Phase transition of a DPPC bilayer induced by an external surface pressure: from bilayer to monolayer behavior. a molecular dynamics simulation study

Comprehensive Study of the Liquid Expanded-Liquid Condensed Phase Transition in 1,2-Dimyristoyl-sn-glycero-3-phospho-l-serine Monolayers: Surface Pressure, Volta Potential, X-ray Reflectivity, and Molecular Dynamics Modeling

An integrated approach is applied to reveal fine changes in the surface-normal structure of 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS) monolayers at the air-lipid-water interface occurring in a liquid expanded (LE)-liquid condensed (LC) transition. The combination of the Langmuir monolayer technique, X-ray reflectometry, and molecular dynamics (MD) modeling provides new insight into the molecular nature of electrostatic phenomena in different stages of lipid compression. A homemade setup with a laboratory X-ray source (λ = 1.54 Å) offers a nondestructive way to reveal the structural difference between the LE and LC phases of the lipid. The electron density profile in the direction normal to the interface is recovered from the X-ray reflectivity data with the use of both model-independent and model-based approaches. MD simulations of the DMPS monolayer are performed for several areas per lipid using the all-atom force field. Using the conventional theory of capillary waves, a comparison is made between the electron density profiles reconstructed from the X-ray data and those calculated directly from MD modeling, which demonstrates remarkable agreement between the experiment and simulations for all selected lipid densities. This confirms the validity of the simulations and allows an analysis of the contributions of the hydrophobic tails and hydrated polar groups to the electron density profile and to the dipole component of the electric field at the interface. According to the MD data, the dependence of the Volta potential on the area per lipid in the monolayer has a different molecular nature below and above the phase transition. In the LE state of the monolayer, the potential is determined mostly by the oriented water molecules in the polar region of the lipid. In the LE-LC transition, these molecules are displaced to the bulk, and their effect on the Volta potential becomes insignificant compared with the contribution of the hydrophobic tails. The hydrophobic tails are highly ordered in the state of the liquid crystal so that their dipole moments entirely determine the growth of the potential upon compression up to the monolayer collapse.

Modulation of dipalmitoylphosphatidylcholine monolayers by dimethyl sulfoxide

The action of the penetration-enhancing agent, dimethyl sulfoxide (DMSO), on phospholipid monolayers was investigated at the air-water interface using a combination of experimental techniques and molecular dynamics simulations. Brewster angle microscopy revealed that DPPC monolayers remained laterally homogeneous at subphase concentrations up to a mole fraction of 0.1 DMSO. Neutron reflectometry of the monolayers in combination with isotopic substitution enabled the determination of solvent profiles as a function of distance perpendicular to the interface for the different DMSO subphase concentrations. These experimental results were compared to those obtained from molecular dynamic (MD) simulations of the corresponding monolayer systems. There was excellent agreement found between the MD-derived reflectivity curves and the measured data for all of the H/D contrast variations investigated. The MD provide a detailed description of the distribution of water and DMSO molecules around the phosphatidylcholine headgroup, and how this distribution changes with increasing DMSO concentrations. Significantly, the measurements and simulations that are reported here support the hypothesis that DMSO acts by dehydrating the phosphatidylcholine headgroup, and as such provide the first direct evidence that it does so primarily by displacing water molecules bound to the choline group.

Surface tension effects on the phase transition of a DPPC bilayer with and without protein: a molecular dynamics simulation

A coarse-grained molecular dynamics simulation was applied to illustrate the phase transition behavior of the pure DPPC bilayer and aquaporin-embedded DPPC bilayer under different surface tensions.

Spatial distribution of lipid headgroups and water molecules at membrane/water interfaces visualized by three-dimensional scanning force microscopy

At biological interfaces, flexible surface structures and mobile water interact with each other to present non-uniform three-dimensional (3D) distributions. In spite of their impact on biological functions, molecular-scale understanding of such phenomena has remained elusive. Here we show direct visualization of such interfacial structures with subnanometer-scale resolution by 3D scanning force microscopy (3D-SFM). We measured a 3D force distribution at an interface between a model biological membrane and buffer solution by scanning a sharp tip within the 3D interfacial space. We found that vertical cross sections of the 3D image taken along a specific lateral direction show characteristic molecular-scale contrasts tilted at 30° to the membrane surface. Detailed analysis of the 3D image reveals that the tilted contrast corresponds to the time-averaged conformation of fluctuating lipid headgroups. On the basis of the obtained results, we discuss the relationships among the hydration structure, headgroup fluctuation, molecular fluidity, and mechanical strength of the membrane. The results demonstrate that 3D-SFM is capable of visualizing averaged 3D distribution of fluctuating surface structures as well as that of mobile water (i.e., hydration structure) at interfaces between biological systems and water.

