Molecular Dynamics Simulations of Liquid Condensed to Liquid Expanded Transitions in DPPC Monolayers

Review of the Interfacial Structure and Properties of Surfactants in Petroleum Production and Geological Storage Systems from a Molecular Scale Perspective

Surfactants play a crucial role in tertiary oil recovery by reducing the interfacial tension between immiscible phases, altering surface wettability, and improving foam film stability. Oil reservoirs have high temperatures and high pressures, making it difficult and hazardous to conduct lab experiments. In this context, molecular dynamics (MD) simulation is a valuable tool for complementing experiments. It can effectively study the microscopic behaviors (such as diffusion, adsorption, and aggregation) of the surfactant molecules in the pore fluids and predict the thermodynamics and kinetics of these systems with a high degree of accuracy. MD simulation also overcomes the limitations of traditional experiments, which often lack the necessary temporal-spatial resolution. Comparing simulated results with experimental data can provide a comprehensive explanation from a microscopic standpoint. This article reviews the state-of-the-art MD simulations of surfactant adsorption and resulting interfacial properties at gas/oil-water interfaces. Initially, the article discusses interfacial properties and methods for evaluating surfactant-formed monolayers, considering variations in interfacial concentration, molecular structure of the surfactants, and synergistic effect of surfactant mixtures. Then, it covers methods for characterizing microstructure at various interfaces and the evolution process of the monolayers' packing state as a function of interfacial concentration and the surfactants' molecular structure. Next, it examines the interactions between surfactants and the aqueous phase, focusing on headgroup solvation and counterion condensation. Finally, it analyzes the influence of hydrophobic phase molecular composition on interactions between surfactants and the hydrophobic phase. This review deepened our understanding of the micro-level mechanisms of oil displacement by surfactants and is beneficial for screening and designing surfactants for oil field applications.

A Review on Investigating the Interactions between Nanoparticles and the Pulmonary Surfactant Monolayer with Coarse-Grained Molecular Dynamics Method

Pulmonary drug delivery has garnered significant attention due to its targeted local lung action, minimal toxic side effects, and high drug utilization. However, the physicochemical properties of inhaled nanoparticles (NPs) used as drug carriers can influence their interactions with the pulmonary surfactant (PS) monolayer, potentially altering the fate of the NPs and impairing the biophysical function of the PS monolayer. Thus, the objective of this review is to summarize how the physicochemical properties of NPs affect their interactions with the PS monolayer. Initially, the definition and properties of NPs, as well as the composition and characteristics of the PS monolayer, are introduced. Subsequently, the coarse-grained molecular dynamics (CGMD) simulation method for studying the interactions between NPs and the PS monolayer is presented. Finally, the implications of the hydrophobicity, size, shape, surface charge, surface modification, and aggregation of NPs on their interactions with the PS monolayer and on the composition of biomolecular corona are discussed. In conclusion, gaining a deeper understanding of the effects of the physicochemical properties of NPs on their interactions with the PS monolayer will contribute to the development of safer and more effective nanomedicines for pulmonary drug delivery.

Phase transitions of fluorotelomer alcohols at the water|alkane interface studied via molecular dynamics simulation

Fluorosurfactants are long-lasting environmental pollutants that accumulate at interfaces ranging from aerosol droplet surfaces to cell membranes. Modeling of adsorption-based removal technologies for fluorosurfactants requires accurate simulation methods which can predict their adsorption isotherm and monolayer structure. Fluorotelomer alcohols with one or two methylene groups adjacent to the alcohol (7 : 1 FTOH and 6 : 2 FTOH, respectively) are investigated using the OPLS-AA force field at the water|hexane interface, varying the interfacial area per surfactant. The acquired interfacial pressure isotherms and monolayer phase behavior are compared with previous experimental results. The results are consistent with the experimental data inasmuch as, at realistic adsorption densities, only 7 : 1 FTOH shows a phase transition between liquid-expanded (LE) and 2D crystalline phases. Structures of the LE and crystalline phases are in good agreement with the sticky disc and Langmuir defective crystal models, respectively, used previously to interpret experimental data. Interfacial pressure of the LE phase agrees well with experiment, and sticky disc interaction parameters indicate no 2D LE-gas transition is present for either molecule. Conformation analysis reveals 7 : 1 FTOH favors conformers where the OH dipole is perpendicular to the molecular backbone, such that the crystalline phase is stabilized when these dipoles align.

