Overview: oxidant and particle photochemical processes above a south-east Asian tropical rain forest (the OP3 project); introduction, rationale, location, characteristics and tools

In April–July 2008, intensive measurements were made of atmospheric composition and chemistry in Sabah, Malaysia, as part of the "Oxidant and particle photochemical processes above a South-East Asian tropical rainforest" (OP3) project. Fluxes and concentrations of trace gases and particles were made from and above the rainforest canopy at the Bukit Atur Global Atmosphere Watch station and at the nearby Sabahmas oil palm plantation, using both ground-based and airborne measurements. Here, the measurement and modelling strategies used, the characteristics of the sites and an overview of data obtained are described. Composition measurements show that the rainforest site was not significantly impacted by anthropogenic pollution, and this is confirmed by satellite retrievals of NO2 and HCHO. The dominant modulators of atmospheric chemistry at the rainforest site were therefore emissions of BVOCs and soil emissions of reactive nitrogen oxides. At the observed BVOC:NOx volume mixing ratio (~100 pptv/pptv), current chemical models suggest that daytime maximum OH concentrations should be ca. 105 radicals cm−3, but observed OH concentrations were an order of magnitude greater than this. We confirm, therefore, previous measurements that suggest that an unexplained source of OH must exist above tropical rainforest and we continue to interrogate the data to find explanations for this.

Mechanism of the antioxidant to pro-oxidant switch in the behavior of dehydroascorbate during LDL oxidation by copper(II) ions

Oxidised low density lipoprotein (LDL) may be involved in the pathogenesis of atherosclerosis. We have therefore investigated the mechanisms underlying the antioxidant/pro-oxidant behavior of dehydroascorbate, the oxidation product of ascorbic acid, toward LDL incubated With Cu2+ ions. By monitoring lipid peroxidation through the formation of conjugated dienes and lipid hydroperoxides, we show that the pro-oxidant activity of dehydroascorbate is critically dependent on the presence of lipid hydroperoxides, which accumulate during the early stages of oxidation. Using electron paramagnetic resonance spectroscopy, we show that dehydroascorbate amplifies the generation of alkoxyl radicals during the interaction of copper ions with the model alkyl hydroperoxide, tert-butylhydroperoxide. Under continuous-flow conditions, a prominent doublet signal was detected, which we attribute to both the erythroascorbate and ascorbate free radicals. On this basis, we propose that the pro-oxidant activity of dehydroascorbate toward LDL is due to its known spontaneous interconversion to erythroascorbate and ascorbate, which reduce Cu2+ to Cu+ and thereby promote the decomposition of lipid hydroperoxides. Various mechanisms, including copper chelation and Cu+ oxidation, are suggested to underlie the antioxidant behavior of dehydroascorbate in LDL that is essentially free of lipid hydroperoxides. (C) 2007 Elsevier Inc. All rights reserved.

Oxo-rhenium(V) complexes with bidentate phosphine ligands: synthesis, crystal structure and catalytic potentiality in epoxidation of olefins using hydrogen peroxide activated bicarbonate as oxidant

Two oxorhenium(V) complexes with bidentate phosphine ligands were synthesized and isolated as [ReOCl3(dppm)] 1 and [ReOCl3(dppp)] 2 [where dppm = 1,1-bis(diphenylphosphino) methane and dppp = 1.3-bis(diphenylphosphino) propanel. Complex 2 was structurally characterized. Both the complexes were used as catalysts in the epoxidation of olefins using NaHCO3 as co-catalyst and H2O2 as terminal oxidant. (c) 2008 Elsevier B.V. All rights reserved.

Novel supramolecular network in tri- and mono-nuclear oxovanadium(V)-salicyl-hydroximate: Synthesis, structure and catalytic oxidation of hydrocarbons using H2O2 as terminal oxidant

