Sequential Oxygen Research Articles

The temperature dependence of the initial sticking probability of oxygen on Pt(111) probed with second harmonic generation

The dissociative adsorption of oxygen on Pt(111) in the temperature range from 220 to 670 K has been investigated by means of second harmonic generation, SHG. The time envelope of the SHG signal was measured while dosing the surface. The sticking data were fitted to a quadratic coverage dependence giving an initial sticking coefficient S 0=0.06(1) at room temperature. Using a model based on two sequential precursors, i.e. physisorbed and molecular chemisorbed oxygen, a good fit of the temperature dependence of the initial dissociative sticking is obtained. A difference between the activation energy for desorption from the physisorbed molecular state and the activation energy for adsorption into the chemisorbed molecular state from the molecular physisorption state is determined to 43.0(5) meV. The fit also gives zero difference between the activation energy for adsorption into the atomic chemisorption state and the activation energy for adsorption into the molecular physisorption state from the molecular chemisorption state.

Mechanism for aromatase inactivation by a suicide substrate, androst-4-ene-3,6,17-trione: The 4β,5β-epoxy-19-oxo derivative as a reactive electrophile irreversibly binding to the active site

Aromatase is a cytochrome P450 enzyme complex that catalyzes the conversion of androst-4-ene-3,17-dione to estrone through three sequential oxygenations of the 19-methyl group. Androst-4-ene-3,6,17-trione (1) is a suicide substrate of aromatase. The inactivation mechanism for steroid 1 has been studied to show that the inactivation reaction proceeds through the 19-oxo intermediate 3. To further clarify the mechanism, 4β,5β-epoxyandrosta-3,6,17,19-tetraone (6) was synthesized as a candidate for a reactive electrophile involved in irreversible binding to the active site of aromatase, upon treatment of compound 3 with hydrogen peroxide in the presence of NaHCO 3. The epoxide 6 inhibited human placental aromatase in a competitive manner ( K i = 30 μM); moreover, it inactivated the enzyme in an active-site-directed manner in the absence of NADPH ( K I = 88 μM, fk inact = 0.071 min −1). NADPH and BSA both stimulated the inactivation rate without a significant change of the K, in either case ( k inact : 0.133 or 0.091 min −1), in the presence of NADPH or BSA, respectively). The substrate androst-4-ene-3,17-dione protected the inactivation, but a nucleophile, l-cysteine, did not. When both the epoxide 6 and its 19-methyl analog 4 were subjected separately to reaction with N-acetyl- l-cysteine in the presence of NaHCO 3, the 19-oxo steroid 6 disappeared from the reaction mixture more rapidly ( T 1 2 = 40 sec ) than the 19-methyl analog 4 ( T 1 2 = 3.0 min ). The results clearly indicate that the 4β,5β-epoxy-19-oxo compound 6, which is possibly produced from 19-oxo-4-ene steroid 3 through the 19-hydroxy-19-hydroperoxide intermediate, is a reactive electrophile that irreversibly binds to the active site of aromatase. BIOCHEM PHARMACOL 52;8:1253–1259, 1996.

Electrical and optical characterisation of double SIMOX structures formed by sequential high energy oxygen implantation into silicon

Novel buried double-layered SIMOX structures have been produced by the sequential high energy implantation of oxygen into silicon. The electrical and the optical properties of the samples were analysed by Spreading Resistance (SR) profiling and by Infrared (IR) reflection, respectively. Results were compared to those of Spectroscopic Ellipsometry (SE) and Rutherford Backscattering Spectroscopy (RBS). The chemical composition (oxygen concentration or SiO 2 fraction) depth profile, the high crystal quality of the Si layers and the nature of the fabricated SiO 2 layers were revealed. The structures can be potentially used as Si optical waveguides.

