Dynamics and instabilities of lipid bilayer membrane shapes

Biological membranes undergo constant shape remodeling involving the formation of highly curved structures. The lipid bilayer represents the fundamental architecture of the cellular membrane with its shapes determined by the Helfrich curvature bending energy. However, the dynamics of bilayer shape transitions, especially their modulation by membrane proteins, and the resulting shape instabilities, are still not well understood. Here, we review in a unifying manner several theories that describe the fluctuations (i.e. undulations) of bilayer shapes as well as their local coupling with lipid or protein density variation. The coupling between local membrane curvature and lipid density gives rise to a ‘slipping mode’ in addition to the conventional ‘bending mode’ for damping the membrane fluctuation. This leads to a number of interesting experimental phenomena regarding bilayer shape dynamics. More importantly, curvature-inducing proteins can couple with membrane shape and eventually render the membrane unstable. A criterion for membrane shape instability is derived from a linear stability analysis. The instability criterion reemphasizes the importance of membrane tension in regulating the stability and dynamics of membrane geometry. Recent progresses in understanding the role of membrane tension in regulating dynamical cellular processes are also reviewed. Protein density is emphasized as a key factor in regulating membrane shape transitions: a threshold density of curvature coupling proteins is required for inducing membrane morphology transitions.     

Dynamics of nematic point defects in a capillary with tilted boundary conditions

The motion of a single point defect in a cylindrical cavity filled with a nematic liquid crystal is described by solving numerically the simplified equations of nematodynamics. Perfect homeotropic anchoring for the director on the lateral boundary would result in the creation of domains with equal elastic energy, escaped upwards or downwards along the cavity axis and separated by point defects of strength ± 1. Defects do not move as long as they are sufficiently far apart. However, small deviations from homeotropic anchoring remove this degeneracy and the energetically favourable domains start to expand at the expense of the others, thus setting the defects in motion along the tube. We present a new numerical approach, which neglects the backflow, for studying the influence of both the pretilt and the elastic anisotropy (K ₃₃ ≠ K ₁₁) on the motion of a defect. We show how even very small pretilt angles (≈1°) result in speeds observed in experiments. For a moderate elastic anisotropy, the velocity of a +1 defect equals the velocity of a -1 defect, whereas for K ₃₃?K ₁₁ a + 1 defect moves faster than a -1 defect. For small pretilts we confirm a good qualitative agreement with an existing analytical approach, which proves inaccurate for large pretilts.     

Mixed hybrid bilayer lipid membrane incorporating valinomycin: improvements in preparation and functioning

The paper deals with the reconstruction of lipid bilayer membranes on a Au-covered polycarbonate membrane. Such a kind of like-biomembranes (namely mixed hybrid bilayer lipid membrane (MHBLM)) are characterised by appreciable long-term stability. Here we describe changes that have been made in the geometry of the experimental device in order to avoid artefacts and render membrane reproduction easier. Incorporation of valinomycin was performed to check the membrane and its stability: conductance and membrane potential following the changes of ion concentration were recorded. This new approach permits increase of successful trials and renders possible, when it breaks, easily formation of a new MHBLM on the same Au-covered polycarbonate membrane support. Finally, the stability shown by the MHBLM renders this system a promising tool for use under flowing conditions.     

Conformation and Interaction of a D,L-alternating peptide with a bilayer membrane: x-ray reflectivity, CD, and FTIR spectroscopy.

Peptides with alternating amino acid configuration provide helical secondary structures that are especially known from the membrane channel and pore-forming gramicidin A. In analogy to this natural D,L-alternating pentadecapeptide, the potential of D,L-alternating peptides for membrane insertion is investigated using the model dodecamer peptide H-(Phe-Tyr)(5)-Trp-Trp-OH. This aromatic peptide is introduced as a novel pore-forming synthetic analogue of gramicidin A. It forms a well-organized homodimer similar to one of the gramicidin A transmembrane motifs. X-ray reflectivity measurements are performed on solid-supported peptide-lipid complexes to obtain information about the influence of the artificial dodecamer peptide on the bilayer parameters. In addition, Fourier-transform infrared (FTIR) and circular dichroism (CD) spectroscopic studies determine the conformational state of H-(Phe-Tyr)(5)-Trp-Trp-OH within the model membrane. Site-specific iodine labeling assists in determining the topology of the membrane-embedded peptide by pinpointing the position of the iodine label within the bilayers.     