Advantages of the classical thermodynamic analysis of single-and multi-component Langmuir monolayers from molecules of biomedical importance-theory and applications

The Langmuir monolayer technique has been successfully used for decades to model biological membranes and processes occurring at their interfaces. Classically, this method involves surface pressure measurements to study interactions within membrane components as well as between external bioactive molecules (e.g. drugs) and the membrane. In recent years, surface-sensitive techniques were developed to investigate monolayers in situ; however, the obtained results are in many cases insufficient for a full characterization of biomolecule-membrane interactions. As result, description of systems using parameters such as mixing or excess thermodynamic functions is still relevant, valuable and irreplaceable in biophysical research. This review article summarizes the theory of thermodynamics of single- and multi-component Langmuir monolayers. In addition, recent applications of this approach to characterize surface behaviour and interactions (e.g. orientation of bipolar molecules, drug-membrane affinity, lateral membrane heterogeneity) are presented.

The lung surfactant activity probed with molecular dynamics simulations

The surface of pulmonary alveolar subphase is covered with a mixture of lipids and proteins. This lung surfactant plays a crucial role in lung functioning. It shows a complex phase behavior which can be altered by the interaction with third molecules such as drugs or pollutants. For studying multicomponent biological systems, it is of interest to couple experimental approach with computational modelling yielding atomic-scale information. Simple two, three, or four-component model systems showed to be useful for getting more insight in the interaction between lipids, lipids and proteins or lipids and proteins with drugs and impurities. These systems were studied theoretically using molecular dynamic simulations and experimentally by means of the Langmuir technique. A better understanding of the structure and behavior of lung surfactants obtained from this research is relevant for developing new synthetic surfactants for efficient therapies, and may contribute to public health protection.

Drug Meets Monolayer: Understanding the Interactions of Sterol Drugs with Models of the Lung Surfactant Monolayer Using Molecular Dynamics Simulations

The lung surfactant monolayer (LSM) is a thin layer of lipids and proteins that forms the air/water interface of the alveoli. The primary function of the LSM is to reduce the surface tension at the air/water interface during breathing. The LSM also forms the main biological barrier for any inhaled particles, including drugs, to treat lung diseases. Elucidating the mechanism by which these drugs bind to and absorb into the LSM requires a molecular-level understanding of any drug-induced changes to the morphology, structure, and phase changes of the LSM.Molecular dynamics simulations have been used extensively to study the structure and dynamics of the LSM. The monolayer is usually simulated in at least two states: the compressed state, mimicking exhalation, and the expanded state, mimicking inhalation. In this chapter, we provide detailed instructions on how to set up, run, and analyze coarse-grained MD simulations to study the concentration-dependent effect of a sterol drug on the LSM, both in the expanded and compressed state.

Modulation of membrane physical properties by natural insecticidal ketones

The insecticidal activity of Mentha oil and its main components has been tested and established for various insects/pests. Several mint ketones have demonstrated to act on GABAA receptors (GABAA-R), a transmembrane channel target of several important insecticides whose activity can be modulated by surface-active compounds and by changes in the physical properties of the lipid membrane. In the present work, we analyze the capacity of monoterpenic ketones most commonly found in Mentha species, pulegone and menthone, to interact with DPPC membranes by molecular dynamics (MD) simulations and Langmuir monolayers. The experimental results indicate that the presence of menthone and pulegone in the subphase modify the interfacial characteristics of DPPC isotherms. The changes were reflected as expansion of the isotherms and disappearance or bringing forward of DPPC phase transition. MD simulation corroborate these results and indicate that both ketones are located at the region of the carbonyl group, at the interface with the acyl chains. Ketone intercalation between lipid molecules would induce an increasing intermolecular interaction, diminishing the film elasticity and causing an ordering effect. Our results suggest that the insecticidal activity of both ketones could involve their interaction with lipid molecules causing disturbance of the cell membrane as postulated for several larvicide compounds, or at least modulating the receptor surrounding.