Phase behavior of lipid bilayers under tension

Given the proposed importance of membrane tension in regulating cellular functions, we explore the effects of a finite surface tension on phase equilibrium using a molecular theory that captures the quantitative structure of the phase diagram of the tensionless DPPC/DOPC/Cholesterol lipid bilayer. We find that an increase in the surface tension decreases the temperature of the transition from liquid to gel in a pure DPPC system by ∼1.0 K/(mN/m), and decreases the liquid-disordered to liquid-ordered transition at constant chemical potentials by approximately the same amount. Our results quantitatively isolate the role of tension in comparison to other thermodynamic factors, such as pressure, in determining the phase behavior of lipid bilayers.

Using molecular dynamics to study liquid phase behavior: simulations of the ternary sodium laurate/sodium oleate/water system

The prediction of surfactant phase behavior has applications in a wide range of areas. An accurate modeling of liquid phase behavior can aid our understanding of colloidal process or be used to design phases that respond in a defined way to their environment. In this work, we use molecular dynamics to model the phase behavior of the ternary sodium laurate/sodium oleate/water system and compare the simulation results to experimental data. Simulations were performed with the GROMOS 53A6 united-atom force field and cover the entire ternary phase diagram, producing micellar, hexagonal, and lamellar phases. The aggregate simulation time for the 33 simulations performed during this study is 4.4 μs. We find that the simulations were able to model the experimentally observed liquid phase behavior accurately, showing that the carboxylate and lipid parameters of the 53A6 force field give very good quality results for the in silico prediction of liquid system phase behavior.

Headgroup mediated water insertion into the DPPC bilayer: a molecular dynamics study

Molecular dynamics simulation was performed on the 1,2-dipalmitoyl-sn-phosphocholine (DPPC) bilayer-water system using the GROMOS96 53a6 united atom force field. The transferability of force field was tested by reproducing the area per lipid within 3% accuracy from the experimental value. The simulation shows that water can penetrate much deeper inside the bilayer almost up to the starting point of the aliphatic chain. There is significant evidence from experiments that water goes deep in the DPPC bilayer, but it has not been reported from theoretical work. The mechanism of insertion of water deep inside the lipid bilayer is still not clear. In this report, for the first time, the mechanism of water insertion deep into the bilayer has been proposed. Water transport occurs by the headgroup and its first solvation shell. The trimethyl ammonium (NMe(3)) group (headgroup of DPPC) has two stable conformations at the bilayer-water interface, one outside the bilayer and another inside it. The NMe(3) group has a large clustering of water around it and takes the water molecules inside the bilayer with it during its entry into the bilayer. The water molecules penetrate into the bilayer with the help of the NMe(3) group present at the headgroup of DPPC and eventually form hydrogen bonds with carbonyl oxygen present deep inside the bilayer. Structural characteristics at the bilayer-water interface region are also reported.

Comparing experimental and simulated pressure-area isotherms for DPPC

Although pressure-area isotherms are commonly measured for lipid monolayers, it is not always appreciated how much they can vary depending on experimental factors. Here, we compare experimental and simulated pressure-area isotherms for dipalmitoylphosphatidylcholine (DPPC) at temperatures ranging between 293.15 K and 323.15 K, and explore possible factors influencing the shape and position of the isotherms. Molecular dynamics simulations of DPPC monolayers using both coarse-grained (CG) and atomistic models yield results that are in rough agreement with some of the experimental isotherms, but with a steeper slope in the liquid-condensed region than seen experimentally and shifted to larger areas. The CG lipid model gives predictions that are very close to those of atomistic simulations, while greatly improving computational efficiency. There is much more variation among experimental isotherms than between isotherms obtained from CG simulations and from the most refined simulation available. Both atomistic and CG simulations yield liquid-condensed and liquid-expanded phase area compressibility moduli that are significantly larger than those typically measured experimentally, but compare well with some experimental values obtained under rapid compression.

Influence of vitamin C on alcohol binding to phospholipid monolayers

The simple model of the biological membrane is provided by well-controlled lipid monolayers at the air-water interface. The Maxwell displacement current technique (MDC) provides novel approach to conformation study of the membrane models. The effect of alcohols is interaction with membrane molecules, mainly with the lipid head group and consequent changes in physical-chemical properties of the membrane. The aim of study is to detect changes in structural, electrical and mechanical properties of dipalmitoyl-phosphatidylcholine (DPPC) monolayer on the subphase of methanol-water and ethanol-water mixtures before and after addition of antioxidant agent, vitamin C. Monolayers properties are investigated by a surface pressure analysis (including mechanical properties evaluation) and the Maxwell displacement current measurement, the dipole moment projection calculation. Surface pressure-area isotherms show similar behaviour of the DPPC monolayer on alcohol-water mixtures independently on presence of vitamin C. Binding/adsorption process induces change of electron density distribution across monolayer and thus the molecular dipole moment. We observe small or negligible binding of methanol molecules on oxygen bonds of DPPC. Thus the antioxidant, vitamin C, has no significant effect. For ethanol-water mixtures is observed recovery of electrical properties in presence of antioxidant agent. We suppose that vitamin C regulates DPPC-ethanol molecules interaction.