Free Energy of Bare and Capped Gold Nanoparticles Permeating through a Lipid Bilayer

Herein, we study the permeation free energy of bare and octane‐thiol‐capped gold nanoparticles (AuNPs) translocating through a lipid membrane. To investigate this, we have pulled the bare and capped AuNPs from bulk water to the membrane interior and estimated the free energy cost. The adsorption of the bare AuNP on the bilayer surface is energetically favorable but further loading inside it requires energy. However, the estimated free‐energy barrier for loading the capped AuNP into the lipid membrane is much higher compared to bare AuNP. We also demonstrate the details of the permeation process of bare and capped AuNPs. Bare AuNP induces the curvature in the lipid membrane whereas capped AuNP creates an opening in the interacting monolayer and get inserted into the membrane. The insertion of capped AuNP induces a partial unzipping of the lipid bilayer, which results in the ordering of the local lipids interacting with the nanoparticle. However, bare AuNP disrupts the lipid membrane by pushing the lipid molecules inside the membrane. We also analyze pore formation due to the insertion of capped AuNP into the membrane, which results in water molecules penetrating the hydrophobic region.     

A molecular dynamics study on heat conduction characteristics in DPPC lipid bilayer

In this paper, nonequilibrium molecular dynamics simulations were performed on a single component 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine lipid bilayer in order to investigate the thermal conductivity and its anisotropy. To evaluate the thermal conductivity, we applied a constant heat flux to the lipid bilayer along and across the membrane with ambient water. The contribution of molecular interaction to the heat conduction was also evaluated. Along the bilayer plane, there is little transfer of thermal energy by the interaction between lipid molecules as compared with the interaction between water molecules. Across the bilayer plane, the local thermal conductivity depends on the constituents (i.e., water, head group, and tail group of lipid molecule) that occupy the domain. Although the intramolecular transfer of thermal energy in the tail groups of lipid molecules works efficiently to promote high local thermal conductivity in this region, the highest thermal resistance appears at the center of lipid bilayer where acyl chains of lipid molecules face each other due to a loss of covalent-bond and low number density. The overall thermal conductivities of the lipid bilayer in the directions parallel and perpendicular to the lipid membrane have been compared, and it was found that the thermal conductivity normal to the membrane is higher than that along the membrane, but it is still smaller than that of bulk water.     

A molecular-dynamics study of lipid bilayers: effects of the hydrocarbon chain length on permeability

In this paper, we investigate the effects of the hydrocarbon chain length of lipid molecules on the permeation process of small molecules through lipid bilayers. We perform molecular-dynamics simulations using three kinds of lipid molecules with different chain length: dilauroylphosphatidylcholine, dimyristoylphosphatidylcholine, and dipalmiltoylphosphatidylcholine. Free-energy profiles of O2, CO, NO, and water molecules are calculated by means of the cavity insertion Widom method and the probability ratio method. We show that the lipid membrane with longer chains has a larger and wider energy barrier. The local diffusion coefficients of water across the bilayers are also calculated by the force autocorrelation function method and the velocity autocorrelation function method. The local diffusion coefficients in the bilayers are not altered significantly by the chain length. We estimate the permeability coefficients of water across the three membranes according to the solubility-diffusion model; we find that the water permeability decreases modestly with increasing chain length of the lipid molecules.     

Templated assembly of biomembranes on silica microspheres using bacteriorhodopsin conjugates as structural anchors