Modeling Gas-Liquid Interfaces by Dissipative Particle Dynamics: Adsorption and Surface Tension of Cetyl Trimethyl Ammonium Bromide at the Air-Water Interface

Adsorption of surfactants at gas-liquid interfaces that causes reduction in the surface tension is a classical problem in colloid and interface science with multiple practical applications in oil and gas recovery, separations, cosmetics, personal care, and biomedicine. Here, we develop an original coarse-grained model of the liquid-gas interface within the conventional dissipative particle dynamics (DPD) framework with the goal of quantitatively predicting the surface tension in the presence of surfactants. As a practical case-study example, we explore the adsorption of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) on the air-water interface. The gas phase is modeled as a DPD fluid composed of fictitious hard-core "gas" beads with exponentially decaying repulsive potentials to prevent penetration of the liquid phase components. A rigorous parametrization scheme is proposed based on matching the bulk and interfacial properties of water and octane taken as the reference compounds. Quantitative agreement between the simulated and experimental surface tension of CTAB solutions is found for a wide range of bulk surfactant concentrations (∼10^-3 to ∼1 mmol/L) with the reduction of the surface tension from ∼72 mN/m (pure water) to the limiting value of ∼37.5 mN/m at the critical micelle concentration. The gas phase DPD model with the proposed parametrization scheme can be extended and applied to modeling various gas-liquid interfaces with surfactant and lipid monolayers, such as bubble suspensions, foams, froths, etc.

Molecular Ordering in Lipid Monolayers: An Atomistic Simulation

We report on atomistic simulations of DPPC lipid monolayers using the CHARMM36 lipid force field (and also the Slipid force field as a control case), combined with a four-point OPC water model. The entire two-phase region where domains of the "liquid-condensed" (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 those 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. In the case of the CHARMM36 force field, 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 and (48.1 ± 0.5)° 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 those in 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, with regard to 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 might be anisotropic with respect to the interfacial domain boundary.

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.

Anisotropic line tension of domains in lipid monolayers

We formulate a simple effective model to describe molecular interactions in a lipid monolayer and calculate the line tension between coexisting domains. The model represents lipid molecules in terms of two-dimensional anisotropic particles on the plane of the monolayer. These particles interact through forces that are believed to be relevant for the understanding of fundamental properties of the monolayer: van der Waals interactions originating from lipid chains and dipolar forces between dipole groups in the molecular heads. The model stresses the liquid-crystalline nature of the ordered phase in lipid monolayers and explains coexistence properties between ordered and disordered phases in terms of molecular parameters. Thermodynamic and interfacial properties of the model are analyzed using density-functional theory. In particular, the line tension at the interface between ordered and disordered phases turns out to be highly anisotropic with respect to the angle between the nematic director and the interface separating the coexisting phases. This important feature mainly results from the tilt angle of lipid chains and, to a lesser extent, from dipolar interactions perpendicular to the monolayer. The role of the two dipolar components, parallel and perpendicular to the monolayer, is assessed by comparing with computer simulation results for lipid monolayers.

Characterisation of N-(Octadecyl)-1,8-naphthalimide Monolayer Compression Using Molecular Dynamics and Experimental Approaches

The development of luminescent surfaces is an active area of supramolecular chemistry, particularly for the development of new sensing platforms. One particularly useful surface deposition method is the Langmuir-Blodgett technique where organic amphiphilic fluorophores (e.g. 1,8-naphthalimides) can form ordered monolayers at an air-water interface before being deposited onto solid supports. The ability to simulate monolayer formation and consequently develop predictability over film formation would allow for significant advances in the luminescent materials field where synthesis might be directed by simulation data. Here, we compare pressure-area isotherms of N-(octadecyl)-1,8-naphthalimide determined experimentally, using the Langmuir-Blodgett technique, and computationally, using three different simulation techniques. We find that all three simulation techniques are able to describe the liquid-condensed/liquid-expanded region of the isotherm, and that the isotherms are highly similar in this region, although the NγT ensemble performs best. Experimental isotherms showed film formation properties that align with the simulation data, suggesting that simulations are a viable means to direct synthesis. Investigation of the underlying structural details disclosed by the simulations reveals the compression-induced ordering at atomic-level detail, which will allow prediction of how functionalisation of the naphthalimides will alter the monolayer compression and mounting process.

