Release Of Halide Ion Research Articles

Molecular Mechanisms Underlying Oxidation Sensitivity of VRAC

The Volume Regulated Anion Channel (VRAC) is a key player in the cell-intrinsic regulatory processes which tend to restore the original cell volume upon osmotic challenges. This is achieved mediating the swelling-induced release of halide ions and organic osmolytes that subsequently drive water efflux through the membrane. Recently identified as a hetero-hexamer formed by LRRC8 proteins, its mechanism of activation remains elusive. Among the relevant triggers identified till now, activation of VRAC has been linked to the action of reactive oxygen species. Using GFP tagged LRRC8 channels expressed in oocytes it was found that oxidation sensitivity is subunit specific: LRRC8A-LRRC8E (8A-8E) heteromers are activated by oxidation, whereas 8A-8C and 8A-8D heteromers are inhibited (Gradogna et al, J Physiol, 2017). Here, using LRRC8 knockout HEK cells, we confirmed oxidation mediated activation of 8A-8E and inhibition of 8A-8C heteromers in a system devoid of background of endogenous LRRC8 genes. To further elucidate the mechanism underlying oxidative activation of 8A-8E, we added charged cysteine reactive compounds (MTSET⁺ or MTSES⁻) to the pipette solution. Both led to an initial dramatic increase of 8A-8E currents, followed by a complete inactivation. Subsequent application of hypotonic solution and oxidizing chloramine-T failed to reactivate the channels, suggesting that activation of 8A-8E by chloramine-T is mediated by intracellular cysteines. This was confirmed by chimeras in which the leucine-rich repeats (LRRs) were swapped between 8E and 8C, resulting in a corresponding interchange of the respective oxidation sensitivity, demonstrating that the relevant residues are located in the LRRs. Taken together, these findings corroborate the hypothesis of a strictly subunit dependent mechanism. Successive subchimeras based on the logic of the bisection method will be used to precisely identify cysteine(s) responsible of oxidation-dependent activation and to deepen the understanding of the in vivo mechanism.

Ligand-Mediated Release of Halides for Color Tuning of Perovskite Nanocrystals with Enhanced Stability.

The rich chemistry of metal halide perovskites has enabled various methods of band structure control and surface passivation. Here we report a highly facile and efficient post-treatment approach for precise color tuning of cesium lead halide perovskite nanocrystals (NCs) with enhanced stability. By utilizing a special multifunctional organic ligand, triphenyl(9-phenyl-9H-carbazol-3-yl)phosphonium bromide (TPP-Carz), carbon-halide bond cleavage can be achieved to release halide ions from halogenated solvents in a controlled manner for color tuning of perovskite NCs via ion exchange. Besides controlled release of halide ions for anion exchange, TPP-Carz can effectively passivate the surfaces of perovskite NCs simultaneously. As a result, perovskite NCs prepared by this post-treatment method with tunable colors over the entire visible spectrum have shown significantly improved luminescence and stability in comparison to the ones prepared using reactive anion precursors without surface passivation by TPP-Carz.

Theoretical and Computational Research on Halide Ion Release Channel of Haloalcohol Dehalogenase From <i>Agrobacterium Radiobacter</i> AD1

ABSTRACT Haloalcohol dehalogenases catalyze the dehalogenation of halohydrins through an intramo-lecular nucleophilic displacement. These enzymes exhibit a huge potential in the synthesis of chiral pharmaceutical intermediates. The three types of haloalcohol dehalogenases, A, B and C can catalyze the reversible dehalogenation reaction. Up-to-date studies suggested that the rate-limiting step of the catalysis process of HheC is the release of halide ions and thus ion release channels are assumed to exist. This article includes pure theoretical and computational studies using classic molecular dynamics simulations, random acceleration molecular dy-namics simulations and specialized toolkit MOLE 2.0 for predicting potential ion release channels. After comprehensive analysis of the computational results, the author found out some sites of interest that might be parts of hypothetical channels; but in order to completely investigate the effects of these key residues to the ion release channels, the author raised some assumptions and recommended some mutation strategies for bench scale experimental re-search for validation and further studies.

Release of Halide Ions from the Buried Active Site of the Haloalkane Dehalogenase LinB Revealed by Stopped-Flow Fluorescence Analysis and Free Energy Calculations

Release of halide ions is an essential step of the catalytic cycle of haloalkane dehalogenases. Here we describe experimentally and computationally the process of release of a halide anion from the buried active site of the haloalkane dehalogenase LinB. Using stopped-flow fluorescence analysis and umbrella sampling free energy calculations, we show that the anion binding is ion-specific and follows the ordering I(-) > Br(-) > Cl(-). We also address the issue of the protonation state of the catalytic His272 residue and its effect on the process of halide release. While deprotonation of His272 increases binding of anions in the access tunnel, we show that the anionic ordering does not change with the switch of the protonation state. We also demonstrate that a sodium cation could relatively easily enter the active site, provided the His272 residue is singly protonated, and replace thus the missing proton. In contrast, Na(+) is strongly repelled from the active site containing the doubly protonated His272 residue. Our study contributes toward understanding of the reaction mechanism of haloalkane dehalogenase enzyme family. Determination of the protonation state of the catalytic histidine throughout the catalytic cycle remains a challenge for future studies.

