Dipalmitoylphosphatidylcholine Molecules Research Articles

Numerical investigations of translocation characteristics of nano-silica lunar dust across pulmonary surfactant monolayer

The interactions between nano-silica lunar dust (NSLD) on the moon surface and pulmonary surfactant (PS) monolayer will pose risks to astronaut health in future manned lunar exploration missions, but the specifics of these interactions are unknown. This study investigates them using the coarse-grained molecular dynamics method considering different sizes (5, 10, and 15 nm) and shapes (sphere, ellipsoid, and cube), with special focus on the unique morphology of NSLDs with bugles. The key findings are as follows: (1) The 10 nm and 15 nm NSLDs embed in the PS monolayer through the major sphere of spherical-type, major ellipsoid of ellipsoidal-type, or one edge of cubic-type NSLDs upon contact the PS monolayer. (2) Adsorbed NSLDs cause a higher Sz value (ASz > 0.84), while embedded NSLDs cause a lower Sz value (0.47 < ASz < 0.83) that decreases with an increase in the number of bulges. (3) The embedding process absorbs 50–342 dipalmitoylphosphatidylcholine (DPPC) molecules, reducing the PS monolayer area by 0.21%–6.05%. NSLDs with bulges absorb approximately 9–126 additional DPPC molecules and cause a 0.05%–3.22% reduction in the PS monolayer area compared to NSLDs without bulges. (4) NSLDs move obliquely or vertically within the PS monolayer, displaying two distinct stages with varying velocities. Their movement direction and speed are influenced by the increasing complexity of NSLD with more bulges on them. In general, larger NSLDs with sharper shapes and increasing complex morphology of more bulges cause more significant damages to the PS monolayer. These findings have implications for safeguarding astronaut health in future manned lunar exploration missions.

Numerical investigations of translocation characteristics of non-spherical silica nanoparticles across pulmonary surfactant monolayer under different respiratory states using coarse-grained molecular dynamics method

Understanding translocation characteristics of non-spherical silica nanoparticles (SiNPs) across pulmonary surfactant (PS) monolayer is of great significance for their applications in inhaled nanoparticles (NPs) drugs, as their shapes may impact drug delivery efficiency. Therefore, this study has performed coarse-grained molecular dynamics (CGMD) simulation to study the characteristics of SiNPs across the PS monolayer and adsorption of dipalmitoylphosphatidylcholine (DPPC) molecules. It is found that SiNPs smaller than 8 nm are more likely to cross the PS monolayer in expansion state than other states and crossing times required for ellipsoidal SiNPs are the shortest, and when SiNPs are in penetrating and embedding states, the number of DPPC molecules adsorbed by bulged-spherical SiNPs is the highest. In conclusion, ellipsoidal SiNPs have the strongest crossing ability and bulged-spherical SiNPs cause the greatest damage. Therefore, it is suggested to adopt ellipsoidal SiNPs and avoid bulged-spherical SiNPs when improving transportation efficiencies of spherical SiNP drug carriers.

Phenylalanine and Tryptophan-Based Surfactants as New Antibacterial Agents: Characterization, Self-Aggregation Properties, and DPPC/Surfactants Vesicles Formulation.

Cationic surfactants based on phenylalanine (CnPC3NH3Cl) and tryptophan (CnTC3NH3Cl) were synthesized using renewable raw materials as starting compounds and a green synthetic procedure. The synthesis, acid-base equilibrium, aggregation properties, and antibacterial activity were investigated. Conductivity and fluorescence were used to establish critical micelle concentrations. Micellization of CnPC3NH3Cl and CnTC3NH3Cl occurred in the ranges of 0.42-16.2 mM and 0.29-4.6 mM, respectively. Since those surfactants have some acidic character, the apparent pKa was determined through titrations, observing increasing acidity with increasing chain length and being slightly more acidic with the phenylalanine than the tryptophan derivatives. Both families showed promising antibacterial efficacy against eight different bacterial strains. Molecular docking studies against the enzyme peptidoglycan glycosyltransferase (PDB ID:2OQO) were used to investigate the potential binding mechanism of target surfactant molecules. According to small angle X-ray scattering (SAXS) results, the surfactants incorporate into DPPC (Dipalmitoyl Phosphatidyl Choline) bilayers without strong perturbation up to high surfactant concentration. Some of the C12TC3NH3Cl/DPPC formulations (40%/60% and 20%/80% molar ratios) exhibited good antibacterial activity, while the others were not effective against the tested bacteria. The strong affinity between DPPC and surfactant molecules, as determined by the DFT (density functional theory) method, could be one of the reasons for the loss of antibacterial activity of these cationic surfactants when they are incorporated in vesicles.

