Role Of Molecular Oxygen Research Articles

The Role of the Oxygen Molecule in the Photolysis of Fullerenes

Experiments designed to clarify the role of molecular oxygen in the photolysis of fullerene are described. The formation of oxygen-fullerene adducts, both in fullerene solutions in various solvents and in micro-crystallites formed by evaporating these solutions, and their photolysis was studied under various experimental conditions. The results confirm that the role of oxygen in the photolysis process in these systems is ambivalent and highly dependent on the conditions under which the irradiation of the fullerenes occurs (wavelength and intensity of the photolyzing radiation, character of the solvent molecules, presence of additional molecules in the solvent-fullerene-oxygen system, etc.).

Spectral, lifetime, fluorescence quenching, energy transfer and photodecomposition of N, N′-bis(2,6-dimethyl phenyl)-3,4:9,10-perylentetracarboxylic diimide (DXP)

The spectral behavior and fluorescence quantum yield of N,N′-bis(2,6-dimethyl phenyl)-3,4:9,10-perylentetracarboxylic diimide (DXP) have been measured in different solvents. The role of molecular oxygen and KI in deactivation of excited singlet state DXP have been studied and the second-order quenching rate constant have been determined. The excitation energy transfer from 9,10-diphenylanthracene (DPA) to DXP has also been studied and the value of energy transfer rate constant, k ET, and critical transfer distance, R 0, indicate a Förster-type energy transfer mechanism. The photochemical reactivity of DXP in carbon tetrachloride has been studied.

Excitation Energy Transfer and Photodecomposition of Some Diolefinic Laser Dyes

Energy transfer from 1,4-bis-(β-pyridyl-2-vinyl)-benzene (P2VB, 1), 2,5-distyryl-pyrazine (DSP, 2), and 2,5-bis-2-(1-naphthylvinyl)-pyrazine (BNVP, 3) to N,N′-bis-(2,5-di-tert-butylphenyl)-3,4:9,10-perylene-bis-dicarboximide (DBPI, 4) has been studied in dimethyl form-amide by steady state emission measurements. It was found that the Stern-Volmer relation is obeyed from which the rate constants of energy transfer were determined as 5.6×1012, 6.8×1012, and 9.1×1012 lċmol−1ċs−1 for 1/4, 2/4, and 3/4, respectively. The corresponding experimental critical transfer distances have been calculated as 5.3, 5.7, and 8.45 nm, indicating a Forster type energy transfer mechanism. The activation energies associated with the fluorescence quenching of 1–3 by 4 in DMF were calculated as 5.5, 5.2, and 4.8 kJċmol−1. The excited state lifetime and the role of molecular oxygen in deactivation of the excited singlet state of 1–3 have been studied and the second order quenching rate constants have been determined. The photochemical quantum yields (λex=365 nm) of 1–3 were studied in CCl4.

Vanadium(IV)-mediated free radical generation and related 2′-deoxyguanosine hydroxylation and DNA damage

Free radical generation, 2′-deoxyguanosine (dG) hydroxylation and DNA damage by vanadium(IV) reactions were investigated. Vanadium(IV) caused molecular oxygen dependent dG hydroxylation to form 8-hydroxyl-2′-deoxyguanosine (8-OHdG). During a 15 min incubation of 1.0 mM dG and 1.0 mM VOSO 4 in phosphate buffer solution (pH 7.4) at room temperature under ambient air, dG was converted to 8-OHdG with a yield of about 0.31%. Catalase and formate inhibited the 8-OHdG formation while Superoxide dismutase enhanced it. Metal ion chelators, DTPA and deferoxamine, blocked the 8-OHdG formation. Incubation of vanadium(IV) with dG in argon did not generate any significant amount of 8-OHdG, indicating the role of molecular oxygen in the mechanism of vanadium(IV)-induced dG hydroxylation. Vanadium(IV) also caused molecular oxygen-dependent DNA strand breaks in a pattern similar to that observed for dG hydroxylation. ESR spin trapping measurements demonstrated that the reaction of vanadium(IV) with H 2O 2 generated OH radicals, which were inhibited by DTPA and deferoxamine. Incubation of vanadium(IV) with dG or with DNA in the presence of H 2O 2 resulted in an enhanced 8-OHdG formation and substantial DNA double strand breaks. Sodium formate inhibited 8-OHdG formation while DTPA had no significant effect. Deferoxamine enhanced the 8-OHdG generation by 2.5-fold. ESR and UV measurements provided evidence for the complex formation between vanadium(IV) and deferoxamine. UV-visible measurements indicate that dG, vanadium(IV) and deferoxamine are able to form a complex, thereby, facilitating site-specific 8-OHdG formation. Reaction of vanadium(IV) with t-butyl hydroperoxide generated hydroperoxide-derived free radicals, which caused 8-OHdG formation from dG and DNA strand breaks. DTPA and deferoxamine attenuated vanadium(IV)/r-butyl-OOH-induced DNA strand breaks.