oxovanadium(V) salicylhydroximate complexes, [VO(SHA)(H2O)]center dot 1.58H(2)O (1) and [V3O3(CSHA)(3) (H2O)(3)]center dot 3CH(3)COCH(3) (2) have been synthesized by reaction of VO43- with N-salicyl hydroxamic acid (SHAHS) and N-(5-chlorosalicyl) hydroxamic acid (CSHAH(3)), respectively, in methanol medium. Compound 1 on reaction with pyridine 2,6-dicarboxylic acid (PyDCH2) yields mononuclear complex [VO(SHAH(2))(PyDC)] (3). Treatment of compound 3 with hydrogen peroxide at low pH (2-3) and low temperature (0-5 degrees C) yields a stable oxoperoxovanadium(V) complex H[VO(O-2)(PyDC)(H2O)]center dot 2.5H(2)O (4). All four complexes (1-4) have been characterized by spectroscopic (IR, UV-Vis, V-51 NMR) and single crystal X-ray analyses. Intermolecular hydrogen bonds link complex 1 into hexanuclear clusters consisting of six {VNO5} octahedra surrounded by twelve {VNO5} octahedra to form an annular ring. While the molecular packing in 2 generates a two-dimensional framework hydrogen bonds involving the solvent acetone molecules, the mononuclear complexes 3 and 4 exhibit three-dimensional supramolecular architecture. The compounds 1 and 2 behave as good catalysts for oxygenation of benzylic, aromatic, carbocyclic and aliphatic hydrocarbons to their corresponding hydroxylated and oxygenated products using H2O2 as terminal oxidant; the process affords very good yield and turnover number. The catalysis work shows that cyclohexane is a very easily oxidizable substrate giving the highest turnover number (TON) while n-hexane and n-heptane show limited yield, longer time involvement and lesser TON than other hydrocarbons. (C) 2008 Elsevier Ltd. All rights reserved.

The pro-inflammatory oxidant hypochlorous acid induces Bax-dependent mitochondrial permeabilisation and cell death through AIF-/EndoG-dependent pathways

At sites of chronic inflammation, such as in the inflamed rheumatoid joint, activated neutrophils release hydrogen peroxide (H2O2) and the enzyme myeloperoxidase to catalyse the formation of hypochlorous acid (HOCl). 3-chlorotyrosine, a marker of HOCl in vivo, has been observed in synovial fluid proteins from rheumatoid arthritis patients. However the mechanisms of HOCl-induced cytotxicity are unknown. We determined the molecular mechanisms by which HOCl induced cell death in human mesenchymal progenitor cells (MPCs) differentiated into a chondrocytic phenotype as a model of human cartilage cells and show that HOCl induced rapid Bax conformational change, mitochondrial permeability and release of intra-mitochondrial pro-apoptotic proteins which resulted in nuclear translocation of AIF and EndoG. siRNA-mediated knockdown of Bax substantially prevented mitochondrial permeability, release of intra-mitochondrial pro-apoptotic proteins. Cell death was inhibited by siRNA-mediated knockdown of Bax, AIF or EndoG. Although we observed several biochemical markers of apoptosis, caspase activation was not detected either by western blotting, fluorescence activity assays or by using caspase inhibitors to inhibit cell death. This was further supported by findings that (1) in vitro exposure of recombinant human caspases to HOCl caused significant inhibition of caspase activity and (2) the addition of HOCl to staurosporine-treated MPCs inhibited the activity of cellular caspases. Our results show for the first time that HOCl induced Bax-dependent mitochondrial permeability which led to cell death without caspase activity by processes involving AIF/EndoG-dependent pathways. Our study provides a novel insight into the potential mechanisms of cell death in the inflamed human joint. (c) 2006 Elsevier Inc. All rights reserved.

Dietary flavonoid (-)epicatechin stimulates phosphatidylinositol 3-kinase-dependent anti-oxidant response element activity and up-regulates glutathione in cortical astrocytes

Flavonoids are plant-derived polyphenolic compounds with neuroprotective properties. Recent work suggests that, in addition to acting as hydrogen donors, they activate protective signalling pathways. The anti-oxidant response element (ARE) promotes the expression of protective proteins including those required for glutathione synthesis (xCT cystine antiporter, gamma-glutamylcysteine synthetase and glutathione synthase). The use of a luciferase reporter (ARE-luc) assay showed that the dietary flavan-3-ol (-)epicatechin activates this pathway in primary cortical astrocytes but not neurones. We also examined the distribution of NF-E2-related factor-2 (Nrf2), a key transcription factor in ARE-mediated gene expression. We found, using immunocytochemistry, that Nrf2 accumulated in the nuclei of astrocytes following exposure to tert-butylhydroquinone (100 mu M) and (-)epicatechin (100 nM). (-)Epicatechin signalling via Nrf2 was inhibited by wortmannin implicating a phosphatidylinositol 3-kinase-dependent pathway. Finally, (-)epicatechin increased glutathione levels in astrocytes consistent with an up-regulation of ARE-mediated gene expression. Together, this suggests that flavonoids may be cytoprotective by increasing anti-oxidant gene expression.