Understanding the Molecular Mechanism of Anesthesia: Effect of General Anesthetics and Structurally Similar Non-Anesthetics on the Properties of Lipid Membranes.

General anesthesia can be caused by various, chemically very different molecules, while several other molecules, many of which are structurally rather similar to them, do not exhibit anesthetic effects at all. To understand the origin of this difference and shed some light on the molecular mechanism of general anesthesia, we report here molecular dynamics simulations of the neat dipalmitoylphosphatidylcholine (DPPC) membrane as well as DPPC membranes containing the anesthetics diethyl ether and chloroform and the structurally similar non-anesthetics n-pentane and carbon tetrachloride, respectively. To also account for the pressure reversal of anesthesia, these simulations are performed both at 1 bar and at 600 bar. Our results indicate that all solutes considered prefer to stay both in the middle of the membrane and close to the boundary of the hydrocarbon domain, at the vicinity of the crowded region of the polar headgroups. However, this latter preference is considerably stronger for the (weakly polar) anesthetics than for the (apolar) non-anesthetics. Anesthetics staying in this outer preferred position increase the lateral separation between the lipid molecules, giving rise to a decrease of the lateral density. The lower lateral density leads to an increased mobility of the DPPC molecules, a decreased order of their tails, an increase of the free volume around this outer preferred position, and a decrease of the lateral pressure at the hydrocarbon side of the apolar/polar interface, a change that might well be in a causal relation with the occurrence of the anesthetic effect. All these changes are clearly reverted by the increase of pressure. Furthermore, non-anesthetics occur in this outer preferred position in a considerably smaller concentration and hence either induce such changes in a much weaker form or do not induce them at all.

Effect of xenon, an apolar general anaesthetic on the properties of the DPPC bilayer

The effect of xenon, a general anaesthetic, on the properties of the fully hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer is investigated by molecular dynamics simulations with two different force fields. Considering the well-known pressure reversal of the anaesthetic effect, we are looking for such changes that are reverted by the increase of the pressure, and hence can be relevant in explaining the molecular mechanism of general anesthesia. The results show that, besides its main preference for staying in the middle of the membrane, xenon also has a secondary preference for positions at the outer boundary of the hydrocarbon region, close to the crowded domain of the polar headgroups. This outer preference is a result of the interplay of the increasing attraction the xenon atoms experience and the decreasing free volume available for them upon going away from the middle of the membrane. It also turns out that any change that might be relevant in explaining the anaesthetic effect is related to xenon atoms staying in this outer preferred position. Since the model we employed does not account for the polarizability of the xenon atoms, and hence misses a part of the thermodynamic driving force of the preference for this outer position, all results should be considered as a lower bound estimate of the real effect. It is found that the xenon atoms induce a lateral swelling of the membrane by pushing the DPPC molecules farther away from each other, which, due to the insensitivity of the membrane thickness to the presence of xenon, naturally results in an increase of also the membrane volume. On the other hand, pressure increases the membrane thickness by increasing the order of the lipid tails, while it also causes a lateral shrinking of the bilayer. The net effect of these two opposite contributions is still a decrease of the membrane volume. In the light of these results, we propose to refine the 70 years old ‘critical volume hypothesis’ to a ‘critical surface area hypothesis’, claiming that general anesthesia occurs if the molar surface area of the membrane exceeds a critical value. Finally, the xenon-induced lateral swelling results in an increase of the empty space in the hydrocarbon region, allowing the DPPC molecules to move more freely, and hence increasing their lateral diffusion coefficient. All these changes are clearly reverted by pressure, and thus may well be relevant in respect of the molecular mechanism of general anesthesia.

