Pace of diffusion through membranes

Summary Although membranes are often viewed as barriers to diffusing particles, in many cases their presence does not slow down diffusion. Investigations of the transit time (mean diffusion time) for cases where the source and the target of diffusing particles are separated by various arrangements of membranes reveal the following facts: (i) The transit time is composed of the sum of the times to diffuse each of the membrane and aqueous regions separately and terms representing the time spent at the vicinity of the interfaces between these regions. (ii) In cases of one dimensional diffusion between aqueous and membranal phases, the transit time is governed by the parameter whereD _m andD _w are the diffusion coefficients in the membrane and water, respectively, and is the membrane/water partition coefficient of the particles. While the former ratio depends mostly on the viscosities of the two phases, the latter parameter is very strongly dependent on the identity of the particles. The diffusion from water to the membrane is faster than from the membrane to water whenever 1$$ " align="middle" border="0"> . The opposite is true when this parameter is smaller than 1. (iii) In case of one dimensional transmembranal diffusion, the transit time shows a minimum when wherel _w1 andl _w2 are the net diffusion distances in the aqueous phases on both sides of the membrane. In this case, if the diffusion proceeds through pores in the membrane, represents the fraction of membrane area that is occupied by the pores.The transit times for three dimensional diffusion into and from a spherical cell are also presented in a simple form. In addition, some of the relations between transit times and other measurable time parameters, such as the course of the decay of gradients and the time lag to establish steady states, are discussed briefly.The conclusions emerging from this analysis, together with the simple expressions for the transit times can make these investigation useful for the understanding of diffusion in systems containing natural or artificial membranes.     

Bilirubin diffusion through lipid membranes

The possibility that bilirubin can diffuse through lipid bilayers is investigated with liposomes prepared from dipalmitoylphosphatidylcholine (DPPC), egg phosphatidylcholine (egg PC) with 22 mole percent cholesterol, and a lipid extract preparation from N115 neuroblastoma cells. Liposomes were prepared with internalized bilirubin and bovine or human serum albumin, and bilirubin efflux into an exogenous solution of human serum albumin was measured. Efflux from DPPC liposomes was significantly higher above the phase transition temperature than below it. This change was dependent on the lipid undergoing a phase transition and could not be accounted for by 6 K change in temperature. Maximum bilirubin efflux from egg PC-cholesterol liposomes was found to depend on the relative internal and external albumin pools, suggesting an equilibrium distribution of bilirubin between them. These observations demonstrate that bilirubin can diffuse freely through these lipid membranes.     

Statistical-mechanical theory of passive transport through partially sieving or leaky membranes

The basic equations for multicomponent transport through partially sieving or leaky membranes are discussed from a statistical-mechanical viewpoint. They have the same mathematical form as the corresponding equations for open membranes, but differ in a discontinuous way from the equations for semipermeable membranes (since a "leak" in a semipermeable membrane constitutes a discontinuous or singular perturbation). Partially sieving membranes can be made to mimic semipermeable behavior through the introduction of characteristic time scales. They may approximate semipermeable behavior at short times, but always deviate at longer times.     

Chiral separation of D,L-mandelic acid through cellulose membranes

This work reports the chiral separation of D,L-mandelic acid with cellulose membranes. Cellulose was chosen as membrane material because it possesses multichiral carbon atoms in its molecular structure unit. The flux and permselective properties of membrane using aqueous solutions of D,L-mandelic acid as feed solution was studied. The top surface and cross-section morphology of the resulting membrane were examined by scanning electron microscopy. When the membrane was prepared with 8.1 wt % cellulose and 8.1 wt % LiCl in the DMA casting solution, and the operating pressure and feed concentration of racemate were 0.0125 MPa and 0.5 mg/ml, respectively, over 90% of enantiomeric excess could be obtained. This is the first report that the cellulose membrane is used for isolating the optical isomers of D,L-mandelic acid. Chirality, 2011.     

Porcine blood cell separation by porous cellulose acetate membranes

Leukocytes were separated from whole porcine blood using laboratory prepared polymeric asymmetric porous membranes from cellulose acetate (CA) and by applying standard blood cell separation methods: centrifugation in a Ficoll solution gradient and in sucrose solution concentration gradient. Leukocytes, obtained by different separation methods were characterised by their quantity, type, viability and growth ability. Membranes prepared by a wet phase inversion process from different cellulose acetate/acetone/water and magnesium chlorate VII systems, were characterised according to: permeability to deionised water, surface morphology and by the determination of the flux of the permeate during the whole porcine blood separation. Cellulose acetate membranes prepared from 300 μm thick cast solution (14.8 wt% of cellulose acetate, 19.9 wt% of water, 2.3 wt% of Magnesium perchlorate, and 63.0 wt% of acetone), have separation characteristics comparable with the standard separation methods; in the dead-end mode filtration, 21.3% of leukocytes from porcine whole blood are separated. The leukocyte number in peripheral blood before separation was 450,000 ml^-1; the number passed through after was 95,000±6620. The main interest of the study was to introduce the CA membrane filters for the continus technological separation of the leukocyte/lymphocytes from animal (= porcine, bovine, horse..) blood. This revised version was published online in June 2006 with corrections to the Cover Date.     

