Página 1 dos resultados de 5816 itens digitais encontrados em 0.021 segundos

Ion transport through homogeneous and heterogeneous ion-exchange membranes in single salt and multicomponent electrolyte solutions

Marti-Calatayud, M. C.; Buzzi, Daniella Cardoso; Garcia-Gabaldón, M; Bernardes, A. M.; Tenorio, Jorge Alberto Soares; Pérez-Herranz, V.
Fonte: Amsterdam Publicador: Amsterdam
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
66.16%
The increasing demand for clean industrial processes has intensified the use of electrodialysis in the treatment of metal containing effluents and encourages the investigation of the different phenomena involved in the transport of metal ions through cation-exchange membranes. Ion sorption, chronopotentiometric and current–voltage characteristics have been obtained to characterize the transport of sodium and iron through homogeneous and heterogeneous cation-exchange membranes. The heterogeneous membranes having a broader pore size distribution showed increased electrical resistances with solutions of trivalent iron, which may be caused by the blockage of the smallest pores by multivalent ions. However, for both types of membranes an unexpected decrease of the electrical resistance with increasing current densities was verified with concentrated solutions of Fe2(SO4)3. This behavior was explained to be a consequence of the dissociation of FeSO4+FeSO4+ ions into more conductive Fe3+ and SO42−SO42− ions as the depleting solution layer becomes diluted. When tested with multicomponent mixtures, the homogeneous perfluorosulfonic membranes show an increased preference for Na+ ions at low current densities and, once Na+ ions are depleted from the membrane surface Fe3+ ions are transported preferentially at higher current densities. On the contrary...

Simulação da dinâmica do íon potássio pelo modelo HYDRUS-1D em condições de solo salino; Dynamic simulations of potassium transport under saline soil conditions using HYDRUS-1D

Santos, Rafaelly Suzanye da Silva
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 18/11/2014 PT
Relevância na Pesquisa
46.26%
A demanda alimentar vem aumentando de acordo com o crescimento populacional e com isso há uma necessidade de que as práticas agrícolas tornem-se cada vez mais intensas e tecnificadas, mediante o incremento, entre outros, da utilização de insumos químicos. Porém, uma vez aplicados de maneira desorganizada, sem a devida preocupação com os possíveis impactos aos recursos naturais, tais produtos podem vir a provocar danos ao solo, contaminação de águas subterrâneas (mediante sua lixiviação) e em alguns casos, podem ser responsáveis pelo aumento da salinidade em alguns solos da região do semiárido do nordeste brasileiro. Nesse sentido, devido ao fato de envolverem processos físicos na natureza, o estudo da dinâmica desses produtos químicos têm motivado diversos pesquisadores a aplicarem ferramentas matemáticas (modelos matemáticos) na área de engenharia de água e solo, buscando entender a correlação entre a dinâmica da água e o movimento de solutos no perfil do solo. Portanto, a presente pesquisa tem como objetivo a aplicação do modelo matemático HYDRUS-1D para a simulação do movimento da água e do íon potássio, em condições de laboratório, utilizando-se colunas preenchidas com dois tipos de material de solos salinos e não saturados: Argissolo Amarelo (S1) e Latossolo Vermelho Amarelo (S2). Além disso...

(In)validity of the constant field and constant currents assumptions in theories of ion transport.

Syganow, A; von Kitzing, E
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /02/1999 EN
Relevância na Pesquisa
46.21%
Constant electric fields and constant ion currents are often considered in theories of ion transport. Therefore, it is important to understand the validity of these helpful concepts. The constant field assumption requires that the charge density of permeant ions and flexible polar groups is virtually voltage independent. We present analytic relations that indicate the conditions under which the constant field approximation applies. Barrier models are frequently fitted to experimental current-voltage curves to describe ion transport. These models are based on three fundamental characteristics: a constant electric field, negligible concerted motions of ions inside the channel (an ion can enter only an empty site), and concentration-independent energy profiles. An analysis of those fundamental assumptions of barrier models shows that those approximations require large barriers because the electrostatic interaction is strong and has a long range. In the constant currents assumption, the current of each permeating ion species is considered to be constant throughout the channel; thus ion pairing is explicitly ignored. In inhomogeneous steady-state systems, the association rate constant determines the strength of ion pairing. Among permeable ions...