Supramolecular Self-Assembly of Dipalmitoylphosphatidylcholine and Carbon Nanotubes: A Dissipative Particle Dynamics Simulation Study.

Dissipative particle dynamics simulations were performed to investigate the self-assembly of dipalmitoylphosphatidylcholine (DPPC) as a model lipid membrane on the surface of carbon nanotubes (CNTs). The influence of surface curvature of CNTs on self-assembly was investigated by performing simulations on solutions of DPPC in water in contact with CNTs of different diameters: CNT (10, 10), CNT (14, 14), CNT (20, 20), and CNT (34, 34). DPPC solutions with a wide range of concentrations were chosen to allow for formation of lipid structures of various surface densities, ranging from a submonolayer to a well-organized monolayer and a CNT covered with a lipid monolayer immersed in a planar lipid bilayer. Our results are indicative of a sequence of phase-ordering processes for DPPC on the surface of CNTs. At low surface coverages, the majority of hydrocarbon tail groups of DPPC are in contact with the CNT surface. Increasing the surface coverage leads to the formation of hemimicellar aggregates, and at high surface coverages close to the saturation limit, an organized lipid monolayer self-assembles. An examination of the mechanism of self-assembly reveals a two-step mechanism. The first step involves densification of DPPC on the CNT surface. Here, the lipid molecules do not adopt the order of the target phase (lipid monolayer on the CNT surface). In the second step, when the lipid density on the CNT surface is above a threshold value (close to saturation), the lipid molecules reorient themselves to form an organized monolayer around the tube. Here, the DPPC molecules adopt stretched conformations normal to the surface, the end hydrocarbon groups adsorb on the surface, and the head groups occupy the outermost part of the monolayer. The saturation density and the degree of lipid ordering on the CNT surface depend on the surface curvature. The saturation density increases with increased surface curvature, and better-ordered structures are formed on less curved surfaces.

Morphological Behavior of DPPC-DMPC Two-dimensional Mixed Monolayer on Water Surface by Dropping Method

We have investigated the morphology of dipalmitoyl phosphatidyl choline (DPPC)-dimyristoyl PC (DMPC) mixed monolayer formed on the water surface by the dropping method using surface tension measurement (STm), Brewster angle microscopy (BAM), and infrared external reflection spectroscopy (IERS). The limiting molecular area (A0) obtained by STm showed that the negative deviation at &lt; mole fraction (xDMPC)= 0.4 whereas the positive deviation at &gt; xDMPC =0.6 when DMPC was added to the DPPC monolayer. BAM images also showed that the condensed rigid monolayer at xDMPC &lt; 0.4 changed to the expandable monolayer at xDMPC &gt; 0.5. Moreover, IER spectra showed that the alkyl chains of both DPPC and DMPC molecules are in trans conformation in mixed monolayer at xDMPC =0.2 whereas gauche conformation existed at xDMPC = 0.8. Above findings have justified the transition of flexible gauche conformation alkyl chains of DMPC molecule into rigid trans conformation by the addition of DPPC molecule at xDMPC &lt; 0.4 whereas alkyl chains retained flexible gauche conformation in both DPPC and DMPC molecules at xDMPC &gt; 0.6.

Altering model cell membranes by means of photoactivated organic functionalized gold nanorods

