Effects Of Dioxygen Research Articles

Anaerobic vs. aerobic preparation of silicon nanoparticles by stirred media milling. The effects of dioxygen, milling solvent, and milling time on particle size, surface area, crystallinity, surface/near-surface composition, and reactivity

Silicon nanoparticles milled anaerobically in heptane or mesitylene are smaller and much more reactive than SiNPs milled aerobically in the same solvents for equal attritor milling times.

Effect of dioxygen on copper(II) binding to α-synuclein

Using the fluorescent amino acid tryptophan (Trp), we have characterized the copper(II) binding of F4W alpha-synuclein in the presence and absence of dioxygen at neutral pH. Variations in Trp fluorescence indicate that copper(II) binding is enhanced by the presence of dioxygen, with the apparent dissociation constant (K(d(app))) changing from 100nM (anaerobic) to 10nM (aerobic). To investigate the possible role of methionine oxidation, complementary work focused on synthetic peptide models of the N-terminal Cu(II)-alpha-syn site, MDV(F/W) and M( *)DV(F/W), where M( *)=methionine sulfoxide. Furthermore, we employed circular dichroism (CD) spectroscopy to demonstrate that the phenyl-to-indole (F-->W) substitution does not alter copper(II) binding properties and to confirm the 1:1 metal-peptide binding stoichiometry. CD comparisons also revealed that Met1 oxidation does not affect the copper-peptide conformation and further suggested the possible existence of a Cu(II)-Trp/Phe (cation-pi) interaction.

Kinetics of the β-picoline oxidation to nicotinic acid over vanadia-titania catalyst. 2. Effect of dioxygen and β-picoline

The effect of the dioxygen and β-picoline concentrations on the oxidation of β-picoline to nicotinic acid was studied at 270°C over a 20% V2O5-80% TiO2 (wt. %) catalyst. The study was performed in a differential reactor at constant concentrations of water and nicotinic acid. A considerable excess of dioxygen was shown to be necessary for the formation of nicotinic acid. It was found that the rates of β-picoline transformation into nicotinic acid, pyridine-3-carbaldehyde, pyridine-3-nitrile, pyridine and carbon oxides as well as selectivities to products and the catalyst state depend strongly on the ratio of dioxygen to β-picoline concentrations. The minimum admissible ratio of these concentrations under industrial conditions was determined.

Oxygen dependence of tyrosine hydroxylase

The effects of dioxygen on tyrosine hydroxylase (TH) activity was studied, measuring the formation of DOPA from tyrosine, (3)H(2)O from 3,5-(3)H-tyrosine, or by direct oxygraphic determination of oxygen consumption. A high enzyme activity was observed during the initial 1-2 min of the reactions, followed by a decline in activity, possibly related to a turnover dependent substoichiometrical oxidation of enzyme bound Fe(II) to the inactive Fe(III) state. During the initial reaction phase, apparent K (m)-values of 29-45 microM for dioxygen were determined for all human TH isoforms, i.e. 2-40 times higher than previously reported for TH isolated from animal tissues. After 8 min incubation, the K (m) (O(2))-values had declined to an average of 20 +/- 4 microM. Thus, TH activity may be severely limited by oxygen availability even at moderate hypoxic conditions, and the enzyme is rapidly and turnover dependent inactivated at the experimental conditions commonly employed to measure in vitro activities.

The reactions of cytidine and 2'-deoxycytidine with SO4.- revisited. Pulse radiolysis and product studies.