Properties of the stochastic energization-relaxation channel model for vectorial ion transport.

Muneyuki, E; Fukami, T A
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /03/2000 EN
Relevância na Pesquisa
46.27%
A model for the primary active transport by an ion pump protein is proposed. The model, the "energization-relaxation channel model," describes an ion pump as a multiion channel that undergoes stochastic transitions between two conformational states by external energy supply. When the potential profile along ion transport pathway is asymmetrical, a net ion flux is induced by the transitions. In this model, the coupling of the conformational change and ion transport is stochastic and loose. The model qualitatively reproduces known properties of active transport such as the effect of ion concentration gradient and membrane potential on the rate of transport and the inhibition of ion transport at high ion concentration. We further examined the effect of various parameters on the ion transport properties of this model. The efficiency of the coupling was almost 100% under some conditions.

Modification of ion transport in lipid bilayer membranes in the presence of 2,4-dichlorophenoxyacetic acid. I. Enhancement of cationic conductance and changes of the kinetics of nonactin-mediated transport of potassium.

Smejtek, P; Paulis-Illangasekare, M
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /06/1979 EN
Relevância na Pesquisa
46.21%
We have found that herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) has the ability to increase the rate of transport of positive ions of several kinds, and to inhibit transport of negatively charged tetraphenylborate ions in lipid bilayer membranes. It has been found that only the neutral form of 2,4-D is transport active, whereas the ionized from of 2,4-D does not modify transport of ions, and does not by itself permeate through lipid membranes. The results suggest that the enhancement of transport of positively charged ions such as tetraphenylarsonium + and nonactin-K+ is dominated by the increase of the ion translocation rate constant. It has been shown that the enhancement of nonactin-mediated transport of K+ by 2,4-D can be accounted for by a simple carrier model. We have observed that a 2,4-D concentration above 3 X 10(-4) M the potassium ion transport in phosphatidylcholine-cholesterol as well as in cholesterol-free glycerolmonooleate membranes is enhanced to such a degree that, depending upon the concentration of potassium ions, it becomes limited by the rate of recombination of K+ with nonactin, and/or by backdiffusion of unloaded nonactin molecules. Furthermore, the effect of 2,4-D is enhanced by ionic strength of aqueous solution. From the changes of kinetic parameters of nonactin-K+ transport...

Changes in ion transport in inflammatory disease

Eisenhut, Michael
Fonte: BioMed Central Publicador: BioMed Central
Tipo: Artigo de Revista Científica
Publicado em 29/03/2006 EN
Relevância na Pesquisa
46.28%
Ion transport is essential for maintenance of transmembranous and transcellular electric potential, fluid transport and cellular volume. Disturbance of ion transport has been associated with cellular dysfunction, intra and extracellular edema and abnormalities of epithelial surface liquid volume. There is increasing evidence that conditions characterized by an intense local or systemic inflammatory response are associated with abnormal ion transport. This abnormal ion transport has been involved in the pathogenesis of conditions like hypovolemia due to fluid losses, hyponatremia and hypokalemia in diarrhoeal diseases, electrolyte abnormalites in pyelonephritis of early infancy, septicemia induced pulmonary edema, and in hypersecretion and edema induced by inflammatory reactions of the mucosa of the upper respiratory tract. Components of membranous ion transport systems, which have been shown to undergo a change in function during an inflammatory response include the sodium potassium ATPase, the epithelial sodium channel, the Cystic Fibrosis Transmembrane Conductance Regulator and calcium activated chloride channels and the sodium potassium chloride co-transporter. Inflammatory mediators, which influence ion transport are tumor necrosis factor...