Effects of precipitation conditions on the membrane morphology and permeation characteristics

The permeability and permselectivity of asymmetric and particulate membranes towards glucose and proteins of various molecular sizes were studied. It was found that the skin layer of asymmetric membranes was permeable to glucose and insulin but effectively prevent the permeation of immunoglobulins. This result parallels our interest for the development of artificial pancreas. It was also found that skinless particulate membranes exhibited not only high permeation rates with respect to albumin and immunoglobulins but also good selectivity between these components. Thus, particulate membranes has the potential to be used in separating albumin from immunoglobulins for treating disorders related to immunoglobulin abnormalities.     

Molecular diffusion and slip boundary conditions at smooth surfaces with periodic and random nanoscale textures

The influence of periodic and random surface textures on the flow structure and effective slip length in Newtonian fluids is investigated by molecular dynamics (MD) simulations. We consider a situation where the typical pattern size is smaller than the channel height and the local boundary conditions at wetting and nonwetting regions are characterized by finite slip lengths. In the case of anisotropic patterns, transverse flow profiles are reported for flows over alternating stripes of different wettability when the shear flow direction is misaligned with respect to the stripe orientation. The angular dependence of the effective slip length obtained from MD simulations is in good agreement with hydrodynamic predictions provided that the stripe width is larger than several molecular diameters. We found that the longitudinal component of the slip velocity along the shear flow direction is proportional to the interfacial diffusion coefficient of fluid monomers in that direction at equilibrium. In case of random textures, the effective slip length and the diffusion coefficient of fluid monomers in the first layer near the heterogeneous surface depend sensitively on the total area of wetting regions.     

Study of a new pervaporation membrane Part I. Preparation and characteristics of the new membrane

A new alcohol dehydration membrane, poly(vinyl alcohol)—chitosan blended composite membrane (PVA-CS) has been prepared. This membrane has high selectivity and promising permeability, especially in separating ethanol—water near the azeotropic region (J_t > 200 g/m² h, _w/e > 500, 70°C). The separating characteristics, which vary with feed composition, operating temperature and the surface structure of the membrane, are determined and the results agree well with the theoretical predictions.

The characteristics of mechanical strength, stability and resistance to water were also determined. The results show that they are considerably enhanced by blending and crosslinking, in comparison with PVA composite membranes.     

Statistical mechanics of bilayer membrane with a fixed projected area

The equilibrium and fluctuation methods for determining the surface tension, sigma, and bending modulus, kappa, of a bilayer membrane with a fixed projected area are discussed. In the fluctuation method the elastic coefficients sigma and kappa are measured from the amplitude of thermal fluctuations of the planar membrane, while in the equilibrium method the free energy required to deform the membrane is considered. The latter approach is used to derive new expressions for sigma and kappa (as well as for the saddle-splay modulus), which relate them to the pair-interactions between the amphiphiles forming the membrane. We use linear response theory to argue that the two routes lead to similar values for sigma and kappa. This argument is confirmed by Monte Carlo simulations of a model membrane whose elastic coefficients are calculated using both methods.     

Study of a new pervaporation membrane Part 2. Performance test and analysis of the new membrane

The compatibility of poly(vinyl alcohol)—chitosan blends was tested and analyzed by their glass transition temperatures with differential scanning calorimetry with a DSC-7 (Epson). Highest selectivity, promising permeability and good mechanical strength for the dehydration of alcohol—water mixtures were obtained at a 4:1 composition of the blend.

The chemical composition, physical structure and morphology of this blended composite membrane were studied by a variety of surface science techniques, including infrared spectrometry (IR), small-angle X-ray diffraction (SAXRD) and scanning electron microscopy (SEM). The surface investigation is needed for the study of the blended composite membrane, and the results confirm well with the characteristics of the new membrane.     