Preparation of a Bicyclic Analog of Qinghao (Artemisinic) Acid via a Lewis Acid Catalyzed Ionic Diels-Alder Reaction Involving a Hydroxy Diene and Cyclic Enone and Facile Conversion into (.+-.)-6,9-Desdimethylqinghaosu

Treatment of 6-methylcyclohex-2-enone (8) and hexa-3,5-dien-1-ol (14) either in dichloromethane at -20 to 0 o C with aluminum chloride (1 equiv) or in acetonitrile at -20 o C with Cu(II) trifluoromethanesulfonate (1 equiv) rapidly provides in a highly stereoselective reaction the hemiacetal Diels-Alder adduct 15, which with a trans ring junction and anti methyl group is considered to arise via an ionic Gassman-type Diels-Alder reaction involving prior formation of a hemiacetal between the alcohol and enone followed by generation of an allylic cation from the hemiacetal mediated by the Lewis acid. The adduct 15 is then converted in straightforward fashion into the methyl ester of the desdimethyl analogue of qinghao (artemisinic) acid, which upon sequential photosensitized oxygenation and then Fe(phen) 3 (PF 6 ) 3 /copper(II) triflate catalyzed cleavage-oxygenation provides (±)-6,9-desdimethylqinghaosu

Sequential Oxygenation of Linoleic Acid in the Fungus Gaeumannomyces graminis: Stereochemistry of Dioxygenase and Hydroperoxide Isomerase Reactions

Linoleic acid is sequentially oxygenated to (7 S,8 S)-dihydroxylinoleic acid by dioxygenase and hydroperoxide isomerase activities present in the fungus Gaeumannomyces graminis (Brodowsky, I. D., Hamberg, M., and Oliw, E. H., J. Biol. Chem. 267, 14738-14745 (1992)). Linoleic acids stereospecifically deuterated at C-7 and C-8 were prepared by biological desaturation of the corresponding stearates and used to determine the stereochemistry of the hydrogen abstractions occurring in the dioxygenase- and hydroperoxide isomerase-catalyzed reactions. The dioxygenase reaction was found to involve stereospecific abstraction of the pro-S hydrogen from C-8 followed by antarafacial insertion of dioxygen to produce (8 R)-hydroperoxylinoleic acid. The hydroperoxide isomerase reaction consisted of conversion of (8 R)-hydroperoxylinoleic acid into (7 S,8 S)-dihydroxylinoleic acid by stereospecific elimination of the pro-S hydrogen from C-7 and intramolecular suprafacial insertion of oxygen at C-7. Accordingly, during the conversion of linoleic acid into (8 R)-hydroperoxylinoleic acid, the absolute configuration of C-8 was inverted, while the conversion of (8 R)-hydroperoxylinoleic acid into (7 S,8 S)-dihydroxylinoleic acid occurred with retention of absolute configuration at C-7.

Metabolic aspects of the 1β-proton and the 19-methyl group of androst-4-ene-3,6,17-trione during aromatization by placental microsomes and inactivation of aromatase

Aromatase catalyzes the conversion of androst-4-ene-3,17-dione to estrogen through sequential oxygenations at the 19-methyl group. Androst-4-ene-3,6,17-trione (AT) is a suicide substrate of aromatase, and the mechanism of inactivation of aromatase has been postulated to involve enzymatic oxygenation at the 19-position. [1β- 3H,4- 14C]-, [19- 3H 3,4- 14C]-, and [1β- 3H,19- 14C]ATs, with high specific activities, were synthesized to study metabolic aspects and the inactivation mechanism. Incubation of the labeled AT with human placental microsomes yielded the 19-oxygenated derivatives, 19-hydroxy-AT and 19-oxo-AT, as well as the aromatization products, 6-oxoestrone and 6-oxoestradiol. A stereospecific 1β-proton elimination occurred during the aromatization of [1β- 3H,4- 14C]AT, and a marked tritium isotope effect was observed in the first hydroxylation at C-19 of [19- 3H 3,4- 14C]AT. After incubation of the three double-labeled ATs, the solubilized proteins were subjected to SDS-PAGE and the 3H/ 14C ratio of the aromatase-bound metabolite in a 46–69 kDa fraction was analyzed. A marked decrease of the 3H/ 14C ratio of the metabolite was observed in the experiment using [19- 3H 3,4- 14C]AT, compared with that of the labeled AT used, but there were no significant changes in the other experiments, indicating that the adduct retains the 1β-proton, the 19-carbon, and one of three 19-methyl protons of AT. Thus, we conclude that further oxygenation of 19-oxo-AT produced by the two initial hydroxylations of AT at C-19 yields not only 6-oxoestrogen (by a mechanism similar to that involved in the aromatization of the natural substrate) but also a reactive electrophile that immediately binds to the active site in an irreversible manner, resulting in inactivation of aromatase.