Cholinergic regulation of epithelial ion transport in the mammalian intestine

Hirota, C L; McKay, D M
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
46.3%
Acetylcholine (ACh) is critical in controlling epithelial ion transport and hence water movements for gut hydration. Here we review the mechanism of cholinergic control of epithelial ion transport across the mammalian intestine. The cholinergic nervous system affects basal ion flux and can evoke increased active ion transport events. Most studies rely on measuring increases in short-circuit current (ISC = active ion transport) evoked by adding ACh or cholinomimetics to intestinal tissue mounted in Ussing chambers. Despite subtle species and gut regional differences, most data indicate that, under normal circumstances, the effect of ACh on intestinal ion transport is mainly an increase in Cl- secretion due to interaction with epithelial M3 muscarinic ACh receptors (mAChRs) and, to a lesser extent, neuronal M1 mAChRs; however, AChR pharmacology has been plagued by a lack of good receptor subtype-selective compounds. Mice lacking M3 mAChRs display intact cholinergically-mediated intestinal ion transport, suggesting a possible compensatory mechanism. Inflamed tissues often display perturbations in the enteric cholinergic system and reduced intestinal ion transport responses to cholinomimetics. The mechanism(s) underlying this hyporesponsiveness are not fully defined. Inflammation-evoked loss of mAChR-mediated control of epithelial ion transport in the mouse reveals a role for neuronal nicotinic AChRs...

EFFECTS OF METABOLIC INHIBITORS AND DRUGS ON ION TRANSPORT AND OXYGEN CONSUMPTION IN ISOLATED FROG SKIN

Huf, Ernst G.; Doss, Norma S.; Wills, Joyce P.
Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 20/11/1957 EN
Relevância na Pesquisa
46.28%
Active ion (NaCl) transport across isolated frog skin is discussed in relation to sodium and potassium composition and to O2 consumption of skin. A distinction is made between processes in skin related to "unidirectional active ion transport" and processes related to "maintenance electrolyte equilibrium;" i.e., ionic composition of skin. Several metabolic inhibitors were found that could be used in separating maintenance electrolyte equilibrium from unidirectional active ion transport. Fluoroacetate (up to 1 x 10–2M/liter) did not affect maintenance electrolyte equilibrium, but severely diminished the rate of active ion transport. This could also be accomplished with azide and diethyl malonate when 1 x 10–3 molar concentrations were used. When applied in higher concentrations, these two inhibitors, and several others, diminished active ion transport, but this was associated with changes in maintenance electrolyte equilibrium (gain of Na+ by and loss of K+ from skin). Similar observations were made when skins were subjected to K+-deficient media. Mersalyl and theophylline, in low concentrations, stimulated active ion transport without leading to changes in maintenance electrolyte equilibrium. Inhibition of active ion transport was found accompanied by decrease...

Ion Transport through Monolayers and Interfacial Films

Miller, I. R.
Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 01/07/1968 EN
Relevância na Pesquisa
56.11%
Ion transport through monolayers and through several molecules of thick films at the mercury/water interface is discussed. The permeability of the monolayer is described by a rate constant, kc. The permeability of a thin but not monomolecular film is expressed as a function of the thickness of the film, the diffusion coefficient of the permeant in the film, and the distribution coefficient between the film and the bulk of the solution. The rate constant kc is expressed in terms of absolute rate processes. In the absence of specific interactions, the activation energy is composed of three terms: (a) electrostatic interaction between the permeating ion and the charged monolayer, (b) monolayer compression work of forming a hole for passage of the ions, and (c) energy of boundary line formation between the monolayer and the hole. The contribution of the third term is especially marked in condensed monolayers. Ions are bound weakly to the monolayers of the dipolar ion lecithin, which complicates the transport problem in this system. The retardation of oxygen reduction by the lecithin monolayer is of particular interest.