A simple method for the simulation of steady-state diffusion through membranes: pressure-tuned,boundary-driven molecular dynamics

We present a novel molecular dynamics-based simulation technique for investigating gas transport through membranes. In our simulations, the main control parameters are the partial pressure for the components on the input side of the membrane and the total pressure on the output side. The essential point of our scheme is that this pressure control should be realised by adjusting the particle numbers in the input and output side control cells indirectly. Although this perturbation is applied sufficiently far from the membrane, the bulk-phase properties are well controlled in a simulation cell of common size. Numerical results are given for silicalite-1 membrane with permeating CH₄, CO₂, H₂ and N₂ gases as well as with binary mixtures of CO₂ with the other three components. To describe interactions between particles, we used the simple shifted and cut Lennard–Jones potential with parameters available in the literature. It is expected that the proposed technique can be applied to several other types of membranes and transported fluids in order to support the development of a deeper understanding of separation processes.     

Determination of diffusion coefficients in a solution by the transport of a solute through porous membranes

The possibility of determining the coefficients of diffusion in solution by the transport of solutes through porous polymeric membrane was studied. Reliable and reproducible results can be obtained by using nucleoporous filters with cylindrical pores. The method enables the selective determining of the diffusion coefficients of solutes being in complex mixtures, which is of special interest for biochemical research. The possibilities of the method are illustrated on the pattern of some globular proteins, polyethylene glycols and proteolytic enzymes.     

Nonelectrolyte diffusion through lecithin-water lamellar phases and red-cell membranes

The longitudinal diffusion of a homologous series of monoamides through lecithin-water lamellar phases with aqueous channel widths of 16–27 Å has been studied. The diffusion coefficients relative to water of the hydrophilic amides, formamide and acetamide, depend logarithmically on solute molar volume, as previously demonstrated in human red cells. Aqueous diffusion of amides in red-cell membranes is similar to that in a lecithin-water phase of aqueous channel width less than 16 Å, the smallest channel width used. Partition coefficients of the lipophilic amides, valeramide and isovaleramide, between lecithin vesicles and water are 1.64 and 1.15 at 20 °C. These data enabled us to compute a valeramide diffusion coefficient of 6.5 · 10⁻⁷cm² · s⁻¹ at 20 °C in the lipid region of a lamellar phase containing 30% water about one order of magnitude greater than the diffusion coefficient of spin-labelled analogs of phosphatidylcholine. The discrimination between the permeability coefficients of valeramide and isovaleramide is more than twice as great in the human red cell as between lipid diffusion coefficients in a phase containing 8% water. This suggests that the lipid region of the human red cell is more highly organized than lipid in the lecithin-water lamellar phase.     

Sodium ion diffusion through liposome membranes containing cerebroside

Na⁺ efflux from liposomes (small unilamellar vesicles, SUV) of various compositions was studied, using ²²Na⁺ and ³H-labelled stachyose in simultaneous dual isotope measurements, stachyose being used as a measure of liposome disintegration. Dialysis was utilised to separate liposomes from extra-liposomal activity.Liposomes were made from egg lecithin and sphingomyelin and from mixtures of egg lecithin, sphingomyelin, cerebroside, sulphatide and cholesterol. All mixtures produced more leaky and less stable SUVs than pure lecithin and pure sphingomyelin. The incorporation of cerebroside is significantly smaller than that of the phospholipids including sphingomyelin. It was found that membranes containing cerebroside had a significantly higher Na⁺ permeability than membranes without cerebroside.     

Selective protein transport through a thin cellulose coated porous membrane

A new composite membrane was designed and studied for permselectivity of various molecular weight proteins. The membrane is composed of a porous substrate membrane [Durapore; poly(vinylidene fluoride)] coated with a thin dense layer of regenerated cellulose. This composite membrane was fabricated by spin coating a cellulose acetate solution onto the membrane, followed by alkaline hydrolysis of the cellulose acetate coating to regenerate cellulose. The coated layer was physically characterized by scanning electron microscopy (SEM) and infrared (IR) spectroscopy. In addition, the water uptake into and permeation properties of macromolecules across the coated and uncoated membranes were studied. A typical composite membrane coating was 0.8 +/- 0.2 mum thick, resulting in a molecular weight cutoff of approximately 40,000 daltons. This composite membrane also demonstrated negligible diffusional lag time for permeants, due to the diffusional barrier. (c) 1994 John Wiley & Sons, Inc.     

