Kinetic and mechanistic investigation of the ozonolysis of 2,4-xylidine (2,4-dimethyl-aniline) in acidic aqueous solution

The ozonolysis of 2,4-xylidine (2,4-dimethyl-aniline) in acidic aqueous solution was investigated by determining the major reaction products and their evolution as a function of the reaction time and their dependence on the pH of the reaction system. 2,4-Dimethyl-nitrobenzene and 2,4-dimethyl-phenol were found to be primary reaction products; their formation might be explained by electron transfer and substitution reactions. 2,4-Dimethyl-phenol was further oxidized yielding 2,4-dimethyl- and/or 4,6-dimethyl-resorcinol by electrophilic addition of HO(center dot) radicals. The best fitting phenomenological kinetic model and the good convergence of calculated and experimentally determined rate constants imply two additional competitive pathways of substrate oxidation: (i) electrophilic addition of HO(center dot) radicals and fast subsequent substitution would also yield the resorcinol derivatives. (ii) Substrate and isolated products are thought to be oxidized by hydrogen abstraction at the benzylic sites, but the corresponding products (alcohols, aldehydes, and carboxylic acids) could not be identified. Fe(II) was added to probe for the presence of H(2)O(2), but had no or only a minor effect on the kinetics of the ozonolysis. (c) 2009 Elsevier B.V. All rights reserved.

Mode of cembranoid action on embryonic muscle acetylcholine receptor

The mechanism of eupalmerin acetate (EUAC) actions on the embryonic muscle nicotinic acetylcholine receptor (nAChR) in BC3H-1 cells was studied by using whole-cell and single-channel patch-clamp current measurements. With whole-cell currents, EUAC did not act as an agonist on this receptor. Coapplication of 30 mu M EUAC with 50 mu M, 100 N, or 500 mu M carbamoylcholine (CCh) reversibly inhibited the current amplitude, whereas, with 20 mu M CCh, current was increased above control values in the presence of EUAC. EUAC concentration curves (0.01-40 N) obtained with 100 mu M and 500 mu M CCh displayed slope coefficients, n(H), significantly smaller than one, suggesting that EUAC bound to several sites with widely differing affinities on the receptor molecule. The apparent rate of receptor desensitization in the presence of EUAC and CCh was either slower than or equal to that obtained with CCh alone. The major finding from single-channel studies was that EUAC did not affect single-channel conductance or the ability of CCh to interact with the receptor. Instead, EUAC acted by increasing the channel closing rate constant. The results are not consistent with the competitive model for EUAC inhibition, with the sequential open-channel block model, or with inhibition by increased desensitization. The data are best accounted for by a model in which EUAC acts by closed-channel block at low concentrations, by positive modulation at intermediate concentrations, and by negative allosteric modulation of the open channel at high concentrations. (c) 2007 Wiley-Liss, Inc.

Heterogeneous photocatalytic degradation of acid dyes in aqueous TiO2 suspensions: Kinetics and toxicity

This work assesses the photocatalytic (TiO2/UV) degradation of a simulated acid dye bath (Yellow 3, Red 51, Blue 74, and auxiliary chemicals). Color and phytotoxicity removal were monitored by spectrophotometry and lettuce (Lactuca sativa) seeds as the test organism, respectively. Mineralization was determined by DOC analyses. Photocatalytic, photolytic, and adsorption experiments were performed, showing that adsorption was negligible. After 240 minutes of irradiation, it was achieved 96% and 78% of color removal with photocatalysis and photolysis, respectively. 37% of mineralization occurred with photocatalysis only. The dye bath was rendered completely non-toxic after 60 minutes of photocatalytic treatment; the same result was only achieved with photolysis after 90 minutes. A kinetic model composed of two first-order in series reactions was used. The first photocatalytic decolorization rate constant was k(1) = 0.062 min(-1) and the second k(2) = 0.0043 min(-1), approximately two times greater than the photolytic ones.