Competitive Adsorption of VOCs and BOM: the Role of Molecular Oxygen

Molecular oxygen further reduced the adsorptive capacity of carbon for the volatile organics studied.In this study, the presence of background organic matter (BOM) was seen to reduce the adsorptive capacity of carbon for chloroform, chlorobenzene, and dibromochloropropane. Adsorption of these compounds was further reduced under oxic conditions. This additional reduction in capacity was likely due to conglomeration of BOM on the carhon surface, which reduced the available surface area for the target volatile organic chemical (VOC). Adsorption isotherms conducted with constant initial concentration ratios of VOC to BOM showed similar behavior. The ideal adsorbed solution theory (IAST) was found to accurately describe anoxic adsorption isotherms for VOCs in water containing BOM but failed to predict this competition for data collected under oxic conditions.

Conversion of isopropyl alcohol to acetone catalysed by Cr2O3 at 473 K: role of molecular oxygen

The conversion of isopropyl alcohol to acetone has been studied over Cr2O3 powder at 473 K in a flowing microcatalytic reactor. Appreciable steady-state catalytic activity was achieved only when oxygen was present in the reactant gas flow. The effect of mild pretreatment of Cr2O3 in O2–N2 at 473 K on the initial activity, and the observed dependence of the process on O2 pressure has been related to the oxidation–reduction cycle of the surface chromium species. Cr(3+n)+ on the surface of Cr2O3 have been considered to be the active species, where n= 1–3.

Role of molecular oxygen in the catalytic behaviour of ZSM-5 zeolite in hydrocarbon transformation

The reactivity of 1-butene over H-ZSM-5 was studied in the absence and in the presence of molecular oxygen. Two ZSM-5 samples were compared; a highly crystalline sample (A) with homogeneous size distribution, and a less crystalline sample (B), of non-homogeneous size distribution. The alkene gives rise to aliphatics, aromatics and carbon oxides; the nature of the prevailing product depends on the reaction temperature. In the presence of oxygen, catalyst deactivation gives rise to a change in the distribution of the products within the first hours of time-on-stream. The rate of deactivation is influenced by the zeolite crystallinity. The addition of small amounts of Fe, Cu or Co into sample B does not substantially modify the catalytic performance, thus suggesting that the ability in oxygen activation is a property of the zeolitic matrix itself, and not of possible traces of metal impurities present in the original material. For comparison the conversion of n-butane and benzene was tested, too. The oxidation of benzene gives rise only to carbon oxides; butane also is converted mainly to carbon oxides, with some formation of aromatics.

Mechanism of the primary photoreaction in the aqueous photochemistry of 4-chlorophenol

The flash-induced photolysis of 4-chlorophenol in water has been studied by high performance liquid chromatography. The primary photoproduct is confirmed as being p-benzoquinone. The role of molecular oxygen in the photochemistry has been investigated and we conclude that it is not involved in the formation of the p-benzoquinone although it does appear to protect the primary photoproduct from secondary photolysis. A photolysis mechanism based on the initial formation of a radical cation is proposed.

Hypoxia and Resistance to Hydrogen Peroxide Confer Resistance to Tumor Necrosis Factor in Murine L929 Cells

The mechanism whereby tumor necrosis factor (TNF) kills mammalian cells is not well understood, although oxidative damage has been suggested by several investigators. Further, it is not known why cells vary in their responsiveness to TNF. We show that the cytotoxic effect of TNF toward TNF-sensitive L929 cells is blocked under hypoxic conditions, suggesting a critical role of molecular oxygen and reactive oxygen species. To test whether cellular resistance to reactive oxygen species could provide resistance to TNF, we derived a variant strain from L929 cells by chronic exposure to an oxidizing agent, hydrogen peroxide (H2O2). These cells exhibit marked resistance to TNF as well as to H2O2. This cross-protection provides additional evidence that mechanisms of resistance to oxidative damage are causally related to TNF-induced cell death. Scatchard analysis of TNF binding did not reveal significant differences between the H2O2-resistant line and the wild-type L929 line. On the other hand, analyses of antioxidant enzymes and glutathione levels in cells of the wild-type and the H2O2-resistant lines revealed several potentially important differences. Before exposure to TNF, the H2O2-resistant variants have elevated catalase activity, decreased activity of total glutathione-S-transferase (GST), and similar superoxide dismutase (SOD) activities. Exposure to TNF led to alteration in CuZnSOD activity, and much more so in the variants than in the wild-type L929 cells. However, no significant change in MnSOD activities in cells of either cell line was observed. Total GST activity was not altered appreciably by TNF in either cell line, but Western analysis showed that the level of alpha GST isozyme was increased and mu GST isozyme decreased in the H2O2-resistant variants. Furthermore, alterations in total glutathione content were observed in both the control and the variant cells.

