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Avaliação de técnicas de separação combinadas para a purificação de xilose visando a obtenção de bioprodutos; Evaluation of combined separation techniques for the xylose purification aiming a production of bioproducts

Magacho, Ana Luísa Ferreira
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 17/02/2009 PT
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O presente trabalho teve como objetivo avaliar o uso combinado de processos de separação, visando a adequação do substrato rico em xilose (hidrolisado de bagaço de cana) para a obtenção de produtos por via fermentativa. Foram estudados processos como coagulação e precipitação seletiva de impurezas coloidais, separação com membranas de microfiltração e ultrafiltração e resinas de troca iônica, tendo como ponto de partida o hidrolisado concentrado 5,56 vezes (hidrolisado H1). Na avaliação dos ensaios de coagulação e precipitação foi utilizado planejamento fatorial fracionado, o qual auxiliou o estudo da performance de agentes coagulantes (policloreto de alumínio e polieletrólito aniônico), em diferentes concentrações, pHs e temperaturas. Como variável resposta foi determinado a redução de compostos fenólicos, resultando numa diminuição final de 32,67% e num modelo matemático que representa os parâmetros envolvidos no processo:[C. Fenólicos] = 13,82 + 4,54xpH + 0,03xPAC - 0,58xpH2 + 0,19xPAC2 - 0,25xpHxPAC. Após a determinação das melhores condições experimentais desta etapa, aplicou-se este modelo numa escala 36 vezes maior, resultando em uma diminuição de 10,49% destes contaminantes, produzindo o hidrolisado H2. Este hidrolisado foi percolado por resinas...

Polímeros impressos com íons: fundamentos, estratégias de preparo e aplicações em química analítica

Marestoni, Luiz Diego; Sotomayor, Maria Del Pilar Taboada; Segatelli, Mariana Gava; Sartori, Lucas Rossi; Tarley, César Ricardo Teixeira
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: 1194-1207
POR
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Chemical imprinting technology has been widely used as a valuable tool in selective recognition of a given target analyte (molecule or metal ion), yielding a notable advance in the development of new analytical protocols. Since their discovery, molecularly imprinted polymers (MIPs) have been extensively studied with excellent reviews published. However, studies involving ion imprinted polymers (IIPs), in which metal ions are recognized in the presence of closely related inorganic ions, remain scarce. Thus, this review involved a survey of different synthetic approaches for preparing ion imprinted adsorbents and their application for the development of solid phase extraction methods, metal ion sensors (electrodes and optodes) and selective membranes.

Development of integrated separation processes with green solvents

Couto, Ricardo Miguel Tomás do
Fonte: Faculdade de Ciências e Tecnologia Publicador: Faculdade de Ciências e Tecnologia
Tipo: Tese de Doutorado
Publicado em //2012 ENG
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Dissertação para obtenção do Grau de Doutor em Química Sustentável; This thesis explores the fractionation of mixtures with unconventional solvents, like supercritical carbon dioxide (scCO2) and room temperature ionic liquids (RTILs), and develops new integrated processes which couple these solvents with membranes. It was explored the feasibility of using reverse osmosis membranes for fractionations in scCO2 with a model mixture of oleic acid and squalene, and it was found that all membranes tested were selective towards oleic acid. The effect of coupling the membrane to a supercritical fluid extraction (SFE) process was also tested, achieving a combined enrichment in squalene of 1.6 times. RTILs were used to fractionate the model mixture oleic acid and squalene. By appropriately choosing the anion and cation of the RTIL it was possible to model the fractionation. With [EMIM][MDEGSO4] a selectivity towards oleic acid of 2.96 was achieved, and with [BMIM][NTF2] the selectivity was 0.86. An indirect method of probing the polarities of RTILs is developed using common organic solvents and Reichardt’s dye and a polarity scale is built. It was found that the most polar RTILs have lower selectivities towards oleic acid. Supported ionic liquid membranes were developed and applied in the fractionation of model mixtures. These membranes had negligible selectivities for fractionating the model mixture of oleic acid and squalene...