A possible non-invasive photothermal therapy employs photothermal agents with high light-to-heat conversion efficiency. Gold nanorods (Au-NRs) can be applied as photothermal agents because of their optical properties and specific tumor-targeting capability. The potential therapeutic applications and toxicological effects of photothermal agents depend on the interaction of the Au-NRs with the cell membrane, the understanding of which is of great importance. Although studies in this area have been performed, the interactions between model cell membranes and photoactivated Au-NRs have not yet been described. In this study, we explain how local excitation of the Au-NRs caused a local temperature increase in their vicinity that affected the properties of a model membrane composed of dipalmitoylphosphatidylcholine (DPPC). Our results showed that the illumination of the Au-NRs changed the DPPC’s organization in the Langmuir monolayer. Upon photoactivation, the mutual distances between the DPPC molecules increased; however, the conformation of the lipid tails remained unchanged. Moreover, the illumination of the membrane at a surface pressure corresponding to that in a native cellular membrane caused a more stable and elastic behavior of the Langmuir monolayer. The results obtained corroborate the theory that photoactivation of Au-NRs influences the packing and phase behavior of mimetic cell membranes. In the long term, it may allow us to understand how the photoactivation of nanoparticles facilitates the transport of medicinal substances.

The Role of Pulmonary Surfactant in COVID-19 Understanding

Background: In Covid-19 the virus infects the respiratory tract in human. When lung tissue becomes diseased, the walls and lining of the alveoli and capillaries are damaged. At this point lung compliance and ventilation decrease. Pulmonary surfactant that is produced and dispersed into alveolar space, has a significant role in understanding how heavily covid-19 interferes and infects lung cells. The importance of pulmonary surfactant in alveoli is to lower surface tension at air/liquid interface in the lung. This is achieved by reducing the work of breathing and preventing alveolar collapse. The main constituent of pulmonary surfactant is dipalmitoylphosphatidylcholine (DPPC) (C40H80NO8P). It is a phospholipid containing two non polar palmitic acid C16 chains as hydrophobic tails linked to a polar head group of a phosphatidylcholine (also known as lecithin).&#x0D; Rationale of the Review and Objective Method: When DPPC molecules are in contact with a polar solvent, micelles which grow further into bilayers are formed considering their cylindrical structures. This trait makes the whole structure of pulmonary surfactant as amphipathic and surface active molecules. The head group of phosphatidylcholine in the pulmonary surfactant is attracted by polar liquid molecules causing a reduction of the liquid surface tension.&#x0D; Conclusion: This review complements the quoted information analysing them theoretically and integrates recent advances in pulmonary surfactant research with the global pandemic.

Interface-Mediated Mechanism of Action-The Root of the Cytoprotective Effect of Immediate-Release Omeprazole.

Omeprazole is usually administered under an enteric coating. However, there is a Food and Drug Administration-approved strategy that enables its release in the stomach. When locally absorbed, omeprazole shows a higher efficacy and a cytoprotective effect, whose mechanism was still unknown. Therefore, we aimed to assess the effect of the absorption route on the gastric mucosa. 2D and 3D models of dipalmitoylphosphatidylcholine (DPPC) at different pH values (5.0 and 7.4) were used to mimic different absorption conditions. Several experimental techniques, namely, fluorescence studies, X-ray scattering methodologies, and Langmuir monolayers coupled with microscopy, X-ray diffraction, and infrared spectroscopy techniques, were combined with molecular dynamics simulations. The results showed that electrostatic and hydrophobic interactions between omeprazole and DPPC rearranged the conformational state of DPPC. Omeprazole intercalates among DPPC molecules, promoting domain formation with untilted phospholipids. Hence, the local release of omeprazole enables its action as a phospholipid-like drug, which can reinforce and protect the gastric mucosa.

Nanoindentation of Graphene/Phospholipid Nanocomposite: A Molecular Dynamics Study

Graphene and phospholipids are widely used in biosensing and drug delivery. This paper studies the mechanical and electronic properties of a composite based on two graphene flakes and dipalmitoylphosphatidylcholine (DPPC) phospholipid molecules located between them via combination of various mathematical modeling methods. Molecular dynamics simulation showed that an adhesion between bilayer graphene and DPCC increases during nanoindentation of the composite by a carbon nanotube (CNT). Herewith, the DPPC molecule located under a nanotip takes the form of graphene and is not destroyed. By the Mulliken procedure, it was shown that the phospholipid molecules act as a “buffer” of charge between two graphene sheets and CNT. The highest values of electron transfer in the graphene/DPPC system were observed at the lower indentation point, when the deflection reached its maximum value.