Atomistic Model for Nearly Quantitative Simulations of Langmuir Monolayers

Lung surfactant and a tear film lipid layer are examples of biologically relevant macromolecular structures found at the air-water interface. Because of their complexity, they are often studied in terms of simplified lipid layers, the simplest example being a Langmuir monolayer. Given the profound biological significance of these lipid assemblies, there is a need to understand their structure and dynamics on the nanoscale, yet there are not many techniques able to provide this information. Atomistic molecular dynamics simulations would be a tool fit for this purpose; however, the simulation models suggested until now have been qualitative instead of quantitative. This limitation has mainly stemmed from the challenge to correctly describe the surface tension of water with simulation parameters compatible with other biomolecules. In this work, we show that this limitation can be overcome by using the recently introduced four-point OPC water model, whose surface tension for water is demonstrated to be quantitatively consistent with experimental data and which is also shown to be compatible with the commonly employed lipid models. We further establish that the approach of combining the OPC four-point water model with the CHARMM36 lipid force field provides nearly quantitative agreement with experiments for the surface pressure-area isotherm for POPC and DPPC monolayers, also including the experimentally observed phase coexistence in a DPPC monolayer. The simulation models reported in this work pave the way for nearly quantitative atomistic studies of lipid-rich biological structures at air-water interfaces.

Understanding the interactions between sebum triglycerides and water: a molecular dynamics simulation study

Sebum triglycerides actively contribute to the water transport across the sebum layerviathe percolating network of triglyceride head groups.

Molecular-level insight into the binding of arginine to a zwitterionic Langmuir monolayer

Arginine molecules bind to a DPPC monolayer, altering the interfacial electrostatic potential and the lateral mobility of the lipids, while having little effect on the compression isotherm of the monolayer.

The impact of auxins used in assisted phytoextraction of metals from the contaminated environment on the alterations caused by lead(II) ions in the organization of model lipid membranes

Auxins are successfully used to improve phytoextraction efficiency of metal ions from the contaminated environment, however, the mechanism of their activity in this field is not explained. Auxins are known to exert various biochemical alterations in the plant membranes and cells, but their activity involves also direct interactions with lipids leading to changes in membrane organization. Following the suggestion that the auxins-induced modifications in membrane properties alleviate toxic effect of metal ions in this paper we have undertaken the comparative studies on the effect of metal ions and metal ions/auxins mixtures on model membrane systems. The experiments were done on lipid monolayers differing in their composition spread on water subphase and on Pb(2+), Indole-3-acetic acid (IAA), 1-Naphthaleneacetic acid (NAA) and Pb(2+)/IAA and Pb(2+)/NAA water solutions. The analysis of the collected data suggests that metal ions and auxins can change fluidity of the lipid systems and weaken the interactions between monolayer components. This manifested in the increase of the mean area per molecule and the excess area per molecule values for the films on Pb(2+), auxins as well as Pb(2+)/auxin solutions as compared to the values on pure water subphase. However, the presence of auxin in the mixture with lead(II) ions makes the alterations induced by sole metal ions weaker. This effect was more pronounced for the membranes of a higher packing. Thus it was proposed that auxins may enhance phytoextraction of metal ions by weakening their destabilizing effect on membrane.

Computer simulations of lung surfactant

Lung surfactant lines the gas-exchange interface in the lungs and reduces the surface tension, which is necessary for breathing. Lung surfactant consists mainly of lipids with a small amount of proteins and forms a monolayer at the air-water interface connected to bilayer reservoirs. Lung surfactant function involves transfer of material between the monolayer and bilayers during the breathing cycle. Lipids and proteins are organized laterally in the monolayer; selected species are possibly preferentially transferred to bilayers. The complex 3D structure of lung surfactant and the exact roles of lipid organization and proteins remain important goals for research. We review recent simulation studies on the properties of lipid monolayers, monolayers with phase coexistence, monolayer-bilayer transformations, lipid-protein interactions, and effects of nanoparticles on lung surfactant. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Structural organization of sterol molecules in DPPC bilayers: a coarse-grained molecular dynamics investigation