Sequential carbon oxygen bond cleavage in chemisorption of CO2 on Fe(100)

Adsorption and dissociation of CO2 on Fe(100) was studied using temperature programmed desorption, x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), high-resolution electron energy-loss spectra (HREELS), and low-energy electron diffraction. CO2 adsorbs on Fe(100) at 110 K to form two distinct adsorbed species: a physisorbed, molecular CO2 species and chemisorbed, bent anionic CO−2 species. These two species were characterized by HREELS losses at 2258 cm−1 for the asymmetric mode of the molecular CO2 state and at 1634 and 1232 cm−1 for asymmetric and symmetric modes of anionic CO2− state. Two features in O(1s) XPS spectrum for CO2 were observed at 534.9 and 531.1 eV binding energy and were assigned to the physisorbed and chemisorbed species, respectively. UPS data also indicate that a fraction of the adsorbed CO2 molecules at 110 K are in a bent CO−2 configuration. The molecular CO2 was found to desorb at 162 K where as the anionic form remains on the surface to dissociate. The dissociation upon heating is shown to proceed through two sequential carbon–oxygen bond breaking events. Strongly chemisorbed CO is produced as an intermediate in the dissociation process.

Features of buried dielectric layers: ion beam synthesis in silicon by a high temperature sequential oxygen and nitrogen implantation with substoichiometric doses

O + and N + ions were sequentially implanted at 150 keV atom −1 and water temperature of 650°C. The total implantation dose was 7.5 × 10 10 17 cm −2 . The O + and N + dose ratio has been chosen as 3:1 and 7:1. The implanted wafers were annealed in a nitrogen flow furnace at 1200°C for 2 h. The two sets of samples were analysed by cross-sectional transmission electron microscopy and X-ray diffractometry. In addition, the samples with a dose ratio of 3:1 were analysed by Auger spectroscopy. It has been shown that the structures obtained are characterized by a highly defective superficial silicon layer, heterogeneity of the chemical composition of the buried layer and the porous region inside this layer. A qualitative model providing an explanation of results obtained has been proposed.

Low-dislocation-density silicon-on-insulator material produced by sequential oxygen implantation and low-temperature annealing

Lattice strain and defect formation in oxygen-implanted silicon (SIMOX) were investigated by high-resolution x-ray diffraction and transmission electron microscopy. At doses of 1×1017 and 3×1017 cm−2 a high density of vacancy-type defects formed in a uniaxially compressed layer at the surface of the as-implanted wafer. Annealing at 900 °C for 0.5 h reduced this negative strain as the defects coarsened into observable cavities. The development of cavities upon annealing was used in a sequential-implantation and low-temperature-annealing process to produce low-threading dislocation density SIMOX. This new process offers several advantages over other methods of producing low-dislocation-density material.

Sequential oxygen transfer from sulphur rich [formula omitted]-dioxo molybdenum(VI) center: Synthesis, Spectroscopy, and electrochemical properties of monomeric Mo(VI),-(V) and -(IV) complexes

Sequential oxygen transfer from sulphur rich [formula omitted]-dioxo molybdenum(VI) center: Synthesis, Spectroscopy, and electrochemical properties of monomeric Mo(VI),-(V) and -(IV) complexes

Peculiarities of buried silicon oxynitride layer synthesis by sequential oxygen and nitrogen ion implantation in silicon

The synthesis of silicon oxynitride buried layers, using the sequential implantation of substoichiometric doses of oxygen and nitrogen ions, has been studied. By using cross-sectional transmission electron microscopy (XTEM), secondary ion mass spectroscopy (SIMS) and infrared transmission spectroscopy we have shown that there is an interrelation between the structure of synthesized layer and the oxygen to nitrogen dose ratio. It has been found that for the chosen implantation and annealing regimes the synthesized structure: consists of buried dielectric, structurally dense, layers which have thicknesses of between 0.12 and 0.185 μm. When the nitrogen to oxygen dose ratio is 3.5, the whole buried layer is amorphous and the interfaces of the buried layer are perfect.

