Orientational disorder and phase transitions in ammonium oxofluorovanadates, (NH4)3VOF5 and (NH4)3VO2F4
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ABSTRACT:
SUBMITTER: Udovenko A
PROVIDER: S-EPMC5744410 | biostudies-literature | 2017 Nov
REPOSITORIES: biostudies-literature
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Project description:Ammonium oxopentafluoroniobate, (NH(4))(2)NbOF(5), was synthesized in a single-crystal form and the structures of its different phases were determined by X-ray diffraction at three temperatures: phase (I) at 297 K, phase (II) at 233 K and phase (III) at 198 K. The distorted [NbOF(5)](2-) octahedra are of similar geometry in all three structures, with the central atom shifted towards the O atom. The structure of (I) is disordered, with three spatial orientations of the [NbOF(5)](2-) octahedron related by a jump rotation around the pseudo-threefold local axis such that the disorder observed is of a dynamic nature. As the temperature decreases, the compound undergoes two phase transitions. The first is accompanied by full anionic ordering and partial ordering of the ammonium groups (phase II). The structure of (III) is completely ordered. The F and O atoms in the structures investigated were identified via the Nb-X (X = O and F) distances. The crystals of all three phases are twinned.
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Project description:Hitherto unknown crystal structures of ?- and ?-Sr2B2O5 are presented and a unified description of the Sr2B2O5 modifications based on the formalism of commensurately modulated structures is created. Data of the ? ? ? ? ?? ? ? thermal phase transitions are revisited and extended. Crystal structures of ?-, ?- and ?-Sr2B2O5 polymorphs resulting from the ? ? (at 565?K) ? ? (at 637?K) ?? ? (at 651?K) ? sequence of reversible first-order phase transitions are studied by high-temperature single-crystal X-ray diffraction, high-temperature X-ray powder diffraction, differential scanning calorimetry and impedance spectroscopy. Out of these phases, the structure of ?-Sr2B2O5 was already known whereas the structures of ?- and ?-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate ?-Sr2B2O5 phase and monoclinic low-temperature ?-Sr2B2O5 as well as high-temperature ?-Sr2B2O5 phase. Taking the ?-Sr2B2O5 phase with space group P21/c as a parent structure, the ?-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the ?-Sr2B2O5 phase was a sixfold superstructure with symmetry P
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Project description:Ammonium transport (Amt) proteins form a ubiquitous family of integral membrane proteins that specifically shuttle ammonium across membranes. In prokaryotes, archaea, and plants, Amts are used as environmental NH4(+) scavengers for uptake and assimilation of nitrogen. In the eukaryotic homologs, the Rhesus proteins, NH4(+)/NH3 transport is used instead in acid-base and pH homeostasis in kidney or NH4(+)/NH3 (and eventually CO2) detoxification in erythrocytes. Crystal structures and variant proteins are available, but the inherent challenges associated with the unambiguous identification of substrate and monitoring of transport events severely inhibit further progress in the field. Here we report a reliable in vitro assay that allows us to quantify the electrogenic capacity of Amt proteins. Using solid-supported membrane (SSM)-based electrophysiology, we have investigated the three Amt orthologs from the euryarchaeon Archaeoglobus fulgidus. Af-Amt1 and Af-Amt3 are electrogenic and transport the ammonium and methylammonium cation with high specificity. Transport is pH-dependent, with a steep decline at pH values of ∼5.0. Despite significant sequence homologies, functional differences between the three proteins became apparent. SSM electrophysiology provides a long-sought-after functional assay for the ubiquitous ammonium transporters.
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Project description:One of the main forms of nitrogen assimilated by microorganisms and plants is ammonium, despite its toxicity at low millimolar concentrations. Ammonium absorption has been demonstrated to be carried out by highly selective plasma membrane-located transporters of the AMT/MEP/Rh family and characterized by the presence of a well conserved hydrophobic pore through which ammonia is proposed to move. However, uncertainties exist regarding the exact chemical species transported by these membrane proteins, which can be in the form of either hydrophobic ammonia or charged ammonium. Here, we present the characterization of PvAMT1;1 from the common bean and demonstrate that it mediates the high affinity (micromolar), rapidly saturating (1 mM) electrogenic transport of ammonium. Activity of the transporter is enhanced by low extracellular pH, and associated with this acidic pH stimulation are changes in the reversal potential and cytoplasm acidification, indicating that PvAMT1;1 functions as an H(+)/NH(4)(+) symporter. Mutation analysis of a unique histidine present in PvAMT1;1 (H125R) leads to the stimulation of ammonium transport by decreasing the K(m) value by half and by increasing the V(max) 3-fold, without affecting the pH dependence of the symporter. In contrast, mutation of the first conserved histidine within the channel modifies the properties of PvAMT1;1, increasing its K(m) and V(max) values and transforming it into a pH-independent mechanism.
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