Carbon monoxide protects against oxidant-induced apoptosis via inhibition of Kv2.1

Oxidative stress induces neuronal apoptosis and is implicated in cerebral ischemia, head trauma, and age-related neurodegenerative diseases. An early step in this process is the loss of intracellular K(+) via K(+) channels, and evidence indicates that K(v)2.1 is of particular importance in this regard, being rapidly inserted into the plasma membrane in response to apoptotic stimuli. An additional feature of neuronal oxidative stress is the up-regulation of the inducible enzyme heme oxygenase-1 (HO-1), which catabolizes heme to generate biliverdin, Fe(2+), and carbon monoxide (CO). CO provides neuronal protection against stresses such as stroke and excitotoxicity, although the underlying mechanisms are not yet elucidated. Here, we demonstrate that CO reversibly inhibits K(v)2.1. Channel inhibition by CO involves reactive oxygen species and protein kinase G activity. Overexpression of K(v)2.1 in HEK293 cells increases their vulnerability to oxidant-induced apoptosis, and this is reversed by CO. In hippocampal neurons, CO selectively inhibits K(v)2.1, reverses the dramatic oxidant-induced increase in K(+) current density, and provides marked protection against oxidant-induced apoptosis. Our results provide a novel mechanism to account for the neuroprotective effects of CO against oxidative apoptosis, which has potential for therapeutic exploitation to provide neuronal protection in situations of oxidative stress.

Sensitivity of global sulphate aerosol production to changes in oxidant concentrations and climate

The oxidation of SO2 to sulphate aerosol is an important process to include in climate models, and uncertainties caused by ignoring feedback mechanisms affecting the oxidants concerned need to be investigated. Here we present the results of an investigation into the sensitivity of sulphate concentrations to oxidant changes (from changes in climate and in emissions of oxidant precursors) and to changes in climate, in a version of HadGAM1 (the atmosphere-only version of HadGEM1) with an improved sulphur cycle scheme. We find that, when oxidants alone are changed, the global total sulphate burden decreases by approximately 3%, due mainly to a reduction in the OH burden. When climate alone is changed, our results show that the global total sulphate burden increases by approximately 9%; we conclude that this is probably attributable to reduced precipitation in regions of high sulphate abundance. When both oxidants and climate are changed simultaneously, we find that the effects of the two changes combine approximately linearly.

Aqueous geochemistry and oxygen isotope compositions of acid mine drainage from the Río Tinto, SW Spain, highlight inconsistencies in current models

The Rio Tinto river in SW Spain is a classic example of acid mine drainage and the focus of an increasing amount of research including environmental geochemistry, extremophile microbiology and Mars-analogue studies. Its 5000-year mining legacy has resulted in a wide range of point inputs including spoil heaps and tunnels draining underground workings. The variety of inputs and importance of the river as a research site make it an ideal location for investigating sulphide oxidation mechanisms at the field scale. Mass balance calculations showed that pyrite oxidation accounts for over 93% of the dissolved sulphate derived from sulphide oxidation in the Rio Tinto point inputs. Oxygen isotopes in water and sulphate were analysed from a variety of drainage sources and displayed delta O-18((SO4-H2O)) values from 3.9 to 13.6 parts per thousand, indicating that different oxidation pathways occurred at different sites within the catchment. The most commonly used approach to interpreting field oxygen isotope data applies water and oxygen fractionation factors derived from laboratory experiments. We demonstrate that this approach cannot explain high delta O-18((SO4-H2O)) values in a manner that is consistent with recent models of pyrite and sulphoxyanion oxidation. In the Rio Tinto, high delta O-18((SO4-H2O)) values (11.2-13.6 parts per thousand) occur in concentrated (Fe = 172-829 mM), low pH (0.88-1.4), ferrous iron (68-91% of total Fe) waters and are most simply explained by a mechanism involving a dissolved sulphite intermediate, sulphite-water oxygen equilibrium exchange and finally sulphite oxidation to sulphate with O-2. In contrast, drainage from large waste blocks of acid volcanic tuff with pyritiferous veins also had low pH (1.7). but had a low delta O-18((SO4-H2O)) value of 4.0 parts per thousand and high concentrations of ferric iron (Fe(III) = 185 mM, total Fe = 186 mM), suggesting a pathway where ferric iron is the primary oxidant, water is the primary source of oxygen in the sulphate and where sulphate is released directly from the pyrite surface. However, problems remain with the sulphite-water oxygen exchange model and recommendations are therefore made for future experiments to refine our understanding of oxygen isotopes in pyrite oxidation. (C) 2009 Elsevier B.V. All rights reserved.