Improving the Sensory Properties of Layered Phospholipid-Graphene Films Due to the Curvature of Graphene Layers.

This article is devoted to the in silico study of the sensory properties of mono- and bilayer phospholipid-graphene films with planar and curved graphene sheets. The DPPC (dipalmitoylphosphatidylcholine) molecules are considered as phospholipid structures. These molecules are part of lipid bilayers, liposomes and cell membranes. To find a way to improve the sensory properties of phospholipid-graphene films, we studied the effect of the curvature of the graphene sheet on the charge transfer and electrical conductivity of the films. The distribution of the electron charge density over the film atoms was calculated using the self-consistent-charge density-functional tight-binding method (SCC-DFTB). The calculation of the current through phospholipid-graphene films was carried out within the framework of the Landauer–Buttiker formalism using the Keldysh nonequilibrium Green function technique. As a result of the calculations, the optimal configuration of the arrangement of DPPC molecules between two graphene layers was established. This configuration provides the maximum possible increase in current to 1 μA at low voltages of ~0.2 V and is achieved for curved graphene with a radius of curvature of ~2.7 nm at individual points of graphene atomic network.

A detailed investigation on interactions between magnetite nanoparticles functionalized with aminated chitosan and a cell model membrane.

Magnetite nanoparticles are promising materials for application in magnetic resonance imaging, targeted drug delivery, enzyme immobilization and cancer therapies based on hyperthermia thanks to their biocompatibility, wide chemical affinity and superparamagnetic properties. However, there is still the lack of the knowledge of interactions between magnetite nanoparticles covered with the bioactive polymers and biological cells. In order to fulfil this gap, we have investigated interactions of newly synthetized magnetite nanoparticles functionalized with aminated chitosan (Fe3O4-aminated chitosan) and a model biological membrane made of dipalmitoylphosphatidylcholine (DPPC) using a Langmuir technique. Surface pressure-mean area per DPPC molecule isotherms and Brewster angle microscope images (BAM) recorded during compression of the two-component Fe3O4-aminated chitosan:DPPC films revealed the strong influence of the Fe3O4-aminated chitosan nanoparticles on the stability, phase state and structure of the phospholipid membrane. The studies on the adsorption/incorporation process of the Fe3O4-aminated chitosan nanoparticles showed that they can adsorb/incorporate into the DPPC model membrane at the surface pressure corresponding to this present in the cellular membrane under the biological conditions (35mN·m-1). The number of the adsorbed/incorporated Fe3O4-aminated chitosan nanoparticles can be regulated by the nanoparticles concentration in the neighbourhood of the DPPC model membrane even at high surface pressure of 35mN·m-1.

Coarse-grained molecular dynamics simulations of the effect of edge activators on the skin permeation behavior of transfersomes

Transfersomes (TRS) can provide sustained drug delivery and themselves are biocompatible, biodegradable and nontoxic. Edge activators (EAs) are key factors for increasing the deformability of TRS, and this active deformation mechanism is of commercial interest, especially at the molecular level. Accordingly, in this paper, the deformability of pure dipalmitoyl phosphatidylcholine (DPPC) vesicles, TRS with sodium cholate as an EA, and DPPC vesicles containing pogostone (POG) were compared via umbrella sampling technology. The DPPC conformation and membrane fluidity of these three types of bilayer systems were evaluated, and the changes in the membrane properties of vesicles caused by EAs were studied. EAs could increase the deformability of TRS by decreasing the deformation energy barrier due to their amphiphilic structures, which was similar to those of DPPC molecules. The membrane properties also changed via treatment with EAs including altering the tail chain angle, disturbing the ordered tail chain arrangement and prompting lateral diffusion of DPPC molecules. In addition, the impact of EAs on DPPC bilayers was further demonstrated to be concentration dependent. An ideal concentration was identified for the lowest amount of EA that offered a gel-liquid-crystalline phase transition of DPPC bilayers. Importantly, POG, a lipophobic transdermal drug, can also affect the skin permeation behavior of vesicles but had weaker effects than EA.