硫醇－磷脂混合双层膜的电化学性质及其与蜂毒素的相互 作用研究

报道了硫醇－磷脂混合双层膜的循环伏安和电化学交流阻抗行为研究,并用电化学方法考察了蜂毒素与其相互作用,实验中通过冷冻表面沾有磷脂溶液的硫醇单层膜制备混合双层膜,研究表明双层膜在电极表面形成致密的绝缘层,阻碍了电极表面的电子传递,在双层膜体系上引入的蜂毒素可在膜表面上形成孔洞,破坏膜的绝缘性,降低膜电阻,增加膜电容,使带负电的探针Fe(CN)6^3-的氧化还原反应速度加快。     

Dynamics of a lipid bilayer induced by electric fields

The dynamics of a lipid bilayer of 1-palmitoyl-2-oleoyl-glycero-3-phospho-ethanolamine, POPE, is investigated under the effect of two electric field intensities. The box of 720 lipids and 13,458 water molecules-plus boundary conditions-undergoes similar re-organizational dynamics in the presence of fields of 0.35 V nm(-1) and 0.5 V nm(-1). Water fingers form followed by some lipid translocation from one layer to the other. The re-organization kinetics is of the second order and is roughly 5 times faster at the higher field. The translocations may occur also upon field switch off, provided that their duration was sufficiently long. Driving few lipid translocations by a macroscopic tool, such as the electric field, appears possible.     

Adsorption of gentamicin onto a bilayer lipid membrane

Addition of the aminoglycoside antibiotic, gentamicin (GM), to one side of a bilayer lipid membrane (BLM) results in a potential difference across the membrane. Evidence is presented that the membrane potential is caused by the adsorption of GM, bearing four positive charges, on the BLM surface. The experimental results are subjected to a quantitative analysis based on the double-layer theory and the Langmuir adsorption isotherm. The adsorption is saturated (i.e., the BLM is fully covered) at the bulk GM concentration of about 80 μmol/1. At this point, the calculated GM-induced increase in the BLM surface charge density is σ = 0.0054 C m⁻², which is equivalent to one positive charge per 50 lipids or one molecule of GM per 200 lipids.     

Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition

A thin, gas-tight palladium (Pd) membrane was prepared by the counter-diffusion chemical vapor deposition (CVD) process employing palladium chloride (PdCl₂) vapor and H₂ as Pd precursors. A disk-shaped, two-layer porous ceramic membrane consisting of a fine-pore γ-Al₂O₃ top layer and a coarse-pore -Al₂O₃ substrate was used as Pd membrane support. A 0.5–1 μm thick metallic membrane was deposited in the γ-Al₂O₃ top layer very close to its surface, as verified by XRD and SEM with a backscattered electron detector. The most important parameters that affected the CVD process were reaction temperature, reactants concentrations and top layer quality. Deposition of Pd in the γ-Al₂O₃ top layer resulted in a 100- to 1000-fold reduction in He permeance of the porous substrate. The H₂ permeation flux of these membranes was in the range 0.5–1.0 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at 350–450°C. The H₂ permeation data suggest that surface reaction steps are rate-limiting for H₂ transport through such thin membranes in the temperature range studied.     

Two components of boundary potentials at the lipid membrane surface: electrokinetic and complementary methods studies

Bilayer lipid membranes (BLM) are commonly used as models for cell membranes to study their interactions with inorganic ions and molecules of biological importance. In this work the principal electrostatic effects at the BLM surface are demonstrated by two methods: by the inner membrane field compensation (IFC) which is applied to planar BLM and sensitive to changes in the total boundary potential; and by electrokinetic measurements in liposome suspensions, sensitive to diffuse (surface) component of this potential. The difference in these two potentials allows us to conclude on changes in the dipole component of the boundary potential caused by structural changes at the membrane–water interface. No difference in the experimental data of both methods was observed for Be²⁺ and other divalent cation adsorption to unchanged phosphatidyl choline (PC) membranes. These data are in a good agreement with the Gouy–Champan–Stern (GCS) theory of diffuse double layer. This theory gives the value of binding constants for Be²⁺ about 400 M⁻¹ and 10⁴ M⁻¹ for DPPC liposomes in the liquid and solid states of the lipids, respectively. Clear isotope effects for normal and heavy water solutions of Be²⁺ were observed both by the electrostatic measurements and by differential scanning calorimetry. In contrast to PC, the electrostatic potentials induced by Be²⁺ and Gd³⁺ adsorption to membranes from phosphatidyiserine (PS) show the difference between the data of mentioned methods — total boundary potential changes are much higher in comparison to the surface potential. Dipole potential changes (about 150 mV) caused by changes in PS head group orientation may be more important in this specific case.     