Quenching of excited states of red-pigment zinc protoporphyrin IX by hemin and natural reductors in dry-cured hams

Zinc protoporphyrin IX (ZnPP), the major red pigment in hams dry-cured without nitrates/nitrites, is an efficient photosensitizer, which upon absorption of visible light forms short-lived excited singlet state ((1)ZnPP*) and by intersystem crossing yields the very reactive triplet-excited state ((3)ZnPP*). Using nano-second laser flash photolysis and transient absorption spectroscopy NADH, ascorbic acid, hemin and dehydroascorbic acid were each found to be efficient quenchers of (3)ZnPP*. The deactivation followed, in homogeneous dimethyl sulfoxide (DMSO) or DMSO:water (1:1) solutions, second-order kinetics. The rate constant for ascorbic acid and NADH for reductive quenching of (3)ZnPP* was at 25 A degrees C found to be 7.5 +/- A 0.1 x 10(4) L mol(-1) s(-1) and 6.3 +/- A 0.1 x 10(5) L mol(-1) s(-1), respectively. The polyphenols catechin and quercetin had no effect on (3)ZnPP*. The quenching rate constant for oxidative deactivation of (3)ZnPP* by dehydroascorbic acid and hemin was at 25 A degrees C: 1.6 +/- A 0.1 x 10(5) L mol(-1) s(-1) and 1.47 +/- A 0.1 x 10(9) L mol(-1) s(-1), respectively. Oxidized glutathione did not act as an oxidative quencher for (3)ZnPP*. After photoexcitation of ZnPP to (1)ZnPP*, fluorescence was only found to be quenched by the presence of hemin in a diffusion-controlled reaction. The efficient deactivation of (3)ZnPP* and (1)ZnPP* by the metalloporphyrin (hemin) naturally present in meat may accordingly inherently protect meat proteins and lipids against ZnPP photosensitized oxidation.

Light-Induced Oxidation of Unsaturated Lipids as Sensitized by Flavins

Triplet-excited riboflavin ((3)RF*) was found by laser flash photolysis to be quenched by polyunsaturated fatty acid methyl esters in tert-butanol/water (7:3, v/v) in a second-order reaction with k similar to 3.0 x 10(5) L mol(-1) s(-1) at 25 degrees C for methyl linoleate and 3.1 x 10(6) L mol(-1) s(-1), with Delta H double dagger = 22.6 kJ mol(-1) and Delta S double dagger = -62.3 J K(-1) mol(-1), for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <10(4) L mol(-1) s(-1). For comparison, beta-casein was found to have a rate constant k similar to 4.9 x 10(8) L mol(-1) s(-1). Singlet-excited flavin was not quenched by the esters as evidenced by insensitivity of steady-state fluorescence to their presence. Density functional theory (DFT) calculations showed that electron transfer from unsaturated fatty acid esters to triplet-excited flavins is endergonic, while a formal hydrogen atom transfer is exergonic (Delta G(HAT)degrees = -114.3, -151.2, and -151.2 kJ mol(-1) for oleate, linoleate, and linolenate, respectively, in acetonitrile). The reaction is driven by acidity of the lipid cation radical for which a pK(a) similar to -0.12 was estimated by DFT calculations. Absence of electrochemical activity in acetonitrile during cyclic voltammetry up to 2.0 V versus NHE confirmed that Delta G(ET)degrees > 0 for electron transfer. Interaction of methyl esters with (3)RF* is considered as initiation of the radical chain, which is subsequently propagated by combination reactions with residual oxygen. In this respect, carbon-centered and alkoxyl radicals were detected using the spin trapping technique in combination with electron paramagnetic resonance spectroscopy. Moreover, quenching of 3RF* yields, directly or indirectly, radical species which are capable of initiating oxidation in unsaturated fatty acid methyl esters. Still, deactivation of triplet-excited flavins by lipid derivatives was slower than by proteins (factor up to 10(4)), which react preferentially by electron transfer. Depending on the reaction environment in biological systems (including food), protein radicals are expected to interfere in the mechanism of light-induced lipid oxidation.