Ascorbate does not protect macrophages against apoptosis induced by oxidised low density lipoprotein

Apoptosis of macrophages and smooth muscle cells is observed in atherosclerotic lesions and may play an important role in the disease progression. Oxidised low density lipoprotein (LDL) is cytotoxic and induces apoptosis in a variety of cell types. We reported previously that ascorbate protects arterial smooth muscle cells from apoptosis induced by oxidised LDL containing the peak levels of lipid hydroperoxides. We now demonstrate that macrophages undergo apoptosis when treated with this species of oxidised LDL, as detected by increased annexin V binding and DNA fragmentation. Ascorbate treatment of macrophages did not protect against the cytotoxicity of oxidised LDL, and modestly increased the levels of annexin V binding and DNA fragmentation. Oxidised LDL treatment also increased the expression of the antioxidant stress protein heme oxygenase-1 in macrophages; however, this increase was markedly attenuated by ascorbate pretreatment. Although apoptosis induced by oxidised LDL was modestly promoted by ascorbate, ascorbate apparently decreased the levels of oxidative stress in macrophages, suggesting that this pro-apoptotic effect was not mediated by a pro-oxidant mechanism, but may instead have been due to intracellular protection of the apoptotic machinery by ascorbate. (c) 2006 Elsevier Inc. All rights reserved.

Molybdenum eta(3)-allyl dicarbonyl complexes as a new class of precursors for highly reactive epoxidation catalysts with tert-butyl hydroperoxide

New Mo(II) diimine derivatives of [Mo(q (3)allyl)X(CO)(2)(CH3CN)(2)] (allyl = C3H5 and C5H5O; X = Cl, Br) were prepared, and [MO(eta(3)-C3H5)Cl(CO)(2)(BIAN)] (BIAN = 1,4-(4-chloro)phenyl-2,3-naphthalene-diazabutadiene) (7) was structurally characterized by single-crystal X-ray diffraction. This complex adopted an equatorial-axial arrangement of the bidentate ligand (axial isomer), in contrast with the precursors, found as the equatorial isomer in the solid and fluxional in solution. The new complexes of the type [Mo(eta(3)-allyl)X(CO)(2)(N-N)l (N-N is a bidentate chelating dinitrogen ligand) were tested for the catalytic epoxidation of cyclooctene using tert-butyl hydroperoxide as oxidant. All catalytic systems were 100% selective toward epoxide formation. While their turnover frequencies paralleled those of related Mo(eta) carbonyl compounds or Mo(VI) compounds bearing similar N-donor ligands, they exhibited similar olefin conversions in consecutive catalytic runs. The acetonitrile precursors were generally more active than the diimine complexes, and the chloro derivatives more active than the bromo ones. Combined vibrational and NMR spectroscopy and computational studies (DFT) were used to investigate the nature of the molybdenum species formed in the catalytic system with [Mo(eta(3)-C3H5)Cl(CO)(2){1,4-(2,6-dimethyl)phenyl-2.3-dimethyldiazabuta diene}] (4) and to propose that the resulting species may be dimeric bearing oxide bridges.

Immobilisation of eta(3)-allyidicarbonyl complexes of Mo-II with bidentate nitrogen ligands within aluminium-pillared clays

Molybdenum(II) complexes [MOX(CO)(2)(eta(3)-allyl)(CH3CN)(2)] (X = Cl or Br) were encapsulated in an aluminium-pillared natural clay or a porous clay heterostructure and allowed to react with bidentate diimine ligands. All the materials obtained were characterised by several solid-state techniques. Powder XRD, and Al-27 and Si-29 MAS NMR were used to investigate the integrity of the pillared clay during the modification treatments. C-13 CP MAS NMR, FTIR, elemental analyses and low-temperature nitrogen adsorption showed that the immobilisation of the precursor complexes was successful as well as the in situ ligand-substitution reaction. The new complex [MoBr(CO)(2)(eta(3)-allyl)(2-aminodipyridyl)] was characterised by single-crystal X-ray diffraction and spectroscopic techniques, and NMR studies were used to investigate its fluxional behaviour in solution. The prepared materials are active for the oxidation of cis-cyclooctene using tert-butyl hydroperoxide as oxidant, though the activity of the isolated complexes is higher. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).