Polímeros impressos com íons: fundamentos, estratégias de preparo e aplicações em química analítica

Marestoni,Luiz Diego; Sotomayor,Maria Del Pilar Taboada; Segatelli,Mariana Gava; Sartori,Lucas Rossi; Tarley,César Ricardo Teixeira
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2013 PT
Relevância na Pesquisa
27.86004%
Chemical imprinting technology has been widely used as a valuable tool in selective recognition of a given target analyte (molecule or metal ion), yielding a notable advance in the development of new analytical protocols. Since their discovery, molecularly imprinted polymers (MIPs) have been extensively studied with excellent reviews published. However, studies involving ion imprinted polymers (IIPs), in which metal ions are recognized in the presence of closely related inorganic ions, remain scarce. Thus, this review involved a survey of different synthetic approaches for preparing ion imprinted adsorbents and their application for the development of solid phase extraction methods, metal ion sensors (electrodes and optodes) and selective membranes.

Comparison of the ion channel characteristics of proapoptotic BAX and antiapoptotic BCL-2

Schlesinger, Paul H.; Gross, Atan; Yin, Xiao-Ming; Yamamoto, Kazuhito; Saito, Mitsuyoshi; Waksman, Gabriel; Korsmeyer, Stanley J.
Fonte: The National Academy of Sciences of the USA Publicador: The National Academy of Sciences of the USA
Tipo: Artigo de Revista Científica
Publicado em 14/10/1997 EN
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The BCL-2 family of proteins is composed of both pro- and antiapoptotic regulators, although its most critical biochemical functions remain uncertain. The structural similarity between the BCL-XL monomer and several ion-pore-forming bacterial toxins has prompted electrophysiologic studies. Both BAX and BCL-2 insert into KCl-loaded vesicles in a pH-dependent fashion and demonstrate macroscopic ion efflux. Release is maximum at ≈pH 4.0 for both proteins; however, BAX demonstrates a broader pH range of activity. Both purified proteins also insert into planar lipid bilayers at pH 4.0. Single-channel recordings revealed a minimal channel conductance for BAX of 22 pS that evolved to channel currents with at least three subconductance levels. The final, apparently stable BAX channel had a conductance of 0.731 nS at pH 4.0 that changed to 0.329 nS when shifted to pH 7.0 but remained mildly Cl− selective and predominantly open. When BAX-incorporated lipid vesicles were fused to planar lipid bilayers at pH 7.0, a Cl−-selective (PK/PCl = 0.3) 1.5-nS channel displaying mild inward rectification was noted. In contrast, BCL-2 formed mildly K+-selective (PK/PCl = 3.9) channels with a most prominent initial conductance of 80 pS that increased to 1.90 nS. Fusion of BCL-2-incorporated lipid vesicles into planar bilayers at pH 7.0 also revealed mild K+ selectivity (PK/PCl = 2.4) with a maximum conductance of 1.08 nS. BAX and BCL-2 each form channels in artificial membranes that have distinct characteristics including ion selectivity...

Distribution of chloride and potassium in cellular and luminal compartments of control and drug-treated turtle thyroid.

Chow, S Y; Woodbury, D M; Yen-Chow, Y C
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /06/1983 EN
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Chloride and potassium activities inside the lumen and the transepithelial potentials of control and drug-treated turtle thyroid follicles were determined simultaneously by ion-selective and conventional 3 M-KCl micro-electrodes, respectively. Water and electrolyte contents of these thyroid tissues were determined chemically after each experiment. Cellular and luminal concentrations of Cl- and K+ in control and drug-treated thyroid glands were derived from the data obtained. Both Cl- and K+ equilibrium potentials across the follicular cell membranes calculated from their concentration gradients are not identical to their corresponding membrane potentials measured directly. Thus, transport of both ions occurs. Thyrotrophin, SITS and ouabain increased both cellular and luminal Cl- concentrations. Acetazolamide increased the cellular but did not alter the concentrations. Furosemide and perchlorate markedly increased the cellular but decreased the luminal Cl- concentrations. There is a discrepancy in the intracellular K+ concentration between values derived from electrometric data as measured by ion-selective micro-electrodes and those calculated from the chemical analyses. The electrometrically determined intracellular K+ concentration is smaller. Intracellular Cl- concentrations derived from measurements with the Cl- ion-selective micro-electrodes were in the same range as those calculated from chemical analyses. Thus...