Photodegradation of Folate Sensitized by Riboflavin

Folate is shown to react with singlet-excited state of riboflavin in a diffusion controlled reaction and with triplet-excited state of riboflavin in a somewhat slower reaction with (3)k(q) = 4.8 x 10(8) L mol(-1) s(-1) in aqueous phosphate buffer at pH 7.4, ionic strength of 0.2 mol L(-1), and 25 degrees C. Singlet quenching is assigned as photo-induced reductive electron transfer from ground state folate to singlet-excited riboflavin, while triplet quenching is assigned as one-electron transfer rather than hydrogen atom transfer from folate to triplet-excited riboflavin, as the reaction quantum yield, phi = 0.32, is hardly influenced by solvent change from water to deuterium oxide, phi = 0.37. Cyclic voltammetry showed an irreversible two-electron anodic process for folate, E = 1.14 V versus NHE at a scan-rate of 50 mV s(-1), which appears to be kinetically controlled by the heterogeneous electron transfer from the substrates to the electrode. Main products of folate photooxidation sensitized by riboflavin were pterin-6-carboxylic acid and p-aminobenzoyl-L-glutamic acid as shown by liquid chromatographic ion-trap mass spectrometry (LC-IT-MS).

Heterogeneous photocatalytic degradation of reactive dyes in aqueous TiO(2) suspensions: Decolorization kinetics

This work assesses the photocatalytic (TiO(2)/UV) degradation of a simulated reactive dye bath (Black 5, Red 239, Yellow 17, and auxiliary chemicals). Color removal was monitored by spectrophotometry. Mineralization was determined by DOC analyses. Photocatalytic, photolytic, and adsorption experiments were performed, showing that adsorption was negligible. After 30 min of irradiation, it was achieved 97% and 40% of color removal with photocatalysis and photolysis, respectively. No mineralization occurred within 30 min. A kinetic model composed of two, first-order in-series reactions was used. The first photocatalytic decolorization rate constant was k(1) = 2.6 min(-1) and the second k(2) = 0.011 min(-1). The fast decolorization of Reactive Black 5 dye is an indication that the number of azo and vinylsulfone groups in the dye molecule maybe a determining factor for the increased photolytic and photocatalytic color removal and degradation rates. (C) 2008 Elsevier B.V. All rights reserved.

Photochemical Production of MicromolarSuperoxideStandards in Aqueous SolutionT

Superoxide(O2-) is a reactive free radical that rapidly undergoes disproportionation to hydrogen peroxide and oxygen. This property makes preparation of superoxide standard for instrument calibration difficult. McDowell et al. (1983) showed photolysis of ketone and alcohol as a convenient method to generate superoxide through triplet and radical intermediates reacting with molecular oxygen. This study expands on this past work and investigates detailed mechanism of the reaction.

Photochemical Generation and Intramolecular Chemistry of β-Alkoxycarbenes

The photolytic phenanthrene-based precursors for both β-methoxycarbene and β-ethoxycarbene were synthesized with and without a deuterium label attached to the a carbon. The incorporation of this deuterium label allowed distinction between a 1, 2-H shift and a 1, 2-O shift pathway to the respective alkyl vinyl ether, without the influence of a primary kinetic isotope effect. Photolyses of these precursors gave rearrangement products of the expected β-alkoxycarbenes. In the case of β-methoxycarbene, no methyl vinyl ether was observed due to its volatility. However, the appearance of aldehyde peaks in the NMR spectra, from an apparent further rearrangement to acetaldehyde through an enol intermediate, indicated that a 1,2-H shift had occurred. Ethyl vinyl ether was isolated following the photolysis of the β-ethoxycarbene precursor. Quantification of the two pathways showed less than 2% undergoing an ethoxy shift to the ethyl vinyl ether. Yield experiments on this photolysis demonstrated a maximum yield of β-ethoxycarbene as 43%, though this decreased as the experiment continued. Computational work on the β-ethoxycarbene system indicates that the triplet scate is more stable than the singlet. In addition, the activation energy to the 1.2-H shift pathway is remarkably low and is clearly consistent with the observed overwhelming preference for this pathway in the experiment.