The reactions of SO4.- with 2'-deoxycytidine 1a and cytidine 1b lead to very different intermediates (base radicals with 1a, sugar radicals with 1b). The present study provides spectral and kinetic data for the various intermediates by pulse radiolysis as well as information on final product yields (free cytosine). Taking these and literature data into account allows us to substantiate but also modify in essential aspects the current mechanistic concept (H. Catterall, M. J. Davies and B. C. Gilbert, J. Chem. Soc., Perkin Trans. 2, 1992, 1379). SO4.- radicals have been generated radiolytically in the reaction of peroxodisulfate with the hydrated electron (and the H. atom). In the reaction of SO4.- with 1a (k = 1.6 x 10(9) dm3 mol-1 s-1), a transient (lambda max = 400 nm, shifted to 450 nm at pH 3) is observed. This absorption is due to two intermediates. The major component (lambda max approximately 385 nm) does not react with O2 and has been attributed to an N-centered radical 4a formed upon sulfate release and deprotonation at nitrogen. The minor component, rapidly wiped out by O2, must be due to C-centered OH-adduct radical(s) 6a and/or 7a suggested to be formed by a water-induced nucleophilic replacement. These radicals decay by second-order kinetics. Free cytosine is only formed in low yields (G = 0.14 x 10(-7) mol J-1 upon electron-beam irradiation). In contrast, 1b gives rise to an intermediate absorbing at lambda max = 530 nm (shifted to 600 nm in acid solution) which rapidly decays (k = 6 x 10(4) s-1). In the presence of O2, the decay is much faster (k approximately 1.3 x 10(9) dm3 mol-1 s-1) indicating that this species must be a C-centered radical. This has been attributed to the C(5)-yl radical 8 formed upon the reaction of the C(2')-OH group with the cytidine SO4(.-)-adduct radical 2b. This reaction competes very effectively with the corresponding reaction of water and the release of sulfate and a proton generating the N-centered radical. Upon the decay of 8, sugar radical 11 is formed with the release of cytosine. The latter is formed with a G value of 2.8 x 10(-7) mol J-1 (85% of primary SO4.-) at high dose rates (electron beam irradiation). At low dose rates (gamma-radiolysis) its yield is increased to 7 x 10(-7) mol J-1 due to a chain reaction involving peroxodisulfate and reducing free radicals. Phosphate buffer prevents the formation of the sugar radical at the SO4(.-)-adduct stage by enhancing the rate of sulfate release by deprotonation of 2b and also by speeding up the decay of the C(5)-yl radical into another (base) radical. Cytosine release in cytidine is mechanistically related to strand breakage in poly(C). Literature data on the effect of dioxygen on strand breakage yields in poly(C) induced by SO4.- (suppressed) and upon photoionisation (unaltered) lead us to conclude that in poly(C) and also in the present system free radical cations are not involved to a major extent. This conclusion modifies an essential aspect of the current mechanistic concept.

Luminescence properties of a cofacial dipalladium porphyrin dimer under argon and in the presence of dioxygen

The preparation and luminescence properties of a dipalladium cofacial porphyrin dimer (DPA) Pd 2 (where DPA is the tetraanion of 1,8-bis(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrin-5-yl)anthracene) are reported and compared together with the photophysical behavior of the known monomeric (OEP)Pd and (TPP)Pd complexes. The effect of dioxygen in the presence and in the absence of the very bulky base, 1-t-butyl-5-phenylimidazole, is also investigated. The title dimer, (DPA) Pd 2, shows fluorescence and phosphorescence in the ps and ms time scale, respectively, with a global intensity lower than that of the porphyrin monomer analogues. The fluorescence sensitivity towards dioxygen quenching appears greater for the dipalladium complex than that of the monoporphyrin derivatives. In the presence of the bulky base, this sensitivity shows a dramatic decrease.

Manganeseporphyrin catalyzed cyclohexene epoxidation by iodosylbenzene: The remarkable effect of the meso-phenyl orthoOH substituent

Catalytic cyclohexene epoxidation with iodosylbenzene by the following Mn-porphyrins bearing ortho substituents on a meso group: MnTTP, MnM 2-OHPTTP(5-(2-hydroxyphenyl)-10,15,20-tritolylporphyrin) and MnM 2-BrPTTP(5-(2-(3-bromo-1-propoxy)phenyl)-10,15,20-tritolylporphyrin), has been studied. The effect of dioxygen and imidazole axial ligand on the yields of epoxide and allylic oxidation products, i.e. alcohol and ketone, was investigated. It was observed that the product selectivity is strongly dependent on the distribution of the Im/MnP species (M, MIm and MIm 2 obtained by spectrophotometric titrations). For MnTPP at Im/MnP=1 the best epoxide yield associated with the maximum MIm concentration was observed. The presence of the OH substituent produced a remarkable effect on the reactivity and catalytic properties of the porphyrin system, i.e. drastic reduction in allylic oxidation, slight increase on the epoxide yield, a decrease on the formation of the MnP/Im complexes and change of the reactivity of the Mn center with PhIO. These results are discussed in terms of a possible electronic interaction of the oxygen ortho substituent and the porphyrin ring.