Reconstitution of ionic channels from inner and outer membranes of mammalian cardiac nuclei.

Rousseau, E; Michaud, C; Lefebvre, D; Proteau, S; Decrouy, A
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /02/1996 EN
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Recent reports suggest that the nuclear envelope possesses specific ion transport mechanisms that regulate the electrolyte concentrations within the nucleoplasm and perinuclear space. In this work, intact nuclei were isolated from sheep cardiac cells. After chromatin digestion, the nuclear envelopes were sonicated and four nuclear vesicle populations were separated by sucrose step gradients (SF1-SF4). These fractions were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their protein content was analyzed by Western blot, using lamin and SEC 61 antibodies. The lamins, which are associated with the inner nuclear membrane, were present in three fractions, SF2, SF3, and SF4, with a lower amount in SF2. The SEC 61 protein, a marker of the rough endoplasmic reticulum, was detected in small amounts in SF1 and SF2. Upon fusion of vesicles into bilayers, the activities of nuclear ionic channels were recorded in 50 mM trans/250 mM cis KCl or CsCl, pH 7.2. Two types of Cl- selective channels were recorded: a large conducting 150-180-pS channel displaying substates, and a low conducting channel of 30 pS. They were both spontaneously active into bilayers, and their open probability was poorly voltage dependent at negative voltages. Retinoic acid (10(-8) M) increases the po of the large Cl- conducting channel...

Ion channels from synaptic vesicle membrane fragments reconstituted into lipid bilayers.

Kelly, M L; Woodbury, D J
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /06/1996 EN
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27.90857%
Cholinergic synaptic vesicles were isolated from the electric organ of Torpedo californica. Vesicle membrane proteins were reconstituted into planar lipid bilayers by the nystatin/ergosterol fusion technique. After fusion, a variety of ion channels were observed. Here we identify four channels and describe two of them in detail. The two channels share a conductance of 13 pS. The first is anion selective and strongly voltage dependent, with a 50% open probability at membrane potentials of -15 mV. The second channel is slightly cation selective and voltage independent. It has a high open probability and a subconductance state. A third channel has a conductance of 4-7 pS, similar to the subconductance state of the second channel. This channel is fairly nonselective and has gating kinetics different from those of the cation channel. Finally, an approximately 10-pS, slightly cation selective channel was also observed. The data indicate that there are one or two copies of each of the above channels in every synaptic vesicle, for a total of six channels per vesicle. These observations confirm the existence of ion channels in synaptic vesicle membranes. It is hypothesized that these channels are involved in vesicle recycling and filling.

Distinct ion channel classes are expressed on the outer nuclear envelope of T- and B-lymphocyte cell lines.

Franco-Obregón, A; Wang, H W; Clapham, D E
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /07/2000 EN
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The outer nuclear membrane, endoplasmic reticulum, and mitochondrial membrane ion channels are poorly understood, although they are important in the control of compartmental calcium levels, cell division, and apoptosis. Few direct recordings of these ion channels have been made because of the difficulty of accessing these intracellular membranes. Using patch-clamp techniques on isolated nuclei, we measured distinct ion channel classes on the outer nuclear envelope of T-cell (human Jurkat) and BFL5 cell (murine promyelocyte) lines. We first imaged the nuclear envelopes of both Jurkat and FL5 cells with atomic force microscopy to determine the density of pore proteins. The nuclear pore complex was intact at roughly similar densities in both cell types. In patch-clamp recordings of Jurkat nuclear membranes, Cl channels (105 +/- 5 pS) predominated and inactivated with negative pipette potentials. Nucleotides transiently inhibited the anion channel. In contrast, FL5 nuclear channels were cation selective (52 +/- 2 pS), were inactivated with positive membrane potentials, and were insensitive to GTPgammaS applied to the bath. We hypothesize that T- and B-cell nuclear membrane channels are distinct, and that this is perhaps related to their unique roles in the immune system.