A surface and catalytic study of heterogenised Os-3(CO)(12) species in MCM-41 structures

Immobilised Os species prepared via chemical vapour deposition (CVD) of Os-3(CO)(12) onto MCM-41 are active and selective catalysts for the dihydroxylation of trans-stilbene in acetone and water, using N-methylmorpholine N-oxide as the oxidant. A detailed temperature programmed decomposition study of the solids enables to identify the active sites as Os-x(CO)(y) surface species. The initial loading of the MCM-41 with the trinuclear precursor, as well as the temperature of the post-synthesis oxidising treatment, are found to have a significant impact on the structure/geometry of the resulting surface species, and thus their catalytic properties. We show how it is also affected by the confined environment of the MCM-41 mesopores and especially the curvature of the 30 Angstrom diameter channels. Finally, a careful study of the catalytic properties of the materials together with a study of the reactivity of the reaction products under similar conditions enable to suggest a mechanism involving the reaction of the oxidant with the osmium carbonyl surface species to form the catalytically active Os-oxo sites, and the formation of an osmoate-type species (through adsorption of the alkene onto the Os-oxo site) which subsequently reacts with the solvent to produce the diol. (C) 2003 Elsevier B.V. All rights reserved.

Heterogenisation of Os species on MCM-41 structure for epoxidation of trans-stilbene

Metal organic chemical vapour deposition technique (MOCVD) has been used to immobilise Os species onto the internal porous structure of MCM-41. Evidence suggests that volatile Os-3(CO)(12) cluster reacts with surface silanol groups of the MCM-41 via an oxidative addition reaction to yield a trinuclear HOs3(CO)(10)(OSi-) surface species. After heat treatment in air or at their very low surface coverage, these triangular sites break up to partially oxidised mononuclear surface species. In the presence of tert-butyl hydroperoxide (TBHP) as an oxidant, we demonstrate that the mononuclear species form extremely active species that catalyse the oxidation of trans-stilbene selectively to the corresponding epoxide. By carefully controlling the parameters of the MOCVD method (loading and calcination temperature), we report a new class of optimised MCM-41 porous heterogeneous catalysts carrying isolated but active Os sites for the selective oxidation of trans-stilbene in liquid phase. The reaction selectivity of the solid supported Os is apparently higher than the soluble homogeneous Os-3(CO)(12) cluster. It is envisaged that our solid supported catalysts not only facilitate separation from products but also offer an excellent utilisation of Os for catalysis. (C) 2003 Elsevier Science B.V. All rights reserved.

Synthesis, structure and solution chemistry of mixed-ligand oxovanadium(IV) and oxovanadium(V) complexes incorporating tridentate ONO donor hydrazone ligands

In the title family, the ONO donor ligands are the acetylhydrazones of salicylaidehyde (H2L1) and 2-hydroxyacetophenone (H2L2) (general abbreviation, H2L). The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of tridentate H2L and a bidentate NN donor [e.g., 2,2'-bipyridine(bpy) or 1,10-phenanthroline(phen), hereafter B] ligands in equimolar ratio afforded the tetravalent complexes of the type [(VO)-O-IV(L)(B)]; complexes (1)-(4) whereas, if B is replaced by 8-hydroxyquinoline(Hhq) (which is a bidentate ON donor ligand), the above reaction mixture yielded the pentavalent complexes of the type [(VO)-O-V(L)(hq)]; complexes (5) and (6). Aerial oxygen is most likely the oxidant (for the oxidation of V-IV -> V-V) in the synthesis of pentavalent complexes (5) and (6). [(VO)-O-IV(L)(B)] complexes are one electron paramagnetic and display axial EPR spectra, while the [(VO)-O-V(L)(hq)] complexes are diamagnetic. The X-ray structure of [(VO)-O-V(L-2)(hq)] (6) indicates that H2L2 ligand is bonded with the vanadium meridionally in a tridentate dinegative fashion through its phenolic-O, enolic-O and imine-N atoms. The general bond length order is: oxo < phenolato < enolato. The V-O (enolato) bond is longer than V-O (phenolato) bond by similar to 0.07 angstrom and is identical with V-O (carboxylate) bond. H-1 NMR spectrum of (6) in CDCl3 solution indicates that the binding nature in the solid state is also retained in solution. Complexes (1)(4) display two ligand-field transitions in the visible region near 820 and 480 nm in DMF solution and exhibit irreversible oxidation peak near +0.60 V versus SCE in DMSO solution, while complexes (5) and (6) exhibit only LMCT band near 535 nm and display quasi-reversible one electron reduction peak near -0.10 V versus SCE in CH2Cl2 solution. The VO3+-VO2+ E-1/2 values shift considerably to more negative values when neutral NN donor is replaced by anionic ON donor species and it also provides better VO3+ binding via phenolato oxygen. For a given bidentate ligand, E-1/2 increases in the order: (L-2)(2-) < (L-1)(2-). (c) 2004 Elsevier B.V. All rights reserved.