Passage of cell through membranes of Millipore diffusion chambers

Diffusion chambers assembled with Millipore filters previously soaked in water are penetrable by peritoneal exudate cells. Those constructed with dry Millipore filters of porosity 0.1 and 0.22 μm are not penetrable by such cells, but they become penetrable when pore size reaches 0.3 μm.     

Unconjugated bilirubin exhibits spontaneous diffusion through model lipid bilayers and native hepatocyte membranes

The liver is responsible for the clearance and metabolism of unconjugated bilirubin, the hydrophobic end-product of heme catabolism. Although several putative bilirubin transporters have been described, it has been alternatively proposed that bilirubin enters the hepatocyte by passive diffusion through the plasma membrane. In order to elucidate the mechanism of bilirubin uptake, we measured the rate of bilirubin transmembrane diffusion (flip-flop) using stopped-flow fluorescence techniques. Unconjugated bilirubin rapidly diffuses through model phosphatidylcholine vesicles, with a first-order rate constant of 5.3 s-1 (t(1)/(2) = 130 ms). The flip-flop rate is independent of membrane cholesterol content, phospholipid acyl saturation, and lipid packing, consistent with thermodynamic analyses demonstrating minimal steric constraint to bilirubin transmembrane diffusion. The coincident decrease in pH of the entrapped vesicle volume supports a mechanism whereby the bilirubin molecule crosses the lipid bilayer as the uncharged diacid. Transport of bilirubin by native rat hepatocyte membranes exhibits kinetics comparable with that in model vesicles, suggesting that unconjugated bilirubin crosses cellular membranes by passive diffusion through the hydrophobic lipid core. In contrast, there is no demonstrable flip-flop of bilirubin diglucuronide or bilirubin ditaurate in phospholipid vesicles, yet these compounds rapidly traverse isolated rat hepatocyte membranes, confirming the presence of a facilitated uptake system(s) for hydrophilic bilirubin conjugates.     

High conversion in asymmetric hydrolysis during permeation through enzyme-multilayered porous hollow-fiber membranes

We describe a novel porous hollow-fiber support for immobilizing aminoacylase in multilayers. Epoxy-group-containing polymer chains were grafted onto a porous hollow-fiber membrane by radiation-induced graft polymerization of glycidyl methacrylate, and subsequently a diethylamino group as an anion-exchange group was introduced into the graft chain. Aminoacylase was adsorbed in multilayers by allowing the amioacylase buffer solution to permeate through the pores across the hollow fiber; the graft chains provided three-dimensional space for the enzymes because of their electrostatic repulsion. The adsorbed enzyme at a degree of multilayer binding of 15 was cross-linked with glutaraldehyde to prevent leakage. An acetyl-DL-methionine solution was allowed to permeate through the pores surrounded by the aminoacylase-immobilized graft chain. Production of L-methionine was observed at a 4.1 mol/h per L of the fiber for a space velocity of 200 h(-1), defined as the flow rate of the effluent penetrating the outside surface of the hollow fiber divided by the membrane volume including the lumen.     

Excess electrical noise during current flow through porous membranes separating ionic solutions

Summary Spectral analysis of electrical noise from various artificial membrane systems suggests that excess noise of anf ^–n spectral form, wheren is approximately unity, is not primarily a bulk phenomenon simply dependent on the number of charge carriers. Measurements from aqueous and nonaqueous electrolytic resistors, comprised of several different ionic species, show only flat power density spectra under applied currents, even at extreme dilutions. Excess noise off ^–n form is observed under applied d-c current in single pore membranes, as previously reported, but is also seen in multipore and polymer mesh membranes. Calculations based on single pore membrane noise data are in significant variance with the bulk charge carrier model proposed by Hooge. These observations suggest that such excess noise occurs in conjunction with anisotropic constraints to ion flow.1 Fishman, H. M., Moore, L. E., Poussart, D. J. M. 1975. Potassium ion conduction noise in squid axon membrane.J. Membrane Biol. (Submitted for publication).     

Supplying CO2 to photosynthetic algal cultures by diffusion through gas-permeable membranes

Summary A method of supplying CO₂ to photosynthetic algal cultures by diffusion through a gas-permeable membrane was developed. The diffusion of CO₂ across a silicone membrane could be described by Fick's Laws of Diffusion, with a permeability constant of 1.92x10^-7 m²/min. By the manipulation of tubing dimensions and the partial pressure or static pressure of CO₂ gas within the tubing, the rate of CO₂ supply could be controlled. Pure CO₂ was applied to the cultivation of Chlorella pyrenoidosa with 100% utilization and without the side effect of CO₂ inhibition.