THE SIGNIFICANCE OF THE STRUCTURE OF A MEMBRANE FOR ITS SELECTIVE PERMEABILITY

Wilbrandt, W.
Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 20/07/1935 EN
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The attempt is made to reconcile the two existing theories on the mechanism of selective ion permeability of the cell membrane by taking into account the molecular arrangement in the substance of the membrane. It is shown that the pore theory and the solubility theory are not contradictory, but two aspects of the same problem, one from the thermodynamic point of view, the other from the point of view of molecular theory. The dried collodion membrane is used as a model in these studies. Its different behavior towards anions and cations is explained on the ground of a quasi-crystalline structure of collodion, the NO3-groups acting as dipoles with the negative charge directed towards the intermolecular spaces, no matter whether these pores are of molecular dimensions or larger. In this way a continuity in the behavior of the ordinary large pored collodion membrane and the dried membrane is established, both theoretically and experimentally. Experiments, with membranes of other cellulose derivatives agree with the mechanism suggested. Membranes of cellophane and ethyl cellulose are negatively charged, membranes of cellulose acetate positively. From solutions of collodion mixed with basic dyestuffs or alkaloids membranes can be obtained more permeable to anions than to cations in contrast to the ordinary collodion membrane. Membranes can be built which give high potential differences even between two identical electrolyte solutions. The asymmetry lies here within the membrane and is artificially produced by gluing together an ordinary collodion membrane with one previously impregnated with a basic dyestuff or an alkaloid.

X-ray crystallographic and mass spectrometric structure determination and functional characterization of succinylated porin from Rhodobacter capsulatus: implications for ion selectivity and single-channel conductance.

Przybylski, M.; Glocker, M. O.; Nestel, U.; Schnaible, V.; Blüggel, M.; Diederichs, K.; Weckesser, J.; Schad, M.; Schmid, A.; Welte, W.; Benz, R.
Fonte: Cold Spring Harbor Laboratory Press Publicador: Cold Spring Harbor Laboratory Press
Tipo: Artigo de Revista Científica
Publicado em /08/1996 EN
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The role of charges near the pore mouth has been discussed in theoretical work about ion channels. To introduce new negative charges in a channel protein, amino groups of porin from Rhodobacter capsulatus 37b4 were succinylated with succinic anhydride, and the precise extent and sites of succinylations and structures of the succinylporins determined by mass spectrometry and X-ray crystallography. Molecular weight and peptide mapping analyses using matrix-assisted laser desorption-ionization mass spectrometry identified selective succinylation of three lysine-epsilon-amino groups (Lys-46, Lys-298, Lys-300) and the N-terminal alpha-amino group. The structure of a tetra-succinylated porin (TS-porin) was determined to 2.4 A and was generally found unchanged in comparison to native porin to form a trimeric complex. All succinylated amino groups found in a mono/di-succinylated porin (MS-porin) and a TS-porin are localized at the inner channel surface and are solvent-accessible: Lys-46 is located at the channel constriction site, whereas Lys-298, Lys-300, and the N-terminus are all near the periplasmic entrance of the channel. The Lys-46 residue at the central constriction loop was modeled as succinyl-lysine from the electron density data and shown to bend toward the periplasmic pore mouth. The electrical properties of the MS-and TS-porins were determined by reconstitution into black lipid membranes...

