Coupling of electron and proton transport in photosynthetic membranes: molecular mechanism

Using the method of Modified Neglect of Diatomic Overlap (MNDO), the electronic structure of plastoquinol (PQH(2)) and plastoquinone (PQ) in neutral, singly (PQ(-)) and doubly (PQ(2-)) reduced states is studied. The conformational analysis performed on these molecules shows that in the lowest energy conformation, the angle between the first link of the tail backbone and the ring plane of neutral and singly reduced PQ and plastoquinol is nearly the same and differs by 15 degrees from that of doubly reduced PQ. Nevertheless, for all states of plastoquinone and for plastoquinol, the total energy changes by less than 0.2 eV when the studied angle is varied from 0 degrees to 180 degrees. As in Rhodobacter sphaeroides, the oxygen of the PQ ring is reported to form a hydrogen bond with a nitrogen in the ring of Histidine (His) L 190. The energy of the PQ-His complex was calculated for different redox states of PQ and for several values of the distance between the molecules (N-O distance from 0.2 to 0.5 nm). For every considered complexes, the total energy dependence on the proton position on the line connecting the N and O atoms was determined, to see if the hydrogen bond is formed. It is shown that for only singly reduced PQ this dependence has a symmetric two-well form, i.e. the hydrogen bond is formed. For neutral and doubly reduced PQ, the curve is also two-well but asymmetric, so that the proton is bound to His or to PQ, correspondingly.On the basis of these results, we propose the following scheme of electron-proton coupling. Negatively charged oxygens of PQ form H-bonds with proton donor groups of the surrounding protein and fix PQ in its pocket. While the negative charges of oxygens increase after quinone reduction, protons shift to PQ oxygens and form strong hydrogen bonds with them. Upon second PQ reduction, protons are torn away from surrounding amino acids and form covalent bonds with the quinol. Resulting PQH(2) detaches from its binding place and is replaced by a neutral PQ. The lacking protons on amino acids in the Q(B) pocket are replaced by a step-by-step transfer from the stroma bulk through the proton channels.     

Nonequilibrium molecular dynamics simulation of water transport through carbon nanotube membranes at low pressure

Nonequilibrium molecular dynamics (NEMD) simulations are used to investigate pressure-driven water flow passing through carbon nanotube (CNT) membranes at low pressures (5.0 MPa) typical of real nanofiltration (NF) systems. The CNT membrane is modeled as a simplified NF membrane with smooth surfaces, and uniform straight pores of typical NF pore sizes. A NEMD simulation system is constructed to study the effects of the membrane structure (pores size and membrane thickness) on the pure water transport properties. All simulations are run under operating conditions (temperature and pressure difference) similar to a real NF processes. Simulation results are analyzed to obtain water flux, density, and velocity distributions along both the flow and radial directions. Results show that water flow through a CNT membrane under a pressure difference has the unique transport properties of very fast flow and a non-parabolic radial distribution of velocities which cannot be represented by the Hagen-Poiseuille or Navier-Stokes equations. Density distributions along radial and flow directions show that water molecules in the CNT form layers with an oscillatory density profile, and have a lower average density than in the bulk flow. The NEMD simulations provide direct access to dynamic aspects of water flow through a CNT membrane and give a view of the pressure-driven transport phenomena on a molecular scale.     

Dynamic mechanism of fatty acid transport across cellular membranes through FadL: molecular dynamics simulations

FadL is an important member of the family of fatty acid transport proteins within membranes. In this study, 11 conventional molecular dynamics (CMD) and 25 steered molecular dynamics (SMD) simulations were performed to investigate the dynamic mechanism of transport of long-chain fatty acids (LCFAs) across FadL. The CMD simulations addressed the intrinsically dynamic behavior of FadL. Both the CMD and SMD simulations revealed that a fatty acid molecule can move diffusively to a high-affinity site (HAS) from a low-affinity site (LAS). During this process, the swing motion of the L3 segment and the hydrophobic interaction between the fatty acid and FadL could play important roles. Furthermore, 22 of the SMD simulations revealed that fatty acids can pass through the gap between the hatch domain and the transmembrane domain (TMD) by different pathways. SMD simulations identified nine possible pathways for dodecanoic acid (DA) threading the barrel of FadL. The binding free energy profiles between DA and FadL along the MD trajectories indicate that all of the possible pathways are energetically favorable for the transport of fatty acids; however, one pathway (path VI) might be the most probable pathway for DA transport. The reasonability and reliability of this study were further demonstrated by correlating the MD simulation results with the available mutagenesis results. On the basis of the simulations, a mechanism for the full-length transport process of DA from the extracellular side to the periplasmic space mediated by FadL is proposed.     

