Model Hydrophobic Ion Exchange Membrane

Two models of hydrophobic ion exchange membranes were examined theoretically with regard to the characteristics of cellulose acetate-nitrate membranes saturated with hydrophobic solvents. The first model, consisting of fixed negative sites dispersed in a homogeneous medium of low dielectric constant, was shown to be invalid for the experimental membranes. The second model, consisting of fixed negative sites in an aqueous channel surrounded by a medium of low dielectric constant, explains many properties of the cellulose acetate-nitrate hydrophobic membranes and was analyzed in some detail. Organic cations can enter the membranes through the hydrophobic phase as well as through the aqueous channels. The mechanism of counterion movement in such a model is assumed to consist of exchange of vacancies and or double-occupied sites positions. The presence of the medium of low dielectric constant around the aqueous channel increases the “self”-energy of the ions in the channel and the electrostatic interaction between a fixed site and a counterion in the membrane. Both these factors can account for the marked dependence of ion mobility in the aqueous channels on the dielectric constant of the surrounding medium. The model predicts membrane preference for monovalent counterions over divalent ones.     

Membrane Potentials and Ion Permeability in a Cation Exchange Membrane

The nonequilibrium electrical potentials across an artificial membrane bathed by solutions of a single salt have been measured and calculated using the Goldman-Hodgkin-Katz equation and the irreversible thermodynamic equation. The latter equation predicts the observed potential differences over a 2500-fold concentration range, while application of a modified Goldman-Hodgkin-Katz equation leads to difficulties.     

The Double Fixed Charge Membrane: Solution-Membrane Ion Partition Effects and Membrane Potentials

An analysis is made of the effect of solution-membrane partition of ions on the electrostatic potential and ion concentration profiles in fixed charge membranes. It is shown that the inclusion of partition effects gives rise to large solution-membrane “Donnan” potentials even when the concentration of fixed charges is of the same order as the concentration of the external solution. This effect renders the system and the simplified analysis of the double fixed charge membrane (FCM) previously given more applicable to biological membranes. An analysis is also given of the voltage dependence of the fluxes of individual ion species in the double FCM when it separates different ionic solutions and an expression is deduced for the membrane resting potential. Although the latter is similar in form to the Goldman-Hodgkin-Katz (GHK) equation the corresponding value of the permeability ratio P_C1/P_K is under certain specified conditions both concentration and potential dependent.     

离子转运相关膜蛋白的几种研究方法

离子转运相关的膜蛋白包括离子通道（Ion channel）和转运蛋白（transporter）等。除了传统的电压钳位法和膜片钳技术外,利用酵母和拟南芥突变体的异源互补技术已成为当今研究离子转运相关膜蛋白及其基因的主要分子生物学手段。     

植物细胞质膜离子通道研究进展

多种有机和无机离子作为重要的营养物质、渗透物质、辅酶和信号分子, 参与植物生殖、生长发育和逆境反应等多种生物学过程。离子通道是离子跨质膜和内膜运动的重要渠道和动态调控因子, 直接影响和调控细胞内离子浓度及亚细胞分布的动态变化。目前, 植物尤其是模式植物拟南芥(Arabidopsis thaliana)的多个离子通道家族被先后鉴定出来, 其中部分离子通道蛋白定位在细胞质膜上, 其基本生物学功能, 诸如蛋白结构、离子选择性和通透性、门控特点、活性调控机理以及不同离子通道之间的协同关系等均取得重要进展。该文概要介绍近年来植物细胞质膜离子通道方面的研究进展。     

植物质膜钾离子转运体研究进展

近年,随着分子生物学技术的不断发展和广泛应用,有关植物质膜钾离子转运体的研究取得重要进展。目前已经克隆到多种质膜钾离子转运体基因并对钾离子转运体生化特性以及结构功能进行广泛研究。研究认为,质膜钾离子转运体可分为钾离子载体和钾离子通道。钾离子通道又可分为内向性Ｋ＾＋通道α亚基、Ｋ＾＋通道β亚基及外生Ｋ＾＋通道等三类。本文对上述质膜钾离子转运体的生化特性以及结构功能研究的进展进行了综述。     

植物质膜钾离子转运体研究进展

近年,随着分子生物学技术的不断发展和广泛应用,有关植物质膜钾离子转运体的研究取得重要进展。目前已经克隆到多种质膜钾离子转运体基因并对钾离子转运体生化特性以及结构功能进行了广泛研究。研究认为,质膜钾离子转运体可分为钾离子载体和钾离子通道。钾离子通道又可分为内向性K+通道α亚基、K+通道β亚基及外向性K+通道等三类。本文对上述质膜钾离子转运体的生化特性以及结构功能研究的进展进行了综述。     