The Ion Permeability Induced in Thin Lipid Membranes by the Polyene Antibiotics Nystatin and Amphotericin B

Cass, Albert; Finkelstein, Alan; Krespi, Vivian
Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 01/07/1970 EN
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Characteristics of nystatin and amphotericin B action on thin (<100 A) lipid membranes are: (a) micromolar amounts increase membrane conductance from 10-8 to over 10-2 Ω-1 cm-2; (b) such membranes are (non-ideally) anion selective and discriminate among anions on the basis of size; (c) membrane sterol is required for action; (d) antibiotic presence on both sides of membrane strongly favors action; (e) conductance is proportional to a large power of antibiotic concentration; (f) conductance decreases ∼104 times for a 10°C temperature rise; (g) kinetics of antibiotic action are also very temperature sensitive; (h) ion selectivity is pH independent between 3 and 10, but (i) activity is reversibly lost at high pH; (j) methyl ester derivatives are fully active; N-acetyl and N-succinyl derivatives are inactive; (k) current-voltage characteristic is nonlinear when membrane separates nonidentical salt solutions. These characteristics are contrasted with those of valinomycin. Observations (a)–(g) suggest that aggregates of polyene and sterol from opposite sides of the membrane interact to create aqueous pores; these pores are not static, but break up (melt) and reform continuously. Mechanism of anion selectivity is obscure. Observations (h)–(j) suggest—NH3+ is important for activity; it is probably not responsible for selectivity...

Voltage-dependent block of anthrax toxin channels in planar phospholipid bilayer membranes by symmetric tetraalkylammonium ions. Effects on macroscopic conductance

Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 01/11/1990 EN
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In a recent paper (Blaustein, R. O., T. M. Koehler, R. J. Collier, and A. Finkelstein, 1989. Proc. Natl. Acad. Sci. USA. 86:2209-2213) we described the general channel-forming properties of the PA65 fragment of anthrax toxin in planar phospholipid bilayer membranes. In the present paper we extend our previous studies of the permeability properties of this channel, using a series of symmetric tetraalkylammonium (TAA) ions. Our main finding is that at micromolar concentrations on either the cis (toxin-containing) or trans side of a membrane containing many (greater than 1,000) channels, these ions, ranging in size from tetramethylammonium to tetrahexylammonium, induce a voltage-dependent reduction of membrane conductance. (We attribute a similar voltage-dependent reduction of membrane conductance by millimolar concentrations of HEPES to a cationic form of this buffer present at micromolar concentrations.) In going from large negative to large positive voltages (on the TAA side) one sees that the conductance first decreases from its value in the absence of TAA, reaches a minimum, and then rises back at larger positive voltages toward the level in the absence of TAA. Our interpretation of this behavior is that these symmetric TAA ions block the cation-selective PA65 channel in a voltage-dependent manner. We postulate that there is a single site within the channel to which TAA ions can bind and thereby block the passage of the major current-carrying ion (potassium). A blocking ion is driven into the site by modest positive voltages...

Detection of Proton Movement Directly across Viral Membranes To Identify Novel Influenza Virus M2 Inhibitors

Sulli, Chidananda; Banik, Soma S. R.; Schilling, Justin; Moser, Allan; Xiang, Xiaoxiao; Payne, Riley; Wanless, Antony; Willis, Sharon H.; Paes, Cheryl; Rucker, Joseph B.; Doranz, Benjamin J.
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /10/2013 EN
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28.04384%
The influenza virus M2 protein is a well-validated yet underexploited proton-selective ion channel essential for influenza virus infectivity. Because M2 is a toxic viral ion channel, existing M2 inhibitors have been discovered through live virus inhibition or medicinal chemistry rather than M2-targeted high-throughput screening (HTS), and direct measurement of its activity has been limited to live cells or reconstituted lipid bilayers. Here, we describe a cell-free ion channel assay in which M2 ion channels are incorporated into virus-like particles (VLPs) and proton conductance is measured directly across the viral lipid bilayer, detecting changes in membrane potential, ion permeability, and ion channel function. Using this approach in high-throughput screening of over 100,000 compounds, we identified 19 M2-specific inhibitors, including two novel chemical scaffolds that inhibit both M2 function and influenza virus infectivity. Counterscreening for nonspecific disruption of viral bilayer ion permeability also identified a broad-spectrum antiviral compound that acts by disrupting the integrity of the viral membrane. In addition to its application to M2 and potentially other ion channels, this technology enables direct measurement of the electrochemical and biophysical characteristics of viral membranes.