Study of mucin interaction with model phospholipid membrane at the air–water interface

The aim of the present study was to determine the interfacial behavior of model biomembrane in the presence of mucin. The Langmuir monolayer technique was used to form a mixed protein–phospholipid film at the air–water interface. Using the surface pressure-area isotherms, the relaxation curves and adsorption experiment, the nature of interactions between mucin and dipalmitoylphosphatidylcholine (DPPC) components of the monolayers under physiological conditions, (characteristic of the vagina and oral cavity), was investigated. It has been shown that at pH 4.2 mucin molecules reveal a strong affinity to the model biomembrane, composed of zwitterionic DPPC, and probably interact with it through the hydrophobic interactions. At pH 6.8 very weak electrostatic interactions are possible between the positively charged quaternary ammonium groups present in the DPPC molecules and the negatively charged mucin molecules. Moreover, from the relaxation experiment we concluded that beneath the DPPC layer there is a layer made of mucin particles due to electrostatic attraction. This layer of mucin beneath the DPPC particles stabilizes the entire system, especially at physiologically significant surface pressure, i.e. at π = 30 mN/m.

Surfactant Charge Modulates Structure and Stability of Lipase-Embedded Monolayers at Marine-Relevant Aerosol Surfaces.

Lipases, as well as other enzymes, are present and active within the sea surface microlayer (SSML). Upon bubble bursting, lipases partition into sea spray aerosol (SSA) along with surface-active molecules such as lipids. Lipases are likely to be embedded in the lipid monolayer at the SSA surface and thus have the potential to influence SSA interfacial structure and chemistry. Elucidating the structure of the lipid monolayer at SSA interfaces and how this structure is altered upon interaction with a protein system like lipase is of interest, given the importance of how aerosols interact with sunlight, influence cloud formation, and provide surfaces for chemical reactions. Herein, we report an integrated experimental and computational study of Burkholderia cepacia lipase (BCL) embedded in a lipid monolayer and highlight the important role of electrostatic, rather than hydrophobic, interactions as a driver for monolayer stability. Specifically, we combine Langmuir film experiments and molecular dynamics (MD) simulations to examine the detailed interactions between the zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayer and BCL. Upon insertion of BCL from the underlying subphase into the lipid monolayer, it is shown that BCL permeates and largely disorders the monolayer while strongly interacting with zwitterionic DPPC molecules, as experimentally observed by Langmuir adsorption curves and infrared reflectance absorbance spectroscopy. Explicitly solvated, all-atom MD is then used to provide insights into inter- and intramolecular interactions that drive these observations, with specific attention to the formation of salt bridges or ionic-bonding interactions. We show that after insertion into the DPPC monolayer, lipase is maintained at high surface pressures and in large BCL concentrations by forming a salt-bridge-stabilized lipase-DPPC complex. In comparison, when embedded in an anionic monolayer at low surface pressures, BCL preferentially forms intramolecular salt bridges, reducing its total favorable interactions with the surfactant and partitioning out of the monolayer shortly after injection. Overall, this study shows that the structure and dynamics of lipase-embedded SSA surfaces vary based on surface charge and pressure and that these variations have the potential to differentially modulate the properties of marine aerosols.

Bicelle-to-Vesicle Transition of a Binary Phospholipid Mixture Guided by Controlled Local Lipid Compositions: A Molecular Dynamics Simulation Study.

An essential step of nanoliposome formation in an aqueous lipid solution is the transition from discoidal lipid aggregate (bicelle) to vesicle. We investigate here the bicelle-to-vesicle transition of a binary lipid mixture of saturated and unsaturated phosphatidylcholine by performing nonequilibrium molecular dynamics simulations with the coarse-grained representation of di-palmitoyl-phosphatidyl-choline (DPPC) and di-linoleoyl-phosphatidyl-choline (DLiPC). When the DPPC molecules of a stable DPPC bicelle are randomly replaced with the DLiPC molecules, the transition occurs for higher apparent DLiPC concentrations. On the other hand, when the DPPC molecules only in the core region of the bicelle are replaced, the transition occurs even for lower apparent DLiPC concentrations. For the bicelle where the head and tail layers are pure DPPC and DLiPC monolayers, respectively, the side of the DLiPC monolayer becomes the concave surface of bending bicelle. Controlling the local lipid compositions in a binary lipid bicelle has the potential to determine the success of vesicle formation and the direction of bicelle bending. Our findings help explain nanoliposome formation with sonication and give useful information for controlling encapsulation efficiencies of nanoliposomes.