Molecular Model of a Cell Plasma Membrane With an Asymmetric Multicomponent Composition: Water Permeation and Ion Effects

We present molecular dynamics simulations of a multicomponent, asymmetric bilayer in mixed aqueous solutions of sodium and potassium chloride. Because of the geometry of the system, there are two aqueous solution regions in our simulations: one mimics the intracellular region, and one mimics the extracellular region. Ion-specific effects are evident at the membrane/aqueous solution interface. Namely, at equal concentrations of sodium and potassium, sodium ions are more strongly adsorbed to carbonyl groups of the lipid headgroups. A significant concentration excess of potassium is needed for this ion to overwhelm the sodium abundance at the membrane. Ion-membrane interactions also lead to concentration-dependent and cation-specific behavior of the electrostatic potential in the intracellular region because of the negative charge on the inner leaflet. In addition, water permeation across the membrane was observed on a timescale of ∼100 ns. This study represents a step toward the modeling of realistic biological membranes at physiological conditions in intracellular and extracellular environments.     

离子通道数学模型

介绍了离子通道记录的记忆性及其反映这种记忆的齐次Markov模型和具有随机环境的Markov模型,并且上述模型较好的解决“Omission”问题,讨论了模型反映的离子通道记录的物理及生理机制,认为离子通道记录记忆性反映离子通道记忆性。     

The Non-Steady-State Membrane Potential of Ion Exchangers with Fixed Sites

A system of equations, based upon the assumption that the only force acting on each ionic species is due to the gradient of its electrochemical potential, is used to deduce, in the non-steady state for zero net current, the expression of the difference of electric potential between two solutions separated by an ion exchange membrane with fixed monovalent sites. The membrane is assumed to be solely permeable to cations or anions, depending on whether the charge of the sites is -1 or +1, and not to permit any flow of solvent. Under the assumptions that the difference of standard chemical potentials of any pair of permeant monovalent species and the ratio of their mobilities are constant throughout the membrane, even when the spacing of sites is variable, explicit expressions are derived for the diffusion potential and total membrane potential as functions of time and of solution activities. The expressions are valid for any number of permeant monovalent species having ideal behavior and for two permeant monovalent species having “n-type” non-ideal behavior. The results show that for a step change in solution composition the observable potential across a membrane having fixed, but not necessarily uniformly spaced, sites becomes independent of time once equilibria are established at the boundaries of the membrane and attains its steady-state value even while the ionic concentration profiles and the electric potential profile within the membrane are changing with time.     

Receptor Displacement in the Cell Membrane by Hydrodynamic Force Amplification through Nanoparticles

We introduce an intrinsically multiplexed and easy to implement method to apply an external force to a biomolecule and thus probe its interaction with a second biomolecule or, more generally, its environment (for example, the cell membrane). We take advantage of the hydrodynamic interaction with a controlled fluid flow within a microfluidic channel to apply a force. By labeling the biomolecule with a nanoparticle that acts as a kite and increases the hydrodynamic interaction with the fluid, the drag induced by convection becomes important. We use this approach to track the motion of single membrane receptors, the Clostridium perfringens ε-toxin (CPεT) receptors that are confined in lipid raft platforms, and probe their interaction with the environment. Under external force, we observe displacements over distances up to 10 times the confining domain diameter due to elastic deformation of a barrier and return to the initial position after the flow is stopped. Receptors can also jump over such barriers. Analysis of the receptor motion characteristics before, during, and after a force is applied via the flow indicates that the receptors are displaced together with their confining raft platform. Experiments before and after incubation with latrunculin B reveal that the barriers are part of the actin cytoskeleton and have an average spring constant of 2.5 ± 0.6 pN/μm before vs. 0.6 ± 0.2 pN/μm after partial actin depolymerization. Our data, in combination with our previous work demonstrating that the ε-toxin receptor confinement is not influenced by the cytoskeleton, imply that it is the raft platform and its constituents rather than the receptor itself that encounters and deforms the barriers formed by the actin cytoskeleton.     

Lyotropic Ion Channel Current Model Compared with Ising Model

Trans-membrane currents in ligand-gated ion channels are calculated in a non-equilibrium, chemically open whole cell system. The model is lyotropic in the sense that dynamics and parameters such as ligand concentration for half-maximal response (scale of response), and threshold for firing in neurons, are nonlinear functions of the reactant concentrations. The derived total current fits recorded data significantly better than those derived from mass action, Ising, and other equilibrium type models, in which the derived response can be displaced from the assessed response by several orders in the ligand concentration. A comparison of the model obtained with an Ising-like model provides a methodology to obtain the non-equilibrium scaling dependence of Ising-like models on the reactant concentrations.     