Phototransformation of linuron and chlorbromuron in aqueous solution

The main photoproducts initially formed in the phototransformation of linuron and chlorbromuron in aqueous solution result from photohydrolysis, i.e. hydroxylation with release of halide ion, and from elimination of a methoxy group. Reductive debromination was also observed with chlorbromuron. The transformation is less specific than with diuron. The orientation of the reaction depends on the wavelength: short wavelengths (254 nm) favour demethoxylation and photohydrolysis in the meta position whereas, with 'black light', i.e. wavelengths longer than 330 nm, photohydrolysis in the para position is the main reaction observed.

Natural Organohalogens in Sediments

Recent fluvial, lacustrine and marine sediments were found to contain significant concentrations of organohalogens which cannot be explained by known anthropogenic halogen compounds. The assumption of a natural source for the major part of these organohalogens is strongly supported by the fact that in lacustrine sediments deposited some hundred years ago – where no industrial chlorinated organic compounds should be expected – concentrations between 30 and 100 mg/kg of adsorbable organic halogens (AOX, expressed as equivalent chlorine) were detected. In anoxic sapropels of the Black Sea, several thousand years old and having a high organic content (mainly derived from marine phytoplankton), organic bromine and iodine occur at levels up to 313 and 465 mg/kg, respectively. AOX concentrations in biogenic sediments from various epochs and representing different stages of coalification (peat – lignite – bituminous coal – anthracite) are clear evidence for naturally occurring organohalogens. Plant material has therefore to be considered as the most important primary source of high molecular weight organohalogens in sediments. In addition, natural low molecular organohalogens produced by bacteria, fungi, algae (such as haloalkanes, terpenes, amino acids and peptides, chlorophenols etc.) will also accumulate in the sediment and become part of its primary organohalogen content. As a secondary source of organohalogens in sediments their formation in the sediment itself has to be considered. Biotic halogenation of organic substrates by haloperoxidases (to occur in algae and other marine organisms but also in terrestrial lichens and fungi) leading to volatile organohalogens has been observed already. Only little knowledge exists on abiotic halogenation. In preliminary investigations in the system trichloroacetic acid – water – humic substances, chlorinated compounds could be identified. A transfer of (low molecular) organohalogens from the sediment into the interstitial water is obvious: If compared with the supernatant lake water, AOX concentrations in interstitial water of Lake Constance sediments are enriched by a factor of 20–70. Under anaerobic conditions bacterial decomposition of organohalogens leads to the release of halide ions into the interstitial water of the sediment. The results presented here fully confirm our previous conclusion [1], that the AOX value cannot be used exclusively as a sum parameter for anthropogenic organic halogen compounds.

Mechanism of inactivation of S-adenosylhomocysteine hydrolase by (E)-5',6'-didehydro-6'-deoxy-6'-halohomoadenosines.

S-Adenosylhomocysteine (AdoHcy) hydrolase is irreversibly inactivated by (E)-5',6'-didehydro-6'-deoxy-6'-halohomoadenosines (EDDHHAs, halogen = I, Br, and Cl). The inactivation is concomitant with the reduction of the enzyme-bound NAD+ (E.NAD+) to NADH, the release of halide ion, and the formation of adenine (Ade) from the EDDHHAs. The mechanism of this inactivation involves two catalytic pathways. Pathway a involves a rapid addition of water to the 5',6'-bond of EDDHHAs and elimination of halide ion, resulting in the formation of 6'-carboxaldehyde 1 which then degrades chemically, resulting in the formation of Ade. Alternatively, 6'-carboxyaldehyde 1 can be oxidized by E.NAD+ to form 3'-keto-6'-carboxaldehyde 3 and the NADH form (inactive) of the enzyme. Like 6'-carboxaldehyde 1, the 3'-keto derivative 3 degrades chemically to form Ade. Pathway b involves the oxidation of EDDHHAs to 3'-keto-EDDHHAs 2 by E.NAD+, as the first step, and the subsequent release of halide ion to form 3'-keto-6'-carboxaldehyde 3. Evidence in support of these mechanisms includes the observations that incubation of EDDHHAs with AdoHcy hydrolase generated large molar excesses of halide ions and Ade, that Ade was shown to eliminate spontaneously from 6'-carboxaldehyde 1, and that the more rapid the halide ion release (Cl- > Br- > I-) from the EDDHHAs or the greater the partition ratios (nonlethal turnovers/lethal event), the lower the enzyme inactivation efficiency.(ABSTRACT TRUNCATED AT 250 WORDS)

The release of halide ions from some primary haloalkylamines

The kinetics for the release of halide ions from some haloalkylamines and the corresponding haloalkylammonium ions have been investigated in aqueous solutions under different conditions. In basic solutions, the free amines react according to a two-term rate law ����������������������� kobs = k1+k2[OH-] and their reactivities are in the order: 2-chloroethylamine ≈ 2-chloro- n-propylamine � 3-chloro-n-propylamine. The products formed are the corresponding iminoalkanes and the possible mechanisms are briefly discussed. In neutral solutions, the haloalkylammonium ions release halide ions by a considerably slower first-order reaction, which is retarded by acid. Since here there is not much difference in reactivity between 2-chloro-n-propylamine and 3-chloro-n-propylamine, the reaction probably involves a single-stage bimolecular attack by the solvent, leading to the formation of aminoalkanols. In both cases, the reactivities of 2-bromoethylamine me greater than those of the corresponding chloro compound.