D software),18 and the theory used is at the semiempirical quantum mechanical method level (PM3 level).19 The theoretical method is a well-known approximate molecular orbital theory and is the third parametrization of the MNDO theory (modified neglect of diatomic overlap; parametric method number 3). The charges are obtained using the the Mulliken population analysis (MPA) technique.20 Thirty different energy-minimized conformations for each of the lipid molecules are obtained, and the corresponding dipole moments are calculated. The orientations of the molecular dipole and the molecular alkyl chains are calculated for each molecule as the conformation and dipole moments are generated using the PM3 method in a medium with a low dielectric constant (magnitude taken as 2). In addition to the tilt from the normal to the surface and the azimuthal projection on the surface, the consideration of the rotation of the molecule around the chain axis (often referred to as “rotation in a cone”) is important to consider. The rotation of the molecule in a cone implies that the molecule should have the possibility to vary the azimuthal angle. The variation of the azimuthal angle from the average values becomes larger with an increase in the degree of rotation of the molecule in a cone. Such rotation is more feasible in the gaseous or liquid-expanded state of the monolayer, and it is gradually more restricted as the monolayer approaches the condensed state at higher surface pressures. Supported by the experimental GIXD data on the lattice structure, in the case of the condensed-phase domains, it can be assumed that the rotation is not significant enough to affect the results of the calculation. The in-plane and out-of-plane components of the dipole moments of all energy-minimized conformations are analyzed in the Results and Discussion. The higher range of surface pressure is considered in the calculation where the role of the dipolar interaction in domain formation is known to be significant. Since we are comparing domain shapes in the LC phase, the higher surface pressures are more significant than the lower surface pressures. Thus, GIXD data at a surface pressure higher than 15-20 mN/m are used in the theoretical calculation. The results are presented in the next section.
III. Results and Discussion Figure 3 shows that DPPC has a higher population of lowenergy conformations with large dipole moments than DPPE.
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Figure 3. Comparison of the total dipole moments of DPPC and DPPE at 20 °C. The calculation is described in section II.
The result can be understood from the fact that the size of the DPPC head group is larger than that of DPPE. The dipole moment of a species depends on its charge separation in the molecular charge distribution. Larger charge separations lead to a concomitant increase of the dipole moment in the molecule, as known for biomolecules such as proteins.21 Since the size of the DPPC head group is significantly larger than that of DPPE, due to the presence of three methyl groups, conformations with a larger charge separation are more probable for DPPC than DPPE. This leads to a greater population of conformations with larger dipole moments in DPPC than in DPPE, as shown in Figure 3. As a result of the relatively greater population of conformations with large dipole moments in DPPC than DPPE, the domain of the former develops elongated arms while that of latter has a round shape. The out-of-plane dipole moment components for DPPC and DPPE obtained for the surface pressure region higher than 15 mN/m are compared in Figure 4, and they are larger for DPPC than for DPPE. The in-plane components for DPPC and DPPE are compared for a similar surface pressure range in Figure 5. On average, the in-plane dipole moment of DPPC is almost double that of DPPE at corresponding surface pressures within (18) CHEM 3D Ultra, version 8.0; Cambridge Soft Corp.: Cambridge, MA. (19) Stewart, J. P. J. Comput. Chem. 1989, 10, 209. (20) Jensen, F. Introd. Comput. Chem. 1999, 81-99. (21) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. ReV. 2000, 100, 2013.
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Figure 4. Comparison of the out-of-plane components of the dipole moments of DPPC and DPPE at 20 °C. The corresponding surface pressures are indicated in the diagram. The calculation is described in section II.
the complete surface pressure range investigated. It is important to note that the tilt from the normal is larger for DPPC than for DPPE over the studied surface pressure range, as shown in Table 1. Consequently, the in-plane dipole moment in DPPC is approximately double that in DPPE, which is due to the larger molecular tilt in the DPPC domain than in the DPPE domain. This correlates with the experimental finding that the enantiomeric
DPPC domains develop elongated arms because of the larger dipolar repulsion whereas the DPPE domains are closed. In the latter case, obviously, the dipolar repulsion cannot overcome the line tension. The larger in-plane component leads to growth in the direction toward the average tilt, whereas relatively low dipolar repulsion in the tilt direction allows growth in various directions, favoring a closed shape in DPPE.
Dipolar Interaction in Phospholipid Monolayers
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Figure 5. Comparison of the in-plane components of the dipole moments of DPPC and DPPE at 20 °C. The corresponding surface pressures are indicated in the diagram. The calculation is described in section II.
IV. Conclusions Summarily, different conformers of DPPC and DPPE were studied using the semiempirical quantum chemical method (PM3).
The in-plane and out-of-plane components of dipole moments were calculated using grazing incidence X-ray diffraction data. The components were compared for comparable surface pressure
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values within the accessible surface pressure range. The result indicates that the dipolar magnitude is significantly larger in DPPC than in DPPE. The enhanced dipolar repulsion correlates with the difference in the domain shapes of the two monolayers. A larger in-plane component leads to growth in the direction toward the average tilt, whereas relatively low dipolar repulsion
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in the tilt direction allows growth in various directions, favoring a closed shaped domain in DPPE. Acknowledgment. This work is supported by a grant from DST, India, to N.N. LA070168Z