We investigate the structural organization of cholesterol (CHOL) analogues in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers using coarse-grained molecular dynamics simulations and the MARTINI forcefield. Different sterol molecules are modelled by increasing (CHOLL) or decreasing (CHOLS) the diameter of the sterol beads employed in the MARTINI model of CHOL. At high sterol concentrations, (xsterol = 0.5), typical of liquid ordered phases, we find that the sterol arrangement and sterol-DPPC interactions strongly depend on the sterol size. Smaller sterols (CHOLS and CHOL) form linear clusters, while the larger sterols (CHOLL) arrange themselves into disc shaped clusters. By combining structural and dynamical properties we also investigate the So→ Ld transition for the CHOLL and CHOLS sterols. We show that small changes in the sterol size significantly affect the stability of the gel phase with the gel phase stabilized by the small sterols, but destabilized by large sterols. The general dependence of the phase behaviour of the membrane with sterol content is reminiscent of the one observed in naturally occurring membranes. The relevance of our results to understand current cholesterol-bilayer structural models is discussed.

Effects of surface pressure on the properties of Langmuir monolayers and interfacial water at the air-water interface

The effects of surface pressure on the physical properties of Langmuir monolayers of palmitic acid (PA) and dipalmitoylphosphatidic acid (DPPA) at the air/water interface are investigated through molecular dynamics simulations with atomistic force fields. The structure and dynamics of both monolayers and interfacial water are compared across the range of surface pressures at which stable monolayers can form. For PA monolayers at T = 300 K, the untilted condensed phase with a hexagonal lattice structure is found at high surface pressure, while the uniformly tilted condensed phase with a centered rectangular lattice structure is observed at low surface pressure, in agreement with the available experimental data. A state with uniform chain tilt but no periodic spatial ordering is observed for DPPA monolayers on a Na(+)/water subphase at both high and low surface pressures. The hydrophobic acyl chains of both monolayers pack efficiently at all surface pressures, resulting in a very small number of gauche defects. The analysis of the hydrogen-bonding structure/dynamics at the monolayer/water interface indicates that water molecules hydrogen-bonded to the DPPA head groups reorient more slowly than those hydrogen-bonded to the PA head groups, with the orientational dynamics becoming significantly slower at high surface pressure. Possible implications for physicochemical processes taking place on marine aerosols in the atmosphere are discussed.

Protein modeling and molecular dynamics simulation of the two novel surfactant proteins SP-G and SP-H

Surfactant proteins are well known from the human lung where they are responsible for the stability and flexibility of the pulmonary surfactant system. They are able to influence the surface tension of the gas–liquid interface specifically by directly interacting with single lipids. This work describes the generation of reliable protein structure models to support the experimental characterization of two novel putative surfactant proteins called SP-G and SP-H. The obtained protein models were complemented by predicted posttranslational modifications and placed in a lipid model system mimicking the pulmonary surface. Molecular dynamics simulations of these protein-lipid systems showed the stability of the protein models and the formation of interactions between protein surface and lipid head groups on an atomic scale. Thereby, interaction interface and strength seem to be dependent on orientation and posttranslational modification of the protein. The here presented modeling was fundamental for experimental localization studies and the simulations showed that SP-G and SP-H are theoretically able to interact with lipid systems and thus are members of the surfactant protein family.

Atomistic potentials for trisiloxane, alkyl ethoxylate, and perfluoroalkane-based surfactants with TIP4P/2005 and application to simulations at the air-water interface

The mechanism of superspreading, the greatly enhanced spreading of water droplets facilitated by trisiloxane surfactants, is still under debate, largely because the role and behavior of the surfactants cannot be sufficiently resolved by experiments or continuum simulations. Previous molecular dynamics studies have been performed with simple model molecules or inaccurate models, strongly limiting their explanatory power. Here we present a force field dedicated to superspreading, extending existing quantum-chemistry-based models for the surfactant and the TIP4P/2005 water model ( Abascal et al. J. Chem. Phys. , 2005 , 123 , 234505 ). We apply the model to superspreading trisiloxane surfactants and nonsuperspreading alkyl ethoxylate and perfluoroalkane surfactants at various concentrations at the air-water interface. We show that the developed model accurately predicts surface tensions, which are typically assumed important for superspreading. Significant differences between superspreading and traditional surfactants are presented and their possible relation to superspreading discussed. Although the force field has been developed for superspreading problems, it should also perform well for other simulations involving polymers or copolymers with water.