Sugar-activated ion transport in canine lingual epithelium. Implications for sugar taste transduction

Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 01/07/1988 EN
Relevância na Pesquisa
46.22%
There is good evidence indicating that ion-transport pathways in the apical regions of lingual epithelial cells, including taste bud cells, may play a role in salt taste reception. In this article, we present evidence that, in the case of the dog, there also exists a sugar- activated ion-transport pathway that is linked to sugar taste transduction. Evidence was drawn from two parallel lines of experiments: (a) ion-transport studies on the isolated canine lingual epithelium, and (b) recordings from the canine chorda tympani. The results in vitro showed that both mono- and disaccharides in the mucosal bath stimulate a dose-dependent increase in the short-circuit current over the concentration range coincident with mammalian sugar taste responses. Transepithelial current evoked by glucose, fructose, or sucrose in either 30 mM NaCl or in Krebs-Henseleit buffer (K-H) was partially blocked by amiloride. Among current carriers activated by saccharides, the current response was greater with Na than with K. Ion flux measurements in K-H during stimulation with 3-O-methylglucose showed that the sugar-evoked current was due to an increase in the Na influx. Ouabain or amiloride reduced the sugar-evoked Na influx without effect on sugar transport as measured with tritiated 3-O-methylglucose. Amiloride inhibited the canine chorda tympani response to 0.5 M NaCl by 70-80% and the response to 0.5 M KCl by approximately 40%. This agreed with the percent inhibition by amiloride of the short-circuit current supported in vitro by NaCl and KCl. Amiloride also partially inhibited the chorda tympani responses to sucrose and to fructose. The results indicate that in the dog: (a) the ion transporter subserving Na taste also subserves part of the response to K...

The Relative Roles of Passive Surface Forces and Active Ion Transport in the Modulation of Airway Surface Liquid Volume and Composition

Tarran, Robert; Grubb, Barbara R.; Gatzy, John T.; Davis, C. William; Boucher, Richard C.
Fonte: The Rockefeller University Press Publicador: The Rockefeller University Press
Tipo: Artigo de Revista Científica
Publicado em 01/08/2001 EN
Relevância na Pesquisa
46.22%
Two hypotheses have been proposed recently that offer different views on the role of airway surface liquid (ASL) in lung defense. The “compositional” hypothesis predicts that ASL [NaCl] is kept low (<50 mM) by passive forces to permit antimicrobial factors to act as a chemical defense. The “volume” hypothesis predicts that ASL volume (height) is regulated isotonically by active ion transport to maintain efficient mechanical mucus clearance as the primary form of lung defense. To compare these hypotheses, we searched for roles for: (1) passive forces (surface tension, ciliary tip capillarity, Donnan, and nonionic osmolytes) in the regulation of ASL composition; and (2) active ion transport in ASL volume regulation. In primary human tracheobronchial cultures, we found no evidence that a low [NaCl] ASL could be produced by passive forces, or that nonionic osmolytes contributed substantially to ASL osmolality. Instead, we found that active ion transport regulated ASL volume (height), and that feedback existed between the ASL and airway epithelia to govern the rate of ion transport and volume absorption. The mucus layer acted as a “reservoir” to buffer periciliary liquid layer height (7 μm) at a level optimal for mucus transport by donating or accepting liquid to or from the periciliary liquid layer...

Ion transport across CF and normal murine olfactory and ciliated epithelium

Grubb, B. R.; Rogers, T. D.; Boucher, R. C.; Ostrowski, L. E.
Fonte: American Physiological Society Publicador: American Physiological Society
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
56.03%
The nasal epithelium of the cystic fibrosis (CF) mouse has been used extensively in CF research because it exhibits ion transport defects similar to those of human CF airways. This tissue is composed of ∼50% olfactory (OE) and ∼50% ciliated epithelium (CE), and on the basis of previous observations, we hypothesized that a significant fraction of the bioelectric signals from murine nasal tissue may arise from OE rather than CE, while CE is the target tissue for CF gene therapy. We compared the bioelectric properties of isolated OE from the nasal cavity and CE from the nasopharynx in Ussing chamber studies. Hyperabsorption of Na+ [amiloride response; CF vs. wild type (WT)] was ∼7.5-fold greater in the OE compared with the CE. The forskolin response in native tissues did not reliably distinguish genotypes, likely due to a cyclic nucleotide-gated cation conductance in OE and a calcium-mediated Cl− conductance in CE. By potential difference assay, hyperabsorption of Na+ (CF vs. WT) and the difference in response to apical 0 Cl− buffer (CF vs. WT) were ∼2-fold greater in the nasal cavity compared with the nasopharynx. Our studies demonstrate that in the CF mouse, both the hyperabsorption of Na+ and the Cl− transport defect are of larger magnitude in the OE than in the CE. Thus...