Membrane proteins are some of the most sophisticated molecules found in nature. These molecules have extraordinary recognition properties; hence, they represent a vast source of specialized materials with potential uses in sensing and screening applications. However, the strict requirement of the native lipid environment to preserve their structure and functionality presents an impediment in building biofunctional materials from these molecules. In general, the purification protocols remove the native lipid support structures found in the cellular environment that stabilize the membrane proteins. Furthermore, the membrane protein structure is often highly complex, typified by large, multisubunit complexes that not only span the lipid bilayer but also contain large (>2 nm) cytoplasmic and extracellular domains that protrude from the membrane. The present study is focused on using a biomimetic approach to build a stable, fluid microenvironment to be used to incorporate larger membrane proteins of interest into a tether-supported lipid bilayer membrane adequately spaced above a substrate passivated to liposome fusion and nonspecific adsorption. Our aim is to reintroduce the supporting structures of the native cell membrane using self-assembled supramolecular complexes constructed on microspheres in an artificial cytoskeleton motif. Central to our architecture is to utilize bacteriorhodopsin (bR), a transmembrane protein, as a biomembrane anchoring molecule to be tethered to surfaces of interest as a sparse structural element in the design. Compared to a typical lipid tether, which inserts into one leaflet of the lipid bilayer, bR anchoring provides an over 8-fold greater hydrophobic surface area in contact with the bilayer. In the work presented here, the silica microsphere surface was biofunctionalized with streptavidin to make it a suitable supporting interface. This was achieved by self-assembly of (p-aminophenyl)trimethoxysilane on the silica surface followed by subsequent conjugation of biotin-PEG3400 (PEG = poly(ethylene glycol) and PEG2000 for further passivation and the binding of streptavidin. We have conjugated bR with biotin-PEG3400 through amine-based coupling to use it as a tether. The biotin-PEG-bR conjugate was further labeled with Texas Red to facilitate localization via fluorescence imaging. Confocal microscopy was utilized to analyze the microsphere surface at different stages of surface modification by employing fluorescent staining techniques. Sparely tethered supported lipid bilayer membranes were constructed successfully on streptavidin-functionalized silica particles (5 mum) using a detergent-based method in which tethered bR nucleates self-assembly of the bilayer membrane. The fluidity of the supported membranes was analyzed using fluorescence recovery after photobleaching in confocal imaging detection mode. The phospholipid diffusion coefficients obtained from these studies indicated that nativelike fluidity was achieved in the tether-supported membranes, thus providing a prospective microenvironment for insertion of membrane proteins of interest.     

Soft lithographic patterning of supported lipid bilayers onto a surface and inside microfluidic channels

We present simple soft lithographic methods for patterning supported lipid bilayer (SLB) membranes onto a surface and inside microfluidic channels. Micropatterns of polyethylene glycol (PEG)-based polymers were fabricated on glass substrates by microcontact printing or capillary moulding. The patterned PEG surfaces have shown 97 +/- 0.5% reduction in lipid adsorption onto two dimensional surfaces and 95 +/- 1.2% reduction inside microfluidic channels in comparison to glass control. Atomic force microscopy measurements indicated that the deposition of lipid vesicles led to the formation of SLB membranes by vesicle fusion due to hydrophilic interactions with the exposed substrate. Furthermore, the functionality of the patterned SLBs was tested by measuring the binding interactions between biotin (ligand)-labeled lipid bilayer and streptavidin (receptor). SLB arrays were fabricated with spatial resolution down to approximately 500 nm on flat substrate and approximately 1 microm inside microfluidic channels, respectively.     

Permeability of water and polar solutes in lipid bilayers

The three commonly used formalisms to describe water and solute permeation in lipid bilayers (namely, solubility-solute properties, activated rate processes and the thermodynamics of the irreversible process theory) are analyzed in the light of experimental results. These approaches are based on the consideration of the lipid bilayer as a composite membrane containing a hydrocarbon core, an H-bonded interfacial network and a fluctuating structure in which pores can appear. The particular structure of the lipid bilayer (i.e., a hydrophobic-hydrophilic leaflet) makes the permeation process of polar solutions more complicated than that occurring in inert polymeric membranes. Thus, the permeation theories of Fick, Henry and Kedem and Katchalsky should be adapted to introduce interfacial and elastic phenomena. A critical analysis of the experimental results available in the current literature opens the possibility to formulate a broader formalism for permeation in lipid membranes.     

Manipulating energy landscapes to tune ordering in biotemplated nanoparticle arrays

Two-dimensional non-close-packed crystals of the protein streptavidin, grown on phospholipid membranes, can serve as nanoscale templates capable of directing the formation of ordered nanoparticle arrays through site-specific electrostatic adsorption. Here we examine the effects of both interparticle and nanoparticle/lipid membrane electrostatic interactions on the degree of structural order exhibited by the templated nanoparticle array. Interparticle electrostatic repulsion is shown to have only marginal influence on nanoparticle ordering. In contrast, the degree of order exhibited by the templated array can be tuned by controlling the charge on the lipid membrane. Analysis of the local and global structure of arrays generated with negatively charged gold nanoparticles (～6 nm) indicate improved long-range order when the lipid membrane supporting the protein crystal is derived from cationic lipid molecules as opposed to zwitterionic phospholipids. Furthermore, as nanoparticle size is reduced (～3 nm), the presence of a charged lipid membrane is found to be essential, as smaller particles do not adhere to streptavidin crystals grown on zwitterionic membranes. These findings demonstrate that the composition of the lipid support can influence the efficacy of directed-assembly processes which utilize protein templates and are important results toward enhancing control over bottom-up nanofabrication applications.     