Role of molecular oxygen in fragmentation processes of abscisic acid methyl ester in electron capture negative ionization

AbstractIon‐molecule reactions with oxygen promote fragmentation in the electron capture negative ionization mass Spectrometry of methylated abscisic acid. It is shown that low‐energy collisions of the molecular anion (m/z 278) with O2 in the center quadrupole of a triple quadrupole mass spectrometer result in the formation of an adduct ion which subsequently fragments to yield two prominent ions, one at m/z 141 which contains the side‐chain and a second at m/z 152 which includes the ring. Two minor products are represented by peaks at m/z 179 and 136. These ion‐molecule reactions can also occur in the ion source when oxygen is present, which may cause fluctuations in relative intensities of the fragment ion peaks. The principal fragments of mass 141 and 152, and also those of mass 179 and 136, do not form in the absence of O2. Product ion mass spectra following collisions of M−· with 18O2 in the center quadrupole show that the side‐chain fragment ion shifts from m/z 141 to m/z 143, indicating retention of an oxygen atom from the gas‐phase reaction with molecular oxygen. It is proposed that on reaction with oxygen, a peroxy radical anion intermediate at m/z 310 is formed which then fragments to yield an enolate anion (m/z 141) or a quinone‐like radical anion (m/z152). The induced fragmentations of the 2‐cis and 2‐trans isomers of methylated abscisic acid appear to be identical. Analysis of isotopically labeled compounds of both abscisic acid and structurally similar metabolites reveal that oxygen‐activated fragmentation in these selected cases provides diagnostic ions which allow one to determine the position(s) of isotopic enrichment in the molecule.

The Role of Molecular Oxygen in the Catalytic Hydrogenation of Hydrazinonucleosides to the Corresponding Aminonucleosides

Abstract The hydrogenolysis of 2-amino-8-hydrazinoadenosine (2a) in water at reflux temperature in the presence of molecular oxygen provided 2,8-diaminoadenosine (3a) in 40-45% yield. 8-Aminoadenosine (3b) was synthesized in an analogous manner from 8-hydrazinoadenosine (2b) in comparable yield. The role of oxygen in these catalytic reductions will be discussed.

ChemInform Abstract: The Enzymatic Conversion of Exomethylene Cephalosporin C into Deacetyl Cephalosporin C and the Role of Molecular Oxygen in Cephalosporin C Biosynthesis.

AbstractCell free extracts from Cephalosporium acremonium containing deacetoxy/deacetyl cephalosporin C synthetase activities are shown to convert exomethylene cephalosporin C (I) directly to deacetyl cephalosporin C (II), a precursor in the cephalosporin C (III) biosynthesis.

The role of molecular oxygen in clavulanic acid biosynthesis: evidence for a bacterial oxidative deamination

Growth of cultures of Streptomyces clavuligerus(ATCC 27064) in an 18O2-containing atmosphere resulted in efficient and equal incorporations of molecular oxygen into the oxazolidine ring and the allylic hydroxy group of clavulanic acid.

The enzymatic conversion of exomethylene cephalosporin c into deacetyl cephalosporin c and the role of molecular oxygen in cephalosporin c biosynthesis

Cell free extracts from Cephalosporium acremonium containing deaoetoxy/-deacetyl cephalosporin C Synthetase activities have been shown to convert Exomethylene cephalosporin C directly to deacetyl cephalosporin C. Labelling experiments in the presence of 13O 2, indicated the source of the 3-methylene-oxygen of the latter was dioxygen. 1

The role of molecular oxygen in cephalosporin C biosynthesis

Cell-free preparations of the deacetoxycephalosporin C/deacetylcephalosporin C synthetase activities from Cephalosporium acremonium CO 728 were shown to convert the unnatural substrate methylenecephalosporin C (1) in the presence of 18O2, into [hydroxy-18O]deacetylcephalosporin C (2b).