MOLECULAR DYNAMICS SIMULATION STUDIES OF ION TRANSPORT ALONG G-QUADRUPLEX DNA CHANNELS

Akhshi, PARISA
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN; EN
Relevância na Pesquisa
56.12%
Guanine-rich DNA and RNA sequences can fold, in the presence of alkali metal ions such as Na+ and K+, into G-quadruplex structures. These alkali metal ions are necessary for the stabilization of G-quadruplex structures. However, little is known about the ion dynamics in G-quadruplex structures. In this thesis, we used molecular dynamics (MD) simulations to study the energetics of ion transport in G-quadruplex DNA channels. In particular, we applied, for the first time, adaptive biasing force (ABF) and umbrella sampling (US) methods to obtain potential of mean force (PMF) profiles for Na+, K+, and NH4+ ion movement along [d(TG4T)]4 and [d(G3T4G4)]2 channels. We found that the ABF and US methods produce very similar PMF profiles, in qualitative agreement with the very limited experimental data in the literature. We found that, within a G-quadruplex channel, K+ and NH4+ ions experience significant energy barriers (13-17 kcal/mol) to cross a G-quartet, whereas the Na+ movement encounters minimal resistance (5-7 kcal/mol). All ions are nearly fully dehydrated inside the channel but quickly become hydrated after exiting the channel. Our simulations suggested that the free energy landscapes for ion movement between the channel exit points and bulk solution are quite flat (ca. 2-4 kcal/mol) regardless of the loop topology in the region. We discovered that the directional symmetry of the ion movement within any G-quadruplex channel depends critically on both the DNA sequence and the folding of the G-quadrupelx structure. While the ion movement inside the [d(TG4T)]4 channel shows the same free energy barrier in either direction...

Salinity tolerance in plants. Quantitative approach to ion transport starting from halophytes and stepping to genetic and protein engineering for manipulating ion fluxes

Volkov, Vadim
Fonte: Frontiers Media S.A. Publicador: Frontiers Media S.A.
Tipo: Artigo de Revista Científica
Publicado em 27/10/2015 EN
Relevância na Pesquisa
46.36%
Ion transport is the fundamental factor determining salinity tolerance in plants. The Review starts from differences in ion transport between salt tolerant halophytes and salt-sensitive plants with an emphasis on transport of potassium and sodium via plasma membranes. The comparison provides introductory information for increasing salinity tolerance. Effects of salt stress on ion transport properties of membranes show huge opportunities for manipulating ion fluxes. Further steps require knowledge about mechanisms of ion transport and individual genes of ion transport proteins. Initially, the Review describes methods to measure ion fluxes, the independent set of techniques ensures robust and reliable basement for quantitative approach. The Review briefly summarizes current data concerning Na+ and K+ concentrations in cells, refers to primary thermodynamics of ion transport and gives special attention to individual ion channels and transporters. Simplified scheme of a plant cell with known transport systems at the plasma membrane and tonoplast helps to imagine the complexity of ion transport and allows choosing specific transporters for modulating ion transport. The complexity is enhanced by the influence of cell size and cell wall on ion transport. Special attention is given to ion transporters and to potassium and sodium transport by HKT...