An Instruction on the In Vivo Shell-Less Chorioallantoic Membrane 3-Dimensional Tumor Spheroid Model

The traditional shell chicken chorioallantoic membrane (CAM) model has been used extensively in cancer research to study tumor growth and angiogenesis. Here we present a combined in vivo tumor spheroid and shell-less CAM three-dimensional model for use in quantitative and qualitative analysis. With this model, the angiogenic and tumorigenic environments can be generated locally without exogenous growth factors. This physiological model offers a stable, static and flat environment that features a large working area and wider field of view useful for imaging and biomedical engineering applications. The short experimental time frame allows for rapid data acquisition, screening and validation of biomedical devices. The method and application of this shell-less model are discussed in detail, providing a useful tool for biomedical engineering research.     

精子膜表面配体依赖性离子通道与精子顶体反应

配体依赖性离子通道是一类由神经递质调控的跨膜离子通道。研究发现它们在精子的顶体反应中起了重要作用,顶体反应是精子完成受精的一个关键步骤。至今已发现3种配体依赖性离子通道受体存在于精子头部的质膜上,它们是乙酰胆碱受体、甘氨酸受体和GABAa受体。尽管乙酰胆碱受体和甘氨酸受体已被清楚的证明参与了ZP3诱导的顶体反应,GABAa受体的功能则相对复杂,需进一步研究。这类受体在精子膜电压变化中起的作用和由此导致的膜电位改变对于精子顶体反应的重要性,为精子顶体反应提供了一个可能的信号传递途径。     

Continuum Approaches to Understanding Ion and Peptide Interactions with the Membrane

Experimental and computational studies have shown that cellular membranes deform to stabilize the inclusion of transmembrane (TM) proteins harboring charge. Recent analysis suggests that membrane bending helps to expose charged and polar residues to the aqueous environment and polar head groups. We previously used elasticity theory to identify membrane distortions that minimize the insertion of charged TM peptides into the membrane. Here, we extend our work by showing that it also provides a novel, computationally efficient method for exploring the energetics of ion and small peptide penetration into membranes. First, we show that the continuum method accurately reproduces energy profiles and membrane shapes generated from molecular simulations of bare ion permeation at a fraction of the computational cost. Next, we demonstrate that the dependence of the ion insertion energy on the membrane thickness arises primarily from the elastic properties of the membrane. Moreover, the continuum model readily provides a free energy decomposition into components not easily determined from molecular dynamics. Finally, we show that the energetics of membrane deformation strongly depend on membrane patch size both for ions and peptides. This dependence is particularly strong for peptides based on simulations of a known amphipathic, membrane binding peptide from the human pathogen Toxoplasma gondii. In total, we address shortcomings and advantages that arise from using a variety of computational methods in distinct biological contexts.     

Low-Friction Graphene Oxide-Based Ion Selective Membrane for High-Efficiency Osmotic Energy Harvesting

Graphene-based laminate membranes with selective ion-transport capability show great potential in renewable osmotic energy harvesting. One of the great challenges is to reduce the overall energy barriers while remain the high ion selectivity in the transmembrane ion transport process. Here, a strategy is proposed to break the trade-off between ion selectivity and permeability in laminar nanochannels using amphiphilic molecules as modifier, which enhances the surface charge density of nanochannel by loading more ion polymer with polar head and lows the frictional force of ion transport with hydrophobic tail. The conversion efficiency can reach to 32% in osmotic energy harvesting (0.5 m /0.01 m concentration gradient) after adopting this modifier. During the process of mixing real river water and seawater, the maximum power density can reach to 13.38 W m⁻². The amphiphilic molecules also bind adjacent nanosheets, endowing the membrane's strong mechanical strength and high stability in aqueous solution. This work can open up a new way to regulate the transmembrane ion transport in 2D laminate membranes.     

Direct Observation of Membrane Insertion by Enveloped Virus Matrix Proteins by Phosphate Displacement

Enveloped virus release is driven by poorly understood proteins that are functional analogs of the coat protein assemblies that mediate intracellular vesicle trafficking. We used differential electron density mapping to detect membrane integration by membrane-bending proteins from five virus families. This demonstrates that virus matrix proteins replace an unexpectedly large portion of the lipid content of the inner membrane face, a generalized feature likely to play a role in reshaping cellular membranes.