A new mechanism of the dioxygen effect on ESR spectra of polyaniline

The effect of dioxygen on the ESR spectra of powders of emeraldine salts at 170–230 K was found to depend on the time of storing of the samples at a specific temperature. The kinetics of changes in the ESR spectra reflects the conformational mobility of the polyaniline chains and its supramolecular structure. A new mechanism of the dioxygen effect on the ESR spectra of polyaniline at 50–100 K associated with the condensation of dioxygen in polyaniline pores was proposed.

Effect of dioxygen on ESR spectra of polyaniline

The effect of dioxygen on the width of ESR lines of powders and films of polyaniline was studied at 15–300 K. It was found that the line broadening can decrease or increase, which corresponds to strong or weak exchange, respectively. For dry polyaniline powders and films, maxdimum broadening was observed at 240 K. The findings were explained by spin exchange during collisions of a mobile polaron with immobile O2 molecules bound to the polymer. The values of exchange interaction between the O2 molecule and the polaron and the mobility of the polarons along the polymer chain were found. The mobility of polarons along the polymer chains decreases in the presence of H2O, and dioxygen predominantly adds to the imine N atom.

An Electron Transfer Reaction in the Imidazo[2,1-b]thiazole Series

The C-alkylation reaction of 5-nitro-6-chloromethylimidazo [2,1-b] thiazole by the 2-nitropropane anion is shown to proceed by the S RN 1 mechanism. This mechanism is confirmed by the inhibitory effects of dioxygen, p-dinitrobenzene, cupric chloride and TEMPO

Acid-Catalyzed One-Electron Reduction of Nitrite to Nitric Oxide by an NADH Analog and 1,1′-Dimethylferrocene in the Absence and Presence of Dioxygen

Abstract One-electron reduction of nitrite to nitric oxide proceeds efficiently by an acid-stable NADH analog, 9,10-dihydro-10-methylacridine (two-electron reductant) as well as 1,1′-dimethylferrocene (one-electron reductant) in the presence of perchloric acid in acetonitrile. The effects of dioxygen on both the one-electron and two-electron reductant systems are compared.

ChemInform Abstract: Electrochemical Reactivity of Manganese(II) Porphyrins. Effects of Dioxygen, Benzoic Anhydride, and Axial Ligands.

AbstractCurrents for electrochemical reductions of Mn(III) porphyrins in oxygenated, aprotic media correspond to passage of between one and two electrons per porphyrin, depending on the experimental time scale, the axial base (Py, N‐methylimidazole) present, and the particular porphyrin (tetraphenyl‐, ‐mesityl‐, and ‐p‐methoxyphenylporphyrin).

Electrochemical reactivity of manganese(II) porphyrins. Effects of dioxygen, benzoic anhydride, and axial ligands

Electrochemical reactivity of manganese(II) porphyrins. Effects of dioxygen, benzoic anhydride, and axial ligands

Electrical and magnetic properties of thin films and single crystals of bis(phthalocyaninato)lutetium

The intrinsic conductivities of thin films and single crystals of bis(phthalocyaninato)lutetium, Pc 2Lu, are reported for the first time. Pc 2Lu is shown to be more conductive than the usual metallophthalocyanines (PcM) by more than six orders of magnitude. Electron spin resonance (ESR) determinations on Pc 2Lu in solution, on sublimed powders or on a monocrystalline solvate demonstrate (i) the radical nature of the complex, (ii) the effect of dioxygen on the spin state, (iii) the extent of interaction between the molecular subunits.

1,10-Phenanthroline and 2,2′-bipyridyl complexes of rhodium(III)

The preparation of cis-[Rh(aa)2Cl2]+(aa = 2,2′-bipyridyl or 1,10-phenanthroline) has been re-examined and shown to be catalytic. The preparation and characterisation of cis-[Rh(aa)2X2]n+(X = I–, CN–, H2O, and NH3) and cis-[Rh(aa)2ClY]m+(Y = H2O, NH3, pyridine, and NO2–) is described and the possible mechanisms of base hydrolysis and of substitution by cyanide and by iodide into cis-[Rh(aa)2Cl2]+ are discussed; the effect of dioxygen on these substitutions is also discussed. The resolution of cis-[M(aa)2Cl2]+(M = Rh or Ir) into optical isomers is described and their stability is discussed. A convenient catalytic preparation of [Rh(aa)3]3+ is given.