Molecular Ordering in Lipid Monolayers: An Atomistic Simulation

Abstract

We report on atomistic simulations of DPPC lipid monolayers using the CHARMM36 lipid force field and four-point OPC water model. The entire two-phase region where domains of the 'liquidcondensed' (LC) phase coexist with domains of the 'liquid-expanded' (LE) phase has been explored.The simulations are long enough that the complete phase-transition stage, with two domains coexisting in the monolayer, is reached in all cases. Also, system sizes used are larger than in previous works. As expected, domains of the minority phase are elongated, emphasizing the importance of anisotropic van der Waals and/or electrostatic dipolar interactions in the monolayer plane. The molecular structure is quantified in terms of distribution functions for the hydrocarbon chains and the PN dipoles. In contrast to previous work, where average distributions are calculated, distributions are here extracted for each of the coexisting phases by first identifying lipid molecules that belong to either LC or LE regions. The three-dimensional distributions show that the average tilt angle of the chains with respect to the normal outward direction is (39.0 ± 0.1) • in the LC phase.In the case of the PN dipoles the distributions indicate a tilt angle of (110.8 ± 0.5) • in the LC phase and (112.5 ± 0.5) • in the LE phase. These results are quantitatively different from previous works, which indicated a smaller normal component of the PN dipole. Also, the distributions of the monolayer-projected chains and PN dipoles have been calculated. Chain distributions peak along a particular direction in the LC domains, while they are uniform in the LE phase. Long-range ordering associated with the projected PN dipoles is absent in both phases. These results strongly suggest that LC domains do not exhibit dipolar ordering in the plane of the monolayer, the effect of these components being averaged out at short distances. Therefore, the only relevant component of the molecular dipoles, as regards both intra-and long-range interdomain interactions, is normal to the monolayer. Also, the local orientation of chain projections is almost constant in LC domains and points in the direction along which domains are elongated, suggesting that the line tension driving the phase transition is anisotropic with respect to the interfacial domain boundary. Both van der Waals interactions and interactions from normal dipolar components seem to contribute to an anisotropic line tension, with dipolar in-plane components playing a negligible role. 29 Mohammad-Aghaie, D.; Bresme, F. Force-field Dependence on the Liquid-Expanded to Liquid-Condensed Transition in DPPC Monolayers. Mol. Simul. 2016, 42, 391-397. 30 Baoukina, S.; Monticelli, L.; Marrink, S. J.; Tieleman, D. P. Pressure-Area Isotherm of a Lipid Monolayer from Molecular Dynamics Simulations. Langmuir 2007, 23, 12617-12623. 31 Choe, S.; Chang, R.; Jeon, J.; Violi, A. Molecular Dynamics Study of a Pulmonary Surfactant Film interacting with a Carbonaceous Nanoparticle. Biophys. J. 2008, 95, 4102-4114. 32 Hauser, H.; Pascher...

Anisotropic line tension of domains in lipid monolayers

“…This is supported by the results of Ref. [9] that the inplane component of the dipoles show almost complete librational motion about the monolayer normal. This could imply that the inplane dipolar components of neighbouring molecules interact weakly, because of strong screening [20] or otherwise.…”

“…ǫ GB ∼ 50 kT (estimated from atomistic force-field simulations [9], and see also [19]), we obtain values of λ ∼ 2 − 15 fN, i. e. within the same order of magnitude as in [15]. This corresponds to molecular projections parallel to the phase boundary.…”

“…Some features of our two-dimensional effective model for a lipid monolayer are inspired by the recent simulation study [9] of an atomistic model for a DPPC monolayer. In this reference the density of interaction units of a molecule, projected on the plane of the monolayer, was studied separately for domains of the LC and LE phases.…”

“…An important source of information comes from atomistic computer simulations. Two such simulations on DPPC monolayers have recently been published [9,10]. The two use the same force fields and the only difference is system size.…”

“…In the present paper we formulate a simple microscopic model based on interacting two-dimensional effective anisotropic particles. The model is inspired by recent computer simulation results that use atomistic force fields [9,10]. The model includes van der Waals interactions between lipid chains, and dipolar forces with perpendicular and parallel components with respect to the monolayer, associated to polar interactions between lipid head groups.…”