Polypyrrole based cation transport membranes

It has been recently reported that the cation transport can be induced across a conducting polymer membrane by the application of a potential gradient. As an extension of that work this article, is a part of an ongoing study to look at the effects of altering various parameters employed in the synthesis of the membranes, on the physical characteristics and cation transport properties of the resulting membranes. The results indicated that the optimal film growth was achieved when using a growth temperature at or below 20°C, a current density between 1 and 3 mA/cm², and a dopant concentration of 0.1 M.     

Ion transport in inhomogeneous ion exchange membranes

The effect on current efficiency produced by fluctuations in an ion-exchange membrane's fixed ion concentration (due to ion clustering) is determined by considering transport in several model membranes containing different fixed ion distributions. Electroneutrality is not assumed, so as to include nonlinear effects due to space charge in the clusters. The Nernst—Planck and Poisson equations are solved using perturbation theory, and the case of small fluctuations in fixed charge density is considered in order to obtain analytic solutions to the perturbation equations. Results show that an inhomogeneous distribution of fixed ions gives rise to increased current efficiency when the counterions and co-ions experience in the membrane unequal forces due to the potential produced by the fixed ions. The length scale over which the inhomogeneities occur is important in determining the current efficiency.     

Enhanced drug transport from unilamellar to multilamellar liposomes induced by molecular recognition of their lipid membranes

Unilamellar PC-based liposomes bearing a recognizable moiety were loaded either with the hydrophilic drug doxorubicin (DXR) or with the hydrophobic drug tamoxiphen (TMX) and allowed to interact with multilamellar PC-based liposomes bearing complementary recognizable groups. It has been established that, due to molecular recognition of these complementary liposomes, effective and fast transport of the drugs occurs from unilamellar to multilamellar liposomes. The transport of TMX is more effective compared to that of DXR. This behavior was observed for both PEGylated and non-PEGylated unilamellar liposomes, and it was attributed to the different sites of solubilization of the drugs in the unilamellar liposomes. PEGylation reduces the transport of both drugs since it inhibits to some extent the molecular recognition effectiveness of the complementary moieties.     

Asymmetric molecular sieve carbon membranes

An asymmetric molecular sieve carbon membrane is obtained by conventional pyrolysis of a thermosetting polymeric film, followed by unequal oxidation. Morphology, pore size distribution, and gas separation characteristics of the membrane are discussed. The transport mechanism for gas permeation is clearly non-Knudsen diffusion since heavier oxygen permeates faster than lighter nitrogen. The proposed mass transfer mechanism is that of a molecular sieve.     

Photoinduced electron transport across protein-containing membranes

It is shown in the present work that a protein globule can mediate the donor-to-acceptor electron transport, when the protein is in the solution or incorporated into the membrane.     

Ion transport modelling through nanofiltration membranes

The aim of this work is to study the transport mechanism of ions through nanofiltration membranes. A model based on extended Nernst–Planck and film theory equations is reported. This model can be characterized by three transport parameters: the water permeability L_p, the salt transmittance Φ and the effective salt transfer coefficient K_eff. The knowledge of the feed and permeate concentration and of the permeate volumetric flux enable us to calculate these transport parameters. The model is used to estimate cadmium salts rejection by a NANOMAX 50 membrane. Experimental and calculated results are shown to be in good agreement. The model is then successfully extended to experimental data reported in the literature.     

Transient response of facilitated transport membranes

The transient flux of permeate is determined for one-dimensional facilitated transport in flat plate, cylindrical, and spherical geometries. The results are presented graphically. The flux is a function of four parameters. The graphical results allow one to determine the permeate flux as a function of time for given set of operating conditions, determine the steady-state value of the permeate flux and the time to reach steady-state.     

Quantum model for transport through membranes

In the present paper a new model for the transport through membranes, introduced previously by Hosur, is considered. In this model, the membrane is assimilated to a potential barrier, and the material flow through the membrane is determined from the transmission coefficient of the barrier, assuming the existence of an energy difference among the molecules placed at both sides of the membrane. An equation for the transport, in the case of small energy differences, is obtained, which is particularized to the cases that the energy difference is caused by a temperature gradient, a concentration gradient, and both gradients acting together. In all cases, under certain limitations, formally identical equations to those of the thermodynamics of irreversible processes are obtained.     

Competitive facilitated transport through liquid membranes

A mathematical model is presented which solves the dimensionless, transient, non-linear partial differential equations governing the competitive facilitated transport of two gases through a liquid membrane. The model incorporates the mass transfer coefficients in the boundary conditions for the free gas concentrations. Several studies were carried out. A comparison of this model with a steady-state “equilibrium core” model was excellent. Through varying the dimensionless parameters, it was found that gas I would have a higher steady-state facilitation factor than gas 2 if k₁ >k₂ and k_-1k_-2. The boundary conditions and mass transfer coefficients were also varied to see their effects on the facilitation factors. The idea of pumping one of the gases against its concentration gradient was shown to be theoretically possible.     