Electro- and Magneto-Modulated Ion Transport through Graphene Oxide Membranes

Sun, Pengzhan; Zheng, Feng; Wang, Kunlin; Zhong, Minlin; Wu, Dehai; Zhu, Hongwei
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
Publicado em 28/10/2014 EN
Relevância na Pesquisa
46.24%
The control of ion trans-membrane transport through graphene oxide (GO) membranes is achieved by electric and magnetic fields. Electric field can either increase or decrease the ion transport through GO membranes depending on its direction, and magnetic field can enhance the ion penetration monotonically. When electric field is applied across GO membrane, excellent control of ion fluidic flows can be done. With the magnetic field, the effective anchoring of ions is demonstrated but the modulation of the ion flowing directions does not occur. The mechanism of the electro- and magneto-modulated ion trans-membrane transport is investigated, indicating that the electric fields dominate the ion migration process while the magnetic fields tune the structure of nanocapillaries within GO membranes. Results also show that the ion selectivity of GO membranes can be tuned with the electric fields while the transport of ions can be enhanced synchronously with the magnetic fields. These excellent properties make GO membranes promising in areas such as field-induced mass transport control and membrane separation.

Estudios experimentales y teóricos del transporte iónico en electrodeposición en celdas delgadas; Experimental and theoretical studies of ion transport in thin-layer electrodeposition cells

Dengra, Silvina
Fonte: Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires Publicador: Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires
Tipo: info:eu-repo/semantics/doctoralThesis; tesis doctoral; info:eu-repo/semantics/publishedVersion Formato: application/pdf
Publicado em //2004 SPA
Relevância na Pesquisa
66.11%
Se estudió el transporte iónico y la morfología de crecimiento en depósitos ramificados en celdas delgadas a través de mediciones experimentales y teóricas, y el modelado computacional. Se usaron en forma simultánea las técnicas de Schlieren y partículas trazadoras para seguir la evolución de los frentes de concentración y los rollos convectivos en regímenes gravitoconvectivos y electroconvectivos dominantes. El modelo teórico macroscópico se basa en primeros principios, la ecuación de Nernst- Planck para el transporte de iones, la ecuación de Poisson para el potencial electrostático y la ecuación de Navier-Stokes para el flujo de fluido. Se presentan simulaciones numéricas para configuraciones de celdas reales usando computación en serie y paralela. En las simulaciones numéricas en paralelo, se hizo uso de técnicas de descomposición por dominios y métodos fuertemente implícitos para su implementación en un cluster tipo Beowulf con MPI bajo Linux. Esto permitió la utilización de mallas muy finas en dorninios distorsionados que se acercan más a la realidad de los experimentos físicos realizados, con el beneficio de un speed up casi lineal para algunos casos. La teoría y las simulaciones numéricas predicen la completa interacción de los frentes...

Salinity tolerance in plants: attempts to manipulate ion transport

Volkov, Vadim
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/11/2014
Relevância na Pesquisa
46.32%
Ion transport is the major determining factor of salinity tolerance in plants. A simple scheme of a plant cell with ion fluxes provides basic understanding of ion transport and the corresponding changes of ion concentrations under salinity. The review describes in detail basic principles of ion transport for a plant cell, introduces set of transporters essential for sodium and potassium uptake and efflux, analyses driving forces of ion transport and compares ion fluxes measured by several techniques. Study of differences in ion transport between salt tolerant halophytes and salt-sensitive plants with an emphasis on transport of potassium and sodium via plasma membranes offers knowledge for increasing salinity tolerance. Effects of salt stress on ion transport properties of membranes show huge opportunities for manipulating ion transport. Several attempts to overexpress or knockout ion transporters for changing salinity tolerance are described. Future perspectives are questioned with more attention given to potential candidate ion channels and transporters for altered expression. The potential direction of increasing salinity tolerance by modifying ion channels and transporters is discussed and questioned. An alternative approach from synthetic biology is to modify the existing membrane transport proteins or create new ones with desired properties for transforming agricultural crops. The approach had not been widely used earlier and leads also to theoretical and pure scientific aspects of protein chemistry...