非支撑磷脂双层膜制备及温度对其力学性质的影响

结合聚苯乙烯球刻蚀和微机电系统技术加工氮化硅纳米多孔膜, 并在其上用囊泡法制备非支撑磷脂双层膜, 通过温控原子力显微术(AFM)的成像模式和力曲线模式对非支撑磷脂双层膜的形貌和力学性质进行研究. 实验结果表明, 该方法制备的非支撑磷脂双层膜具有流动性, 能进行自我修复, 该特点有利于提供足够的非支撑磷脂双层膜区域用于其性质研究; 非支撑磷脂双层膜的膜破力和粘滞力均随着温度的升高而减小, 即膜的机械稳定性随着温度的升高而降低. 非支撑磷脂双层膜膜破力小于支撑磷脂双层膜的膜破力, 并且非支撑磷脂双层膜粘滞力随温度的变化趋势与支撑磷脂双层膜的变化趋势相反.     

Incorporation of Na(+),K(+)-ATP-ase into the thiolipid biomimetic assemblies via the fusion of proteoliposomes

The black lipid membranes (BLMs) are artificial membrane systems that have been widely used in the study of different biological processes. In this paper the planar bilayer lipid membranes have been used to study the behavior of thiolipid molecules-dipalmitoyl-phosphatidyl-ethanolamine-mercaptopropionamide (DPPE-MPA) and cholesteryl 3-mercaptopropionate (Chs-MPA)-as compared to classical BLM made of natural lipids. We present our experiments on black thiolipid bilayer (BTM) formation from a thiolipid solution and basic results of pump currents generated by sodium-potassium pump-Na(+),K(+)-ATP-ase-introduced to such bilayer systems via proteoliposome adsorption with subsequent fusion. Our results imply that no substantial difference exists between BLMs formed from classical lipids and those made from thiolipids used in this study. The same thiolipid molecules were subsequently used for the formation of covalently bound, tethered bilayer lipid membranes (t-BLMs) on polycrystalline gold electrodes. Similarly, as in the case of BLMs, we took advantage of proteoliposome adsorption/fusion to obtain a t-BLM system with reconstituted enzyme. The vesicle fusion on hydrophobic or hydrophilic substrates is one of the main ways to obtain a bilayer system with incorporated biological species. In this paper we present also our preliminary results of electrochemical experiments using rapid solution exchange technique on such t-BLMs systems and their comparison with painted solid supported membranes (SSMs) and BLMs. We have also followed the process of vesicles fusion onto thiolipid monolayer by means of in situ atomic force microscopy in tapping mode (TM-AFM). On the basis of these experiments, we conclude that DPPE-MPA and Chs-MPA molecules used in our experiments preserve lipid properties, allowing for at least partial reconstitution of Na(+),K(+)-ATP-ase into such t-BLMs. On the other hand, the relatively compact organization on polycrystalline gold and the hydrophobic nature of the first monolayer of tethered thiolipids slows down the proteoliposome fusion onto such monolayers and consequently hinders the protein insertion. However, this effect can be overcome by mechanical stimulus that facilitates proteoliposome delamination onto the self-assembled monolayer.     

Biomolecular simulations of membranes: physical properties from different force fields

Phospholipid force fields are of ample importance for the simulation of artificial bilayers, membranes, and also for the simulation of integral membrane proteins. Here, we compare the two most applied atomic force fields for phospholipids, the all-atom CHARMM27 and the united atom Berger force field, with a newly developed all-atom generalized AMBER force field (GAFF) for dioleoylphosphatidylcholine molecules. Only the latter displays the experimentally observed difference in the order of the C2 atom between the two acyl chains. The interfacial water dynamics is smoothly increased between the lipid carbonyl region and the bulk water phase for all force fields; however, the water order and with it the electrostatic potential across the bilayer showed distinct differences between the force fields. Both Berger and GAFF underestimate the lipid self-diffusion. GAFF offers a consistent force field for the atomic scale simulation of biomembranes.     