Oxygen dissolution and transport in cobaltporphyrin-bound organophosphazene membranes

Rubbery poly(organophosphazene)s were synthesized, and were combined with cobaltporphyrin (CoP) which binds molecular oxygen rapidly and reversibly. The apparent oxygen-binding equilibrium constant (K_app) is in proportion to the physical solubility coefficient of oxygen in the polymer, although the reduced equilibrium and thermodynamic parameters are not dependent on the polymer matrix species. Diffusivity of oxygen via the fixed CoP(D_C) is enhanced for poly(organophosphazene) membranes with a larger oxygen diffusion constant. Poly(organophosphazene) membranes with both a large K_app and D_C yield high oxygen permeability.     

Ion transport and clustering in nafion perfluorinated membranes

A theory of ion-clustering is presented for Nation resins. An expression for the cluster diameter is derived; it correctly describes the experimental variations in cluster diameter with cation form, equivalent weight, and water content of the polymers. The theory further suggests that short channels connecting adjacent clusters are thermodynamically stable in support of the cluster-network model proposed by one of us earlier. The percolative aspects of ion transport and an ionic insulator-to-conductor transition in Nation resins are discussed.     

Modeling molecular transport in slit pores

We examine the transport of methane in microporous carbon by performing equilibrium and nonequilibrium molecular dynamics simulations over a range of pore sizes, densities, and temperatures. We interpret these simulation results using two models of the transport process. At low densities, we consider a molecular flow model, in which intermolecular interactions are neglected, and find excellent agreement between transport diffusion coefficients determined from simulation, and those predicted by the model. Simulation results indicate that the model can be applied up to fluid densities of the order to 0.1-1 nm(-3). Above these densities, we consider a slip flow model, combining hydrodynamic theory with a slip condition at the solid-fluid interface. As the diffusion coefficient at low densities can be accurately determined by the molecular flow model, we also consider a model where the slip condition is supplied by the molecular flow model. We find that both density-dependent models provide a useful means of estimating the transport coefficient that compares well with simulation.     

A study of lithium transport in aluminum membranes

We employed the Devanathan–Stachurski experimental methodology for direct measurement of the rate of lithium transport in metals that can be used as negative electrodes in lithium–ion batteries. The measured Li transport rate in aluminum appears to be in relatively good agreement with previously reported results obtained using standard electrochemical techniques. However, Li transport rate measurements in aluminum membranes of different thicknesses reveal anomalies with regard to the standard diffusion controlled mass transfer model. We attribute this effect to a complex Li transport mechanism in Al membranes upon alloying.     

Facilitated transport mechanism in fixed site carrier membranes

Facilitated transport of gases, such as O₂, has been observed for fixed site carrier membranes. Carrier-impregnated ion-exchange membranes have also been used to produce facilitated transport of CO₂, H₂S and olefins when the carriers are not completely mobile. The actual mechanism for this facilitated transport has not been determined previously. The facilitated transport is provided by the exchange of solute between the free solute region and the bound complexing agent combined with gradients in the free solute and complexed carrier. The ratio of mobility for exchange between the regions is the dimensionless equilibrium constant of the reversible reaction. An expression is also derived for the relationship between the effective diffusion coefficient of the complex in facilitated transport, D_AB, and the diffusion coefficients for exchange between free and complexed solute, D_DH and D_HD. Since D_AB can be determined from experimental results, D_DH and D_HD can be calculated. The results also show the morphology dependence of D_AB. The diffusion coefficient of the free solute D_A also changes with the loading of the complexing agent in the film. This morphology dependence can explain why the gas permeability can decrease when the concentration of the complexing agent increases in the membrane.     

Gas transport in polyurethane-polystyrene interpenetrating polymer network membranes. I. Effect of synthesis temperature and molecular structure variation

The gas (oxygen and nitrogen) transport characteristics of the interpenetrating polymer network (IPN) membranes of polyurethane/polystyrene were studied. The effect of synthesis temperature, composition, molecular weight of the polyol and aromatic content (of MDI, TDI and HDI) on the gas permeability were analyzed. In the IPN synthesis, first polyurethane was polymerized thermally, and then polystyrene was polymerized by photolytic methods at different temperatures. The permeability coefficient decreased and the separation factor increased with decreasing synthesis temperature due to the miscibility increase. The permeability coefficient showed a minimum value and the separation factor showed a maximum value at ca.25 wt.% polyurethane composition. The permeability coefficient decreased and the separation factor increased with increasing aromatic content in polyurethane component. The morphology and density behavior of the IPN's agreed well with the permeability data. The tensile strength of the membrane increased with decreasing synthesis temperature and with increasing crosslink density and polystyrene content.