Silicon transport during oxidation

Abstract We have investigated the transport of silicon across the silicon/silicon dioxide boundary, in the light of recently proposed reactive layer models of silicon oxidation. Using sequential 18O2-enriched and natural oxygen gas oxidations at 600, 850 and 950°C we have placed an upper limit of ∼0·2 of a monolayer on the distance silicon diffuses into the oxide during oxidation. Thus the formation of new oxide occurs essentially at the silicon/silica interface.

18O isotope studies on the redistribution of oxygen in noncontinuous buried layers formed by high-dose oxygen ion implantation

Abstract The implantation process of forming a buried oxide layer in a silicon matrix (SIMOX) has been modelled by a number of workers [1,2] to predict the oxygen depth distribution. In the experiments reported in this work we show the results from sequential oxygen isotope experiments in which the total dose results in a peak concentration just below that for stoichiometric SiO2. The total dose (18O+16O) ranges from 0.3 × 1018 to 1.2 × 1018 O/cm2 at 200 keV. Our results show that for these substoichiometric doses the peak concentration for the first implanted species is shifted deeper, relative to the second implant, especially for the higher doses. These results have been compared with the predictions from the model of layer formation proposed by Jager et al. [2] and good agreement is found.

Oxygen and carbon isotope ratios in gastropod shells as indicators of paleoenvironments in the afar region of ethiopia

The oxygen and carbon isotope ratios in the shells of ancient freshwater gastropods, coupled with geological, geographical and hydrological data from their area and time of origin, can yield information concerning general climatic conditions prevailing at the time the gastropod lived. In addition, sequential oxygen isotope analyses along the growth spiral of the shell provides indications of seasonal variability in the life span of the organisms. Such data were collected for some freshwater gastropods collected from two lakes and two springs in the Afar region of Ethiopia, spanning late Pleistocene to modern age. Episodes of high rainfall in the Ethiopian highlands were clearly evident in the depleted δ 18O values in some shells, whiel arid spells produced enriched levels in others. Hot springs introduced an added complication to the isotope record. This study, on a region of particularly complex hydrology and highly variable environmental history, indicates the utility of stable isotope analyses as proxies for past climates, but also emphasizes the problems involved.

Preparation of superconducting Bi-Sr-Ca-Cu-O thin films by sequential electron beam evaporation and oxygen annealing

Superconducting films with nominal composition Bi2Sr2Ca1Cu2Ox have been prepared on 〈100〉 MgO substrates by sequential electron beam evaporation of Cu, Bi, and Sr2/3 Ca1/3 F2, followed by annealing in flowing wet, then dry, O2. X-ray diffraction data show that the films contain the two Bi-Sr-Ca-Cu-O phases that have been identified in the literature as a superconducting phase with c∼31 Å and a semiconducting phase with c∼24 Å. Both phases are strongly textured with the c axis perpendicular to the substrate. For the best film, which was annealed at 870 °C for 30 min, zero resistance was observed at 90 K, and the critical current density increased from 0.8×105 A/cm2 at 77 K to 2.3×105 A/cm2 at 4.2 K.

The roles of innate oxygen and adsorbed oxygen in the chemistry of soot oxidation by oxygen atoms at 298 K

We have previously reported work examining the oxidation of soot by oxygen atoms at 298 K. We found that this oxidation produced CO 2 and CO gasification products, but that the chemistry was dominated by large net oxygen atom adsorption on the soot. In the present work we have used O 16 and O 18 in various sequential oxygen atom exposure experiments to study the roles of oxygen which is part of the as-prepared soot (hereafter referred to as innate oxygen) and adsorbed oxygen in the chemistry of this soot oxidation. We find that oxygen innate in the soot appears exclusively in the CO 2 gasification products. Adsorbed oxygen appears in both CO 2 and CO gasification products. A significant fraction of gasification product molecules contain exclusively oxygen atoms which had been adsorbed previously on the soot. That is, the oxygen atom responsible for the evolution of a gasification product molecule may not appear in this product molecule. We interpret these results to mean that the kinetically limiting reaction step in this soot gasification is product desorption; heat liberated by adsorption of an oxygen atom on the soot produces sufficient local surface energy to allow the activated desorption of CO 2 and CO.