Different Permeability of Potassium Salts across the Blood-Brain Barrier Follows the Hofmeister Series

Breschi, Gian Luca; Cametti, Massimo; Mastropietro, Alfonso; Librizzi, Laura; Baselli, Giuseppe; Resnati, Giuseppe; Metrangolo, Pierangelo; de Curtis, Marco
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Publicado em 28/10/2013 EN
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The passage of ions across biological membranes is regulated by passive and active mechanisms. Passive ion diffusion into organs depends on the ion-pairing properties of salts present in the serum. Potassium ions could affect brain activity by crossing the blood-brain barrier (BBB) and its accumulation in the extracellular cerebral space could precipitate seizures. In the present study, we analyze passive diffusion of a series of potassium salts in the in vitro isolated guinea pig brain preparation. Different potassium counter-anions confer ion-pairing and lipophilicity properties that modulate membrane diffusion of the salt. Extracellular recordings in different cortical areas demonstrated the presence of epileptiform activities that strongly relate to anion identity, following the qualitative order of the Hofmeister series. Indeed, highly lipophilic salts that easily cross the BBB enhanced extracellular potassium concentration measured by ion-selective electrodes and were the most effective pro-epileptic species. This study constitutes a novel contribution for the understanding of the potential epileptogenicity of potassium salts and, more generally, of the role of counter-anions in the passive passage of salts through biological membranes.

Nanostructured silicon membranes for control of molecular transport

Srijanto, Bernadeta R.; Retterer, Scott T.; Fowlkes, Jason D.; Doktycz, Mitchel J.
Fonte: American Vacuum Society Publicador: American Vacuum Society
Tipo: Artigo de Revista Científica
EN
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A membrane that allows selective transport of molecular species requires precise engineering on the nanoscale. Membrane permeability can be tuned by controlling the physical structure and surface chemistry of the pores. Here, a combination of electron beam and optical lithography, along with cryogenic deep reactive ion etching, has been used to fabricate silicon membranes that are physically robust, have uniform pore sizes, and are directly integrated into a microfluidic network. Additional reductions in pore size were achieved using plasma enhanced chemical vapor deposition and atomic layer deposition of silicon dioxide to coat membrane surfaces. Cross sectioning of the membranes using focused ion beam milling was used to determine the physical shape of the membrane pores before and after coating. Functional characterization of the membranes was performed by using quantitative fluorescence microscopy to document the transport of molecular species across the membrane.

CO₂ and ion transport via plant aquaporins.

Zhao, Manchun
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //2013
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28.046196%
CO₂ diffusion across membranes is one of the rate limiting steps during photosynthesis, therefore understanding the process of CO₂ permeation across membranes is important. The question of whether CO₂ transport across membranes can be facilitated by aquaporins is very controversial. Previous research where aquaporins were heterologously expressed in either Xenopus oocytes or yeast protoplasts showed that some plasma membrane intrinsic proteins (PIPs) or animal aquaporins could facilitate CO₂ transport. However, others have demonstrated using molecular simulation approaches and biophysical calculations that the unstirred layer poses the major rate limiting step for CO₂ diffusion across membranes, and that it is unlikely that CO₂ permeates via the water pathway in aquaporins, because this pathway exhibits a greater energy barrier compared to that for the lipid bilayer. If water and CO₂ share the same pathway through aquaporins or if the presence and activity of aquaporins somehow affects CO₂ permeation, there should be a correlation between water permeability and CO₂ permeability. Therefore, by employing the stopped-flow technique and using pea plasma membrane vesicles isolated from pea leaves, this thesis explored the links between CO₂ permeability and water permeability. Plasma membrane vesicles from pea plants that were grown in different conditions showed considerable variability in water permeability. The very high and variable (between preparations) water permeability (0.06 to 0.18 m s⁻¹) plus the low activation energy (10.8 KJ mol⁻¹) of water transport indicated aquaporins dominated water flow...