The effect of the dipalmitoylphosphatidylcholine lipid bilayer state on the adsorption of phenyltins

The nonspecific adsorption of amphiphilic molecules onto the membrane depends both on the properties of the adsorbate and the state of the lipid bilayer. Electrostatic interactions drive the adsorption of charged molecules and hydrophobicity determines partition of the adsorbate into the membrane, whereas the steric compatibility of the lipid bilayer and the amphiphilic molecule is an additional factor to be accounted for when considering interaction between the adsorbate and the membrane. The adsorption of phenyltins was evaluated from changes in Fluorescein‐PE fluorescence intensity. The pH sensitivity of fluorophore, located at the membrane surface, was utilized to detect charges introduced onto the membrane by adsorbing compounds. It has been shown that the state of the membrane affects phenyltin adsorption in accordance with the number of phenyl rings on the molecule. Furthermore, the membrane surface topology determines interfacially located triphenyltin adsorption, with a much weaker effect on deeply embedded diphenyltin. When the dipalmitoylphosphatidylcholine (DPPC) model membrane is in the ripple phase, with complex surface morphology, phenyltin adsorption is greatly enhanced. Results presented in this paper show that steric constraints imposed on rigid and bulky amphiphilic compounds by ordered alkyl chains and membrane surface topology affect nonspecific molecule adsorption onto the membrane. Copyright © 2000 John Wiley & Sons, Ltd.     

Lipid bilayer membrane with atomic step structure: supported bilayer on a step-and-terrace TiO2(100) surface

The formation of a supported planar lipid bilayer (SPLB) and its morphology on step-and-terrace rutile TiO 2(100) surfaces were investigated by fluorescence microscopy and atomic force microscopy. The TiO 2(100) surfaces consisting of atomic steps and flat terraces were formed on a rutile TiO 2 single-crystal wafer by a wet treatment and annealing under a flow of oxygen. An intact vesicular layer formed on the TiO 2(100) surface when the surface was incubated in a sonicated vesicle suspension under the condition that a full-coverage SPLB forms on SiO 2, as reported in previous studies. However, a full-coverage, continuous, fluid SPLB was obtained on the step-and-terrace TiO 2(100) depending on the lipid concentration, incubation time, and vesicle size. The SPLB on the TiO 2(100) also has step-and-terrace morphology following the substrate structure precisely even though the SPLB is in the fluid phase and an approximately 1-nm-thick water layer exists between the SPLB and the substrate. This membrane distortion on the atomic scale affects the phase-separation structure of a binary bilayer of micrometer order. The interaction energy calculated including DLVO and non-DLVO factors shows that a lipid membrane on the TiO 2(100) gains 20 times more energy than on SiO 2. This specifically strong attraction on TiO 2 makes the fluid SPLB precisely follow the substrate structure of angstrom order.     

Discretization of unknown functions: A new method to solve partial differential equations with mixed boundary conditions

In this paper, we develop a new numerical technique to obtain an approximate solution of partial differential equations subject to mixed boundary conditions (MBCs). The approach has been applied to a class of differential equations which frequently arise in a large variety of problems such as heat conduction, potential theory, and diffusion‐controlled chemical reactions. In our approach, based on the discretization of unknown functions (DF), the solution is expressed as a series expansion and the determination of the series coefficients is reduced to the solution of a system of algebraic equations. The main advantages of the DF procedure are: (a) the smoothness of the function and of its first derivative in the different domains, whereas the other numerical methods generally show a highly oscillating behavior; (b) the fast convergence of the series expansion. This method has been applied to solve diffusion problems in different coordinate systems (trigonometric, cylindrical and spherical). The obtained results have been compared with the analytical solution (when available) as well as with other numerical methods commonly used to solve MBCs problems. This revised version was published online in July 2006 with corrections to the Cover Date.