Analytical Methods in Photoelectrochemical Treatment of Phenol

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Photodegradation of sulfamethoxazole in various aqueous media: Persistence, toxicity and photoproducts assessment

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

The sensitization of the ligand-field state of the hexaammineruthenium(II) complex ion by organic compounds

The sensitized photolysis of [Ru(NH3)(6)](2+) by the organic dye rhodamine B and biacetyl was studied under conditions in which only the sensitizer absorbs. The reaction products resulting from ammonia aquation and Ru(II) to Ru(III) oxidation are the same for direct and sensitized photolysis. The energy transfer rate constant, calculated from the fluorescence quenching of rhodamine B, is similar to that estimated from the limiting quantum yield of the photosensitized photoaquation of the complex. Both reactions originate from a common reactive low-lying ligand-field (LF) state, which is also responsible for the direct photolysis reactions. This state, which leads directly to photoaquation, seems to have a certain charge transfer to solvent (CTTS) character, which is responsible for the photo-oxidation products. Sensitization is effective with rhodamine B (17 450 cm(-1)) and biacetyl (19 000 cm(-1)), whereas no reaction is observed with neutral red (16 900 cm(-1)). These results show that the excited state responsible for the photochemical reactions lies in the energy range between 16 900 cm(-1) and 17 700 cm(-1) and possesses spin-orbit character.

Degradation of the herbicide tebuthiuron using solar photo-Fenton process and ferric citrate complex at circumneutral pH

The influence of pH on the degradation of the herbicide tebuthiuron (TBH) was investigated using in situ generated Fe(III)-citrate complexes (Fe:cit) submitted to the photo-Fenton process under solar irradiation. Using Fe:cit in a wide pH range (2.5-7.5), 100-78% TBH oxidation was achieved respectively from a UV dose of 2.0 J cm(-2) (15 min). Moreover, the oxidation of TBH obtained in the presence of Fe:cit at pH 6.0 was higher than that obtained using Fe(NO3)3 at pH 2.5. A similar behavior is observed for the removal of total organic carbon (TOC) in TBH solutions. In the presence of Fe:cit, 20% and 85% of TOC was removed at pH 7.5 and 2.5, respectively, after 7.5 J cm-2 irradiation, while no mineralization was observed employing Fe(NO3)(3) for the same UV dose. Using Fe(NO3)(3), mineralization was observed only after 11 J cm-2 (8%). A higher mineralization rate was obtained with Fe(NO3)(3) only when a concentration three times higher was employed at pH 2.5. Besides the high efficiency of TBH degradation observed using the ferric citrate complex in the solar photo-Fenton process, it also offers the advantage of application at a pH of up to 7.5. (c) 2007 Elsevier B.V. All rights reserved.

A Validated Stability-Indicating LC Method for Orbifloxacin in the Presence of Degradation Products

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Kinetics and mechanism of the forward and reverse reactions between N,N′-dimethyl-4,4′-bipyridinium and hexacyanoferrate(II)

The kinetics of the hexacyanoferrate(III)-N,N′-dimethyl-4,4′-bipyridinium radical (MV+) reaction was studied by a laser flash photolysis technique. The radical was generated, in the presence of Fe(CN)6 3-, by quenching the excited state *Ru(bpy)3 2+ with MV2+. The second-order rate constant for the Fe(CN)6 3--MV+ reaction is (7.6 ± 0.5) × 109 M-1 s-1 at 23°C and ionic strength 0.10 M. Comparison with the rate constants calculated for the diffusion-controlled reaction (4.7 × 109 M-1 s-1) and the activation-controlled reaction (5.2 × 1012 M-1 s-1, on the basis of self-exchange rate constants of 8.0 × 105 M-1 s-1 and 1.9 × 104 M-1 s-1 for the MV2+/+ and Fe(CN)6 3-/4- couples, respectively) leads to the conclusion that the Fe(CN)6 3--MV+ reaction is diffusion controlled. The rate constant for the Fe(CN)6-MV2+ reaction, calculated from the rate constant for the Fe(CN)6 3--MV+ reaction and the appropriate equilibrium constant, is 2.4 × 10-5 M-1 s-1 at 23°C and ionic strength 0.10 M. Microscopic reversibility considerations require that the Fe(CN)6 4--MV2+ reaction be controlled by the dissociation of the successor complex Fe(CN)6 3-|MV+. The thermal and optical electron transfers in the ion pair Fe(CN)6 4-|MV2+ and in related systems are analyzed and discussed. © 1982 American Chemical Society.