Ion-transport catalysis: catalyzed isomerizations of NNH+ and NNCH3+

Chalk, Andrew; Radom, Leo
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.15%
High-level ab initio calculations using the G2(**) and G2(ZPE=MP2) methods have been employed to examine the effect of interaction with a range of neutral molecules (X) on the barrier to the degenerate proton-transport reactions in NNH+ (X = Ar, HF, CO, N2, H2O) and methyl cation-transport reactions in NNCH3+ (X = HF, H2, N2, HCl, H2O). It is found that the barriers to both proton and methyl cation transport are lowered from their values in the isolated ions of 182 and 152 kJ mol-1, respectively, by interaction with species having values of the proton or methyl cation affinity, respectively, lower than that of molecular nitrogen. Interaction with species that have larger values of proton or methyl cation affinities leads to a further lowering of the barrier, but transfer to the neutral molecule X becomes the more energetically favorable process in such cases. It is found that the ideal catalyst for ion transport should have an ion affinity close to but less than that of molecular nitrogen and have a large dipole moment.

The role of natriuretic peptides in ion transport mechanisms

Kourie, Joseph
Fonte: John Wiley & Sons Inc Publicador: John Wiley & Sons Inc
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.21%
Natriuretic peptides (NP) act as ligands on the guanylyl cyclase family of receptors. The NP binding site on these receptors is extracellular and the guanylyl cyclase and protein kinase domains are intracellular. The guanylyl cyclase receptor catalyzes the synthesis of the second messenger molecule, cGMP, which activates protein kinase. This in turn is involved in the phosphorylation of various ion transport proteins. Ion transport proteins, which are modulated by NP and are thought to underlie the natriuretic and diuretic actions of NP, include: (a) calcium-activated K+ channels; (b) ATP- sensitive K+ channels; (c) inwardly-rectifying K+ channels; (d) outwardly- rectifying K+ channels; (e) L-type Ca2+ channels; (f) Cl- channels including cystic fibrosis transmembrane conductance regulator Cl- channels; (g) Na+-K+2Cl- co-transporter; (h) Na+-K+ ATPase; (i) Na+ channels; (j) stretch-activated channels; and (k) water channels. It appears that NP modulate the kinetics, rather than the conductance, of ion channels. Some of these channels, like the Ca2+, ATP-sensitive K+ and stretch-activated channels, are also involved in NP secretion. In addition, the structural properties of the NP, e.g., ovCNP-22 and ovCNP-39, appear to confer on them the ability to form ion channels. These CNP-formed ion channels can modify the trans-membrane signal transduction and second messenger systems underlying NP-induced pathological effects.

The importance of dehydration in determining ion transport in narrow pores

Richards, Laura A.; Schafer, Andrea I.; Richards, Bryce; Corry, Ben
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.25%
The transport of hydrated ions through narrow pores is important for a number of processes such as the desalination and filtration of water and the conductance of ions through biological channels. Here, molecular dynamics simulations are used to systematically examine the transport of anionic drinking water contaminants (fluoride, chloride, nitrate, and nitrite) through pores ranging in effective radius from 2.8 to 6.5 Ã to elucidate the role of hydration in excluding these species during nanofiltration. Bulk hydration properties (hydrated size and coordination number) are determined for comparison with the situations inside the pores. Free energy profiles for ion transport through the pores show energy barriers depend on pore size, ion type, and membrane surface charge and that the selectivity sequence can change depending on the pore size. Ion coordination numbers along the trajectory showed that partial dehydration of the transported ion is the main contribution to the energy barriers. Ion transport is greatly hindered when the effective pore radius is smaller than the hydrated radius, as the ion has to lose some associated water molecules to enter the pore. Small energy barriers are still observed when pore sizes are larger than the hydrated radius due to re-orientation of the hydration shell or the loss of more distant water. These results demonstrate the importance of ion dehydration in transport through narrow pores...