Structural properties of POPC monolayers under lateral compression: computer simulations analysis

1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a lipid comprising a saturated and an unsaturated acyl chain, belongs to the class of glycerophosphatidylcholines, major lipids in eukaryotic cell membranes. To get insight into the structural properties of this lipid within monolayers as membrane models, we performed molecular dynamics (MD) simulations of POPC monolayers under compression at the air/water interface. MD simulations were carried out at 300 K and at different surface pressures using the all-atom general Amber force field (GAFF). A good agreement was found between the simulated data and experimental isotherms. At surface pressures greater than 15 mN/m, two orientations of the head groups clearly appear: one nearly parallel to the monolayer interface and another one pointing toward the water. On the basis of the analysis of headgroup dihedral angles, we propose that the conformational variations around the bonds connecting the phosphorus atom to the adjacent oxygens are involved in these two orientations of the headgroup. The glycerol group orientation is characterized by a large distribution centered around 50° with respect to the monolayer normal. The acyl chains are predominantly in trans configuration from 7.5 to 43 mN/m surface pressures. Moreover, the calculated order parameter profiles of both chains suggest an independent behavior of the saturated and unsaturated chains that could be correlated with the formation of chain-type clusters observed along the simulated trajectories.

Simulations of lipid monolayers

A lipid monolayer lining a boundary between two immiscible phases forms a complex interface with inhomogeneous distribution of forces. Unlike lipid bilayers, monolayers are formed in asymmetric environment and their properties depend strongly on lipid surface density. The monolayer properties are also affected significantly by the representation of the pure interface. Here we give a brief theoretical introduction and describe methods to simulate lipid monolayers starting from force-fields and system setup to reproducing state points on the surface tension (pressure)-area isotherms and transformations between them.

Molecular dynamics simulations of the Ca2+-pump: a structural analysis

We report large scale molecular dynamics computer simulations, ∼100 ns, of the ion pump Ca(2+)-ATPase immersed in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. The structure simulated here, E1, one of the several conformations resolved using X-ray diffraction techniques, hosts two Ca(2+)-ions in the hydrophobic domain. Our results indicate that protonated residues lead to stronger ion-residue interactions, supporting previous conclusions regarding the sensitivity of the Ca(2+) behaviour to the protonated state of the amino acid binding sites. We also investigate how the protein perturbs the bilayer structure. We show that the POPC bilayer is ∼12% thinner than the pure bilayer, near the protein surface. This perturbation decays exponentially with the distance from the protein with a characteristic decay length of 0.8 nm. We find that the projected area per lipid also decreases near the protein. Using an analytical model we show that this change in the area is only apparent and it can be explained by considering the local curvature of the membrane. Our results indicate that the real area per lipid near the protein is not significantly modified with respect to the pure bilayer result. Further our results indicate that the local deformation of the membrane around the protein might be compatible with the enhanced protein activity observed in experiments over a narrow range of membrane thicknesses.

Molecular dynamics simulation of hydrated DPPC monolayers using charge equilibration force fields

We present results of molecular dynamics simulations of a model DPPC-water monolayer using charge equilibration (CHEQ) force fields, which explicitly account for electronic polarization in a classical treatment of intermolecular interactions. The surface pressure, determined as the difference between the monolayer and pure water surface tensions at 323 K, is predicted to be 22.92 ±1.29 dyne/cm, just slightly below the broad range of experimental values reported for this system. The surface tension for the DPPC-water monolayer is predicted to be 42.35 ±1.16 dyne/cm, in close agreement with the experimentally determined value of 40.9 dyne/cm. This surface tension is also consistent with the value obtained from DPPC monolayer simulations using state-of-the-art nonpolarizable force fields. The current results of simulations predict a monolayer-water potential difference relative to the pure water-air interface of 0.64 ±0.02 Volts, an improved prediction compared to the fixed-charge CHARMM27 force field, yet still overestimating the experimental range of 0.3 to 0.45 Volts. As the charge equilibration model is a purely charge-based model for polarization, the current results suggest that explicitly modeled polarization effects can offer improvements in describing interfacial electrostatics in such systems.