Influence of Cholesterol on the Dynamics of Hydration in Phospholipid Bilayers

We investigate the dynamics of interfacial waters in dipalmitoylphosphatidylcholine (DPPC) bilayers upon the addition of cholesterol, by molecular dynamics simulations. Our data reveal that the inclusion of cholesterol modifies the membrane aqueous interfacial dynamics: waters diffuse faster, their rotational decay time is shorter, and the DPPC/water hydrogen bond dynamics relaxes faster than in the pure DPPC membrane. The observed acceleration of the translational water dynamics agrees with recent experimental results, in which, by means of NMR techniques, an increment of the surface water diffusivity is measured upon the addition of cholesterol. A microscopic analysis of the lipid/water hydrogen bond network at the interfacial region suggests that the mechanism underlying the observed water mobility enhancement is given by the rupture of a fraction of interlipid water bridge hydrogen bonds connecting two different DPPC molecules, concomitant to the formation of new lipid/solvent bonds, whose dynamics is faster than that of the former. The consideration of a simple two-state model for the decay of the hydrogen bond correlation function yielded excellent results, obtaining two well-separated characteristic time scales: a slow one (∼250 ps) associated with bonds linking two DPPC molecules, and a fast one (∼15 ps), related to DPPC/solvent bonds.

The interaction of trace heavy metal with lipid monolayer in the sea surface microlayer

Lipid molecules and trace heavy metals are enriched in sea surface microlayer and can be transferred into the sea spray aerosol. To better understand their impact on marine aerosol generation and evolution, we investigated the interaction of trace heavy metals including Fe3+, Pb2+, Zn2+, Cu2+, Ni2+, Cr3+, Cd2+, and Co2+, with dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface. Phase behavior of the DPPC monolayer on heavy metal solutions was probed with surface pressure–area (π–A) isotherms. The conformation order and orientation of DPPC alkyl chains were characterized by infrared reflection–absorption spectroscopy (IRRAS). The π–A isotherms show that Zn2+ and Fe3+ strongly interact with DPPC molecules, and induce condensation of the monolayers in a concentration-dependent manner. IRRAS spectra show that the formation of cation-DPPC complex gives rise to conformational changes and immobilization of the headgroups. The current results suggest that the enrichment of Zn2+ in sea spray aerosols is due to strong binding to the DPPC film. The interaction of Fe3+ with DPPC monolayers can significantly influence their surface organizations through the formation of lipid-coated particles. These results suggest that the sea surface microlayer is capable of accumulating much higher amounts of these metals than the subsurface water. The organic and metal pollutants may transfer into the atmosphere by this interaction.

Mechanisms underlying interactions between PAMAM dendron-grafted surfaces with DPPC membranes.

Biofouling is a pervasive problem which demands the creation of smart, antifouling surfaces. Towards this end, we examine the interactions between a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer and a polyamidoamine (PAMAM) dendron-grafted surface. In addition, we investigate the impact of dendron generation on the system behavior. To resolve the multiscale dynamical processes occurring over a large spatial scale, we employ Molecular Dynamics simulations with a coarse-grained implicit solvent force field. Our results demonstrate the transient and equilibrium system dynamics to be determined by the PAMAM dendron generation along with the underlying mechanisms. Higher generation dendrons are observed to favor penetration of the DPPC molecules into the dendron branches, thereby enabling sustained interactions between the membrane and the dendron-grafted surface. Under equilibrium, the membrane adopts a bowl-shaped morphology whose dimensions are determined by the dendron generation and density of interactions. The results from our study can be used to guide the design of novel surfaces with selective antifouling properties which can prevent the adsorption of microorganisms onto lipid membranes.