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. 6-8 The 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. 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(CH 3 ) 3 present in the head group, the DPPE headgroup has + NH 3 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 sub- divided by curved defect lines at which the orientation jumps * 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. (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. 6991 Langmuir 2007, 23, 6991-6996 10.1021/la070168z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/26/2007