The effect of cholesterol on the adsorption of phenyltin compounds onto phosphatidylcholine and sphingomyelin liposome membranes

Organometallic compounds are widely spread in the human environment sometimes, causing a substantial health risk. Their amphiphilic character enables them to intercalate and penetrate cell membranes, potentially affecting various vital cell functions. Compound adsorption onto the membrane depends on the compound properties, as well as on the membrane composition and state. When adsorbing onto the lipidic surface, phenyltins localize at areas where lipid bilayer organization is compatible with compound spatial requirements. The lipid bilayer is a dynamic and laterally nonuniform structure with complex local and global architecture correlated with a variety of cell functions. The selective binding of a toxic compound to selected membrane areas may, therefore, interfere with some types of cellular process. We present experimental results concerning phenyltin adsorption onto the lipid bilayer surface measured with the fluorescent probe fluorescein‐PE. Model lipid bilayers were formed from lipid mixtures mimicking various plasma membrane regions. The adsorption of Ph₃SnCl and P₂SnCl₂ onto the phosphatidylcholine–cholesterol bilayer was qualitatively different from sphingomyelin–cholesterol. The results presented indicate that phenyltins are likely to accumulate in areas containing phosphatidylcholine, outside of lipid rafts. Copyright © 2004 John Wiley & Sons, Ltd.     

Recent progress in the application of atomic force microscopy for supported lipid bilayers

In the past two decades, atomic force microscopy has been widely used for studying supported lipid bilayer related research, including the structure and dynamics of membranes and membrane proteins, and the interaction of membranes with chemical and biological molecules. The focus of this minireview is on the recent progress in the application of atomic force microscopy for supported lipid bilayers. Such progress mainly includes the application in the following aspects: submolecular-resolution imaging, in situ observation, and nanomechanics measurement.     

Aqueous dispersions of cubic lipid–water phases

Inverse lipid–water phases such as cubic phases can form kinetically stable dispersions by fragmentation in water. Cubic lipid phases can be dispersed by polar lipids favoring lamellar phases or by block copolymers, which can close the bilayer at the surface so that the hydrocarbon chain core is not exposed to water. Monodisperse particles based on glycerol monooleate, with their bilayer curved as the P-, D- or G-minimal surface, have been prepared in this way. Their inner bilayer conformation and outer shape have been examined, mainly by X-ray diffraction and cryo transmission electron microscopy. There is also a different type of cubic lipid bilayer particles with a periodicity in the micrometer range, which have been identified in phospholipid–water dispersions and in cell membrane assemblies. The mechanism behind formation in vivo of such cubic membranes, which also follow the P-, D- and G-surfaces, is discussed. Other lipid–water dispersions with lower symmetry are finally considered; dispersions formed by the inverse hexagonal phase and the dispersed state of a tetragonal bilayer structure formed by lung surfactants.     

Mechanism of unassisted ion transport across membrane bilayers

To establish how charged species move from water to the nonpolar membrane interior and to determine the energetic and structural effects accompanying this process, we performed molecular dynamics simulations of the transport of Na+ and Cl- across a lipid bilayer located between two water lamellae. The total length of molecular dynamics trajectories generated for each ion was 10 ns. Our simulations demonstrate that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. Once the ion crosses the midplane of the bilayer the deformation "switches sides"; the initial defect slowly relaxes, and a defect forms in the outgoing side of the bilayer. As a result, the ion remains well solvated during the process; the total number of oxygen atoms from water and lipid head groups in the first solvation shell remains constant. A similar membrane deformation is formed when the ion is instantaneously inserted into the interior of the bilayer. The formation of defects considerably lowers the free energy barrier to transfer of the ion across the bilayer and, consequently, increases the permeabilities of the membrane to ions, compared to the rigid, planar structure, by approximately 14 orders of magnitude. Our results have implications for drug delivery using liposomes and peptide insertion into membranes.     

Dynamics and state of lipid bilayer-internal water unraveled with solution state 1H dynamic nuclear polarization

The dynamics and state of lipid bilayer-internal hydration water of unilamellar lipid vesicles dispersed in solutions is characterized. This study was enabled by a recently developed technique based on Overhauser dynamic nuclear polarization (DNP)-driven amplification of (1)H nuclear magnetic resonance (NMR) signal of hydration water. This technique can, in the full presence of bulk water, selectively quantify the translational dynamics of hydration water within ～10 ? around spin labels that are specifically introduced to the local volume of interest within the lipid bilayer. With this approach, the local apparent diffusion coefficients of internal water at different depths of the lipid bilayer were determined. The modulation of these values as a response to external stimuli, such as the addition of sodium chloride or ethanol and the lipid phase transitions, that alter the fluctuations of bilayer interfaces together with the activation energy values of water diffusivity shows that water is not individually and homogeneously solvating lipid's hydrocarbon tails in the lipid bilayer. We provide experimental evidence that instead, water and the lipid membrane comprise a heterogeneous system whose constituents include transient hydrophobic water pores or water structures traversing the lipid bilayer. We show how these transient pore structures, as key vehicles for passive water transport can better reconcile our experimental data with existing literature data on lipid bilayer hydration and dynamics.     

