Selectivity

About: Selectivity is a research topic. Over the lifetime, 9445 publications have been published within this topic receiving 227360 citations.

The Structure of the Potassium Channel: Molecular Basis of K+ Conduction and Selectivity

Abstract: The potassium channel from Streptomyces lividans is an integral membrane protein with sequence similarity to all known K+ channels, particularly in the pore region. X-ray analysis with data to 3.2 angstroms reveals that four identical subunits create an inverted teepee, or cone, cradling the selectivity filter of the pore in its outer end. The narrow selectivity filter is only 12 angstroms long, whereas the remainder of the pore is wider and lined with hydrophobic amino acids. A large water-filled cavity and helix dipoles are positioned so as to overcome electrostatic destabilization of an ion in the pore at the center of the bilayer. Main chain carbonyl oxygen atoms from the K+ channel signature sequence line the selectivity filter, which is held open by structural constraints to coordinate K+ ions but not smaller Na+ ions. The selectivity filter contains two K+ ions about 7.5 angstroms apart. This configuration promotes ion conduction by exploiting electrostatic repulsive forces to overcome attractive forces between K+ ions and the selectivity filter. The architecture of the pore establishes the physical principles underlying selective K+ conduction.

Particle Size Effects in the Catalytic Electroreduction of CO2 on Cu Nanoparticles

Abstract: A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2–15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity–selectivity–size relations provide novel insights in the CO2 electroreduction r...

Tunable photocatalytic selectivity of hollow TiO2 microspheres composed of anatase polyhedra with exposed {001} facets.

Abstract: A fluoride mediated self-transformation method is proposed for the synthesis of hollow TiO2 microspheres (HTS) composed of anatase polyhedra with exposed ca. 20% {001} facets. Importantly, HTS exhibit tunable photocatalytic selectivity in decomposing azo dyes in water. The fluorinated HTS show preferential decomposition of methyl orange (MO) in comparison to methylene blue (MB). In contrast, the surface-modified HTS by either NaOH washing or calcinations at 600 °C favor decomposition of MB over MO. The surface chemistry and the surface structure at the atomic level are key factors in tuning the adsorption selectivity and, consequently, photocatalytic selectivity of HTS toward azo dyes.

High-efficiency oxygen reduction to hydrogen peroxide catalysed by oxidized carbon materials

Abstract: Hydrogen peroxide (H2O2) is a valuable chemical with a wide range of applications, but the current industrial synthesis of H2O2 involves an energy-intensive anthraquinone process. The electrochemical synthesis of H2O2 from oxygen reduction offers an alternative route for on-site applications; the efficiency of this process depends greatly on identifying cost-effective catalysts with high activity and selectivity. Here, we demonstrate a facile and general approach to catalyst development via the surface oxidation of abundant carbon materials to significantly enhance both the activity and selectivity (~90%) for H2O2 production by electrochemical oxygen reduction. We find that both the activity and selectivity are positively correlated with the oxygen content of the catalysts. The density functional theory calculations demonstrate that the carbon atoms adjacent to several oxygen functional groups (–COOH and C–O–C) are the active sites for oxygen reduction reaction via the two-electron pathway, which are further supported by a series of control experiments. The direct synthesis of hydrogen peroxide via oxygen reduction is an attractive alternative to the anthraquinone process. Here, a general trend linking oxygenation of carbon surfaces with electrocatalytic performance in peroxide synthesis is demonstrated, and computational studies provide further insight into the nature of the active sites.

Calcium channel characteristics conferred on the sodium channel by single mutations.

Abstract: THE sodium channel, one of the family of structurally homologous voltage-gated ion channels1, differs from other members, such as the calcium and the potassium channels, in its high selectivity for Na+. This selectivity presumably reflects a distinct structure of its ion-conducting pore. We have recently identified two clusters of predominantly negatively charged amino-acid residues, located at equivalent positions in the four internal repeats of the sodium channel as the main determinants of sensitivity to the blockers tetrodotoxin and saxitoxin2. All site-directed mutations reducing net negative charge at these positions also caused a marked decrease in single-channel conductance2. Thus these two amino-acid clusters probably form part of the extracellular mouth and/or the pore wall of the sodium channel. We report here the effects on ion selectivity of replacing lysine at position 1,422 in repeat III and/or alanine at position 1,714 in repeat IV of rat sodium channel II (ref. 3), each located in one of the two clusters, by glutamic acid, which ccurs at the equivalent positions in calcium channels. These amino-acid substitutions, unlike other substitutions in the adjacent regions, alter ion-selection properties of the sodium channel to resemble those of calcium channels. This result indicates that lysine 1,422 and alanine 1,714 are critical in deter mining the ion selectivity of the sodium channel, suggesting that these residues constitute part of the selectivity filter of the channel.