Formation of lipoxin B by the pure reticulocyte lipoxygenase via sequential oxygenation of the substrate.

The pure reticulocyte lipoxygenase converts 15LS-hydroxy-5,8,11,13(Z,Z,Z,E)-icosatetraenoic acid (15LS-HETE) methyl ester to a complex mixture of products containing 5DS,14LR,15LS-trihydro(pero)xy-6E,++ +8Z,10E,12E-icosatetraenoate methyl ester (lipoxin B methyl ester), 5DS,15LS-DiH(P)ETE methyl ester and four 8,15LS-DiH(P)ETE methyl ester isomers [DiH(P)ETE = dihydro(pero)xy-icosatetraenoic acid]. After a short incubation period (15 min) 5DS,15LS-DiH(P)ETE methyl ester was found to be the main product, whereas after a 3-h incubation lipoxin B methyl ester was the predominant product. The reaction shows a remarkable stereoselectivity since only small amounts of other trihydroxy tetraenes are formed. Anaerobiosis, heat inactivation of the enzyme, or incubation in the presence of lipoxygenase inhibitors (icosatetraynoic acid, nordihydroguaiaretic acid) completely abolished the reaction. The complete steric structure of the major tetraene product (lipoxin B methyl ester) was established by ultraviolet spectroscopy, HPLC on four different types of columns, gas chromatography/mass spectrometry, gas/liquid chromatography of the ozonolysis fragments of the menthoxycarbonyl derivatives, and by 400-MHz 1H-NMR. Atmospheric oxygen was incorporated at carbon-5 and carbon-14 into the major product. 5DS,15LS-DiH(P)ETE methyl ester was shown to be an intermediate in the synthesis. Lipoxin B was also formed during the oxygenation of arachidonic acid, 15LS-HETE and 5DS,15LS-DiHETE. The results presented here indicate that lipoxin B can be formed by pure lipoxygenases via a sequential oxygenation of arachidonic acid or its hydro(pero)xy derivatives.

Current concepts in the management of the adult respiratory distress syndrome.

AbstractThe adult respiratory distress syndrome is the most severe pulmonary complication that can follow multiple trauma and critical illness. It is caused by an insult to the pulmonary circulation which results in an increased capillary permeability and subsequent interstitial edema. An increased venoarterial shunt occurs resulting in arterial hypoxemia. The mortality rate is at least 50% if treatment is not begun before profound hypoxemia occurs. Recognition of abnormal cardiopulmonary function before overt hypoxemia develops is important. This is best accomplished by attentive monitoring of arterial blood gases. Sequential arterial oxygen tensions obtained on 40% and 100% inspired oxygen concentration are the best guides to selecting patients who have a high probability of pulmonary deterioration. Aggressive cardiovascular monitoring and early treatment with low levels of positive‐end expiratory pressure offer this group of patients an improved change of survival.

On the stability of sub-stoichiometric uranium oxides

The oxidation of clean, high-purity polycrystalline uranium metal surfaces for low exposures to dry oxygen was studied with AES and XPS in an attempt to substantiate claims for the formation of a stable UO surface phase at ambient temperatures. We found no evidence for such a surface phase and found instead that grossly sub-stoichiometric surface oxides were formed after sequential oxygen saturation and heating.

Formation of lipoxin B by the pure reticulocyte lipoxygenase

The pure reticulocyte lipoxygenase converts 5,15-DiHETE via a lipoxygenase reaction to 5,14,15-trihy-droxy-6, 8,10,12-eicosatetraenoic acid (a lipoxin B isomer) as shown by GC/MS analysis of its trimethylsilyl ether. With arachidonic acid, 15-HETE and 15-HETE methyl ester this lipoxin B isomer was also formed. The results presented here indicate that pure mammalian lipoxygenases are able to form lipoxins via sequential multiple oxygenation of arachidonic acid or its hydroxy derivatives.