Relationship of gangliosides to the structure and function of thyrotropin receptors: their absence on plasma membranes of a thyroid tumor defective in thyrotropin receptor activity.

Fishman, P H; Aloj, S M; Kohn, L D; Brady, R O
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /11/1976 EN
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Plasma membranes derived from rat thyroid tumor (1-8R) which is unresponsive to thyrotropin but is responsive to dibutyryl adenosine 3':5'-cyclic monophosphate bind less than 20% of the [125I] thyrotropin which can be bound to plasma membranes from normal rat thyroids under conditions which optimize tumor membrane binding relative to normal thyroid membranes. In addition, the binding is different from thyrotropin binding to normal thyroid membranes both in its altered sensitivity to changes in hydrogen ion concentration and in a decreased sensitivity to competition by unlabeled thyrotropin. This reduced capacity to bind [125I] thyrotropin cannot be attributed to degradation of the hormone by membrane-associated proteases. Although the supernatant phase of the thyroid tumor homogenates contains a soluble component which inhibits [125I] thyrotropin binding to thyrotropin receptors on plasma membranes, its level is the same as in homogenates of normal thyroid tissue. Trypsin digestion does not expose thyrotropin receptors in a manner analogous to that seen in normal thyroid tissue. The major ganglioside in the tumor membranes is N-acetylneuraminylgalactosylglucosylceramide and the membranes lack the N-acetylgalactosaminyltransferase required for the synthesis of more complex gangliosides. In contrast...

Thermodynamics of Ion Solvation and Differential Adsorption at Liquid-Liquid Interfaces and Membranes

Kung, William; Solis, Francisco J.; de la Cruz, Monica Olvera
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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We construct a mean-field formulation of the thermodynamics of ion solvation in immiscible polar binary mixtures. Assuming an equilibrium planar interface separating two semi-infinite regions of different constant dielectric medium, we study the electrostatic phenomenon of differential adsorption of ions at the interface. Using general thermodynamic considerations, we construct the mean-field $\Omega$-potential and demonstrate the spontaneous formation of an electric double-layer around the interface necessarily follow. In our framework, we can also relate both the bulk ion densities in the two phases and the distribution potential across the interface to the fundamental Born free energy of ion polarization. We further illustrate this selective ion adsorption phenomenon in respective examples of fully permeable membranes that are neutral, negative, or positive in charge polarity.; Comment: 8 pages, 4 figures, RevTeX, submitted

Ion Channels, Natural Nanovalves

Eisenberg, Bob
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/06/2012
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28.019514%
Ion channels are proteins with holes down their middle that control the flow of ions and electric current across otherwise impermeable biological membranes. The flow of sodium, potassium, calcium (divalent), and chloride ions have been central issues in biology for more than a century. The flow of current is responsible for the signals of the nervous system that propagate over long distances (meters). The concentration of divalent calcium ions is a 'universal' signal that controls many different systems inside cells. The concentration of divalent calcium and other messenger ions has a role in life rather like the role of the voltage in different wires of a computer. Ion channels also help much larger solutes (e.g., organic acid and bases; perhaps polypeptides) to cross membranes but much less is known about these systems. Ion channels can select and control the movement of different types of ions because the holes in channel proteins are a few times larger than the (crystal radii of the) ions themselves. Biology uses ion channels as selective valves to control flow and thus concentration of crucial chemical signals. For example, the concentration of divalent calcium ions determines whether muscles contract or not. Ion channels have a role in biology similar to the role of transistors in computers and technology. Ion Channels Control Concentrations Important To Life The Way Computers Control Voltages Important To Computers.; Comment: as prepared for the Encyclopedia of Applied Electrochemistry (Springer)