Dynamics of a bilayer membrane with membrane-solvent partial slip boundary conditions

We discuss the dynamics of a bilayer membrane with partial slip boundary conditions between the monolayers and the bulk fluid. Using Onsager’s variational principle to account for the associated dissipations, we derive the coupled dynamic equations for the membrane height and the excess lipid density. The newly introduced friction coe?cients appear in the renormalized fluid viscosities. For ordinary lipid bilayer membranes, we find that it is generally justified to ignore the e?ects of permeation and parallel slip at the membrane surface.     

Properties and behaviour of tetracyclic allopsoralen derivatives inside a DPPC lipid bilayer model

Allopsoralens are angular psoralen derivatives presenting advantages over the parent compound because of monofunctional DNA-photobinding and consequent lower toxicity. Allopsoralen molecules with three different substituents and different protonation states were studied using the molecular dynamics technique. The location of these molecules when inside the lipid bilayer is of major importance because their photochemical properties can change with the environment. Also, the ability of psoralens to form photoadducts with unsaturated phospholipids depends on the preference of the molecules to locate themselves closer to the bilayer middle were the double bond functionality can be found. Herein we show that the allopsoralens tend to accumulate inside the lipid bilayer closer to the water interface when protonated or closer to the interface middle otherwise. Allopsoralens containing one amine terminated carbon chain tend to have different rotational and orientational behaviour and an orientation preference close to the ones shown by the lipids. The size and chemical nature of the substituent also affect the molecular mobility and capacity to interact with water molecules and the nitrogen or phosphorus atoms of the lipids.     

Formation of Ag nanostrings induced by lyotropic liquid-crystalline phospholipid multilayer

Morphological variation of the Ag nanoparticles embedded in a lyotropic phospholipid (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, DOPE) membrane during hydration was investigated. Hydration at 5 °C resulted in transformation of the Ag nanoparticles into a bundle of Ag nanostrings as the Ag nanoparticles conformed to the H(II) phase of the DOPE molecules. Above 30 °C, the nanoparticles quickly coarsened into large polygonal-shaped particles since high mobility of the lipid molecules overwhelmed the tendency for the Ag nanoparticles to order. The result provided an insight into the long-term stability of nanoparticles trapped in different lipid membranes depending on the structural ordering of the molecules.     

Water concentration profiles in membranes measured by ESEEM of spin-labeled lipids

Electron spin-echo envelope modulation (ESEEM) spectroscopy of phospholipids spin-labeled systematically down the sn-2 chain was used to detect the penetration of water (D2O) into bilayer membranes of dipalmitoyl phosphatidylcholine with and without 50 mol % cholesterol. Three-pulse stimulated echoes allow the resolution of two superimposed 2H-ESEEM spectral components of different widths, for spin labels located in the upper part of the lipid chains. Quantum chemical calculations (DFT) and ESEEM simulations assign the broad spectral component to one or two D2O molecules that are directly hydrogen bonded to the N-O group of the spin label. Classical ESEEM simulations establish that the narrow spectral component arises from nonbonded water (D2O) molecules that are free in the hydrocarbon chain region of the bilayer membrane. The amplitudes of the broad 2H-ESEEM spectral component correlate directly with those of the narrow component for spin labels at different positions down the lipid chain, reflecting the local H-bonding equilibria. The D2O-ESEEM amplitudes decrease with position down the chain toward the bilayer center, displaying a sigmoidal dependence on position that is characteristic of transmembrane polarity profiles established by other less direct spin-labeling methods. The midpoint of the sigmoidal profile is shifted toward the membrane center for membranes without cholesterol, relative to those with cholesterol, and the D2O-ESEEM amplitude in the outer regions of the chain is greater in the presence of cholesterol than in its absence. For both membrane types, the D2O amplitude is almost vanishingly small at the bilayer center. The water-penetration profiles reverse correlate with the lipid-chain packing density, as reflected by 1H-ESEEM intensities from protons of the membrane matrix. An analysis of the H-bonding equilibria provides essential information on the binding of water molecules to H-bond acceptors within the hydrophobic interior of membranes. For membranes containing cholesterol, approximately 40% of the nitroxides in the region adjacent to the lipid headgroups are H bonded to water, of which ca. 15% are doubly H bonded. Corresponding H-bonded populations in membranes without cholesterol are ca. 20%, of which ca. 6% are doubly bonded.