The Role of Molecular Oxygen in the Photodynamic Effect of Phthalocyanines

Phthalocyanines are a class of mammalian cell photosensitizers which may be useful in photodynamic therapy for cancer. Chloroaluminum phthalocyanine tetrasulfonate was incubated with Chinese hamster cells in culture and exposed to white light at different concentrations of oxygen. The ability of the cells to form colonies served as an end point for the photobiological effect of the dye. The efficiency of photoinactivation of the sensitized cells decreased with decreasing oxygen concentration. Very little photoinactivation was observed when the atmosphere equilibrated with the cells was oxygen-free nitrogen. At an oxygen partial pressure of 2.5 mm Hg, photoinactivation was reduced by 50% compared to ambient atmosphere. In an attempt to understand the nature of the interaction between excited dyes and oxygen, the ability of several phthalocyanines to photogenerate singlet oxygen was measured. Thus phthalocyanines containing paramagnetic ions (copper, iron, vanadyl) do not generate 1O2 in contradistinction to diamagnetic metals (zinc and aluminum). The latter are efficient photosensitizers, while the former have little if any photobiological activity. In spite of this correlation, singlet oxygen may not be the intermediate involved in cytotoxicity. The reasons are discussed.

Role of molecular oxygen in lignin peroxidase reactions

Homogeneous lignin peroxidase (diarylpropane oxygenase) oxidized veratryl alcohol to veratryl aldehyde under anaerobic conditions in the presence of either H 2O 2, m-chloroperoxybenzoic acid (mCPBA), or p-nitroperoxybenzoic acid (pNPBA). Lignin peroxidase also oxidized the 1-(3′,4′-diethoxyphenyl)-1,2-dihydroxy-(4″-methoxyphenyl)-propane I under anaerobic conditions in the presence of mCPBA to yield 3,4-diethoxybenzaldehyde III and 1-(4′-methoxyphenyl)-1,2-dihydroxyethane IV. In contrast to what occurs under aerobic conditions, under anaerobic conditions no 2-hydroxy-1-(4′-methoxyphenyl)-1-oxoethane V was obtained. During the diarylpropane I cleavage under anaerobic conditions, 18O from H 2 18O was incorporated into the α-position of the phenylglycol IV. Lignin peroxidase also hydroxylated 1-(4′-ethoxy-3′-methoxyphenyl)propane II at the α-position to yield 1-(4′-ethoxy-3′-methoxyphenyl)-1-hydroxypropane VI under anaerobic conditions in the presence of mCPBA. During the phenylpropane II hydroxylation under anaerobic conditions, 18O from H 2 18O was incorporated into the α-position of VI. These results are rationalized according to a mechanism involving an initial one-electron oxidation of the diarylpropane I by the lignin peroxidase compound I to form a benzene π cation radical which undergoes α,β cleavage to produce a benzaldehyde and a C 6C 2 benzylic radical. The latter is then attacked by O 2 to form a hydroperoxy radical which may decompose through a tetroxide to form the phenylglycol IV and phenylketol V. Under anaerobic conditions the C 6C 2 benzylic radical is probably oxidized to a carbonium ion which would be subsequently attacked by H 2O to yield the phenylglycol V.

Activation of molecular hydrogen by transition metal complexes. Communication 6. Role of molecular oxygen in formation of palladium complexes that are active in the hydrogenation of unsaturated compounds

1. In order to form complexes that will catalyze the hydrogenation of unsaturated hydro-carbons in a system consisting of (Ph3P)2Pd2, HOAc, and H2, the presence of oxygen is necessary. Under these conditions, the hydrogen and oxygen form water and H2O2, which oxidizes the bis-(triphenylphosphine)dipalladium to palladium acetate. The palladium acetate, by reaction with the bis(triphenylphosphine)dipalladium and hydrogen, gives a complex that is active in hydrogenation. 2. A cluster with the composition [(PPh)2Pd5]n, which is active in the hydrogenation of unsaturated hydrocarbons, has been isolated as an individual substance.

Incorporation of molecular oxygen into androgens microbiologically derived from progesterone

Role of molecular oxygen in the course of the sidechain cleavage of progesterone by a microorganism, Cladosporium resinae was investigated with the double tracer method using 18O 2 as a tracer of molecular oxygen, and 14C-labeled progesterone as a tracer of the substrate. Then, it was confirmed that, when progesterone was directly transformed to testosterone acetate by the microorganism, this enzymic reaction required molecular oxygen which was incorporated into the testosterone acetate as the oxygen atom located at 17β-position of testosterone.

Oxygen Effects and Formation of Peroxides in Aqueous Solutions

The present status of knowledge of the chemical action of ionizing radiations on aqueous systems and the action of ionizing radiations on water are reviewed. Physical and chemical primary processes in the action of radiation on water, the role of molecular oxygen in radiation-induced reactions, the radiation- induced formation of hydroperoxides, and oxygen effects with densely ionizing radiations are discussed. It is concluded that the chemical effects of ionizing radiations on aqueous systems can be considerably influenced by the presence of molecular oxygen. 22 references. (C.H.)