89 resultados para P450-catalyzed Hydroxylation


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In this review article, we give a general introduction on the mechanisms involved in organic chemiluminescence, where three basic models for excited state formation are presented. The chemiluminescence properties of 1,2-dioxetanes - four membered ring peroxides - are briefly outlined in the second part. In the main part, the mechanisms involved in the decomposition of 1,2-dioxetanes and analogous peroxides are discussed: (i) the unimolecular decomposition of 1,2-dioxetanes; (ii) the electron transfer catalyzed decomposition of peroxides by an intermolecular CIEEL (Chemically Initiated Electron Exchange Luminescence) mechanism; (iii) 1,2-dioxetane decomposition catalyzed by an intramolecular electron transfer mechanism (intramolecular CIEEL). Special emphasis is given to the latter subject, where recent examples with potential analytical applications are presented.

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5-Aminolevulinic acid (ALA) is a heme precursor accumulated in acute intermittent porphyria (AIP), which might be associated with hepatocellular carcinoma (HCC) in symptomatic patients. Under metal catalyzed oxidation, ALA and its cyclic dimerization product, 3,6-dihydropyrazine-2,5-dipropanoic acid, produce reactive oxygen species that damage plasmid and calf thymus DNA bases, increase the steady state level of 8-oxo-7,8-dihydro-2´-deoxyguanosine in liver DNA and promote mitochondrial DNA damage. The final product of ALA, 4,5-dioxovaleric acid (DOVA), is able to alkylate guanine moieties, producing adducts. ALA and DOVA are mutagenic in bacteria. This review shows an up-to-date literature data that reinforce the hypothesis that the DNA damage induced by ALA may be associated with the development of HCC in AIP patients.

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Several polyunsaturated fatty acids (PUFA) belonging to the ômega 6 series, such as cis-6,9,12 gamma-linolenic acid, as well as those of the ômega 3 series, such as cis-5,8,11,14,17-eicosapentaenoic acid and cis-4,7,10,13,16,19-docosahexaenoic acid are of considerable interest due to their nutritional and therapeutic properties. Methods used for the concentration of PUFA from natural sources include urea adduct formation, solvent winterization, supercritical fluid extraction and lipase-catalyzed reaction. Lipases are known to have little reactivity on PUFA and these acids can be enriched by selective hydrolysis, direct esterification of glycerol with PUFA and interesterification. Since lipase reactions are advantageous with respect to fatty acid, positional specificities and mild incubation condition, these enzymes are considered to be suitable for the production of PUFA concentrates for medical purposes.

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Today satellites propulsion is based on the use of monopropellant and/or bipropellant chemical systems. The maneuvering of satellite is based on the hydrazine decomposition micropropulsors catalyzed by metallic iridium supported on g-alumina. This reaction is a surface reaction and is strongly exothermic and implies that the operation of the micropropulsor is controlled by the mass and heat diffusions. For this reason and for the fact that the propulsor operation is frequently in pulsed regime, the catalyst should support high pressure and temperature variations within a short time period. The performance and the durability of the commercial catalyst are jeopardized by the low thermal conductivity of the alumina. The low thermal conductivity of the alumina support restricts the heat diffusion and leads to the formation of hot spots on the catalyst surface causing the metal sintering and/or fractures of the support, resulting in loss of the activity and catalyst destruction. This work presents the synthesis and characterization of new carbon composite support for the active element iridium, in substitution of the commercial catalysts alumina based support. These supports are constituted of carbon nanofibers (30 to 40 nm diameter) supported on a macroscopic carbon felt. These materials present high thermal conductivity and mechanical resistance, as well as the easiness to be shaped with different macroscopic shapes. The mechanical stability and the performance of the iridium supported on the carbon composite support, evaluated in a laboratory scale test in hydrazine decomposition reaction, are superior compared to the commercial catalyst.

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Biological nitrogen fixation, catalyzed by nitrogenases, contributes about half of the nitrogen needed to global agriculture. For forty years synthetic chemists and theoreticians have tried to understand and model the structure and function of this important metalloenzyme. Ten years after the first report on the crystal structure of the MoFe protein, scientists still have not been able to synthesize a chemical equivalent of the FeMo cofactor nor the structure knowledge revealed the key to its catalytic activity. This paper with 104 references presents a review of the most relevant advances in chemical nitrogen fixation and their relation with the nitrogenases.

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Protein phosphorylation-dephosphorylation catalyzed by the opposing and dynamic action of protein kinases and phosphatases probably, is the most crucial chemical reaction taking place in living organisms. Protein phosphatases are classified according to their substrate specificity and sensitivity to inhibitory or activator agents, into two families of protein phosphatases: serine/threonine phosphatases and tyrosine phosphatases (PTPs). PTPs can be divided into 3 groups: tyrosine specific phosphatases, dual and low molecular weight phosphatases. The role of tyrosine phosphorylation in mitogenic signaling is well documented, and one would predict that vanadate, pervanadate and other oxidant agents (protein tyrosine phosphatase inhibitors) may act as a growth stimulator.

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The development of cobalt catalysts to produce hydrogen from ethanol is the goal of this investigation. Co/Al2O3 catalysts were prepared by impregnation and characterized by atomic absorption, nitrogen adsorption, X-ray diffraction, Raman spectroscopy, temperature programmed reduction and carbon analysis. The catalysts contained Co3O4 oxide and Co3+ and Co2+ species interacting with alumina. The cobalt load affects the crystal size and the crystalline structure and higher Co loads influence the reaction mechanism, changing the selectivity of the catalysts, decreasing the amount of CO produced and avoiding the formation of products catalyzed by the support. The ethanol conversion was 50-70% with 10-<1% of CO in the hydrogen.

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The aim of this study is to evaluate the crystal structure of binary mixtures of palm kernel fat and fish oil, before and after chemical and enzymatic interesterification. The crystal structure was analyzed by polarized light microscopy. The addition of fish oil didn't change the palm kernel fat crystallization characteristics, spherullites of types A and B being observed. However, due to chemical and enzymatic interesterification, smaller crystals were obtained. There was no difference between chemical and enzymatic interesterification, probably as a function of acyl migration in discontinuous processes catalyzed by lipases.

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The oxidation of sulfite catalyzed by transition metal ions produces reactive oxysulfur species that can damage plasmid and isolated DNA in vitro. Among the four DNA bases, guanine is the most sensitive to one-electron oxidation promoted by the species formed in the autoxidation of sulfite (HSO5-, HO•, SO3•-, SO4•- and SO5•-) due to its low reduction potential and ability to bind transition metal ions capable to catalyze oxidative processes. Some oxidative DNA lesions are promutagenic and oxidative DNA damage is proposed to play a crucial role in certain human pathologies, including cancer.

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Initially, all major factors that affect the rate of the AldH-catalyzed reaction (enzyme concentration, substrate concentration, temperature and pH) were investigated. Optimal activity was observed between pH values of 7.5 and 9.5 in the temperature range of 25 to 50 ºC. Kinetic parameters, such as Km (2.92 µmol L-1) and Vmax (1.33 10-2 µmol min-1) demonstrate a strong enzyme-substrate affinity. The sensors were based on screen-printed electrodes modified with the Meldola Blue-Reinecke salt (MBRS) combination. Operational conditions (NAD+ and substrate contents, enzyme loading and response time) were optimized. Also, two enzyme immobilization procedures were tested: entrapment in poly(vinyl alcohol) bearing styrylpyridinium groups (PVA-SbQ) and crosslinking with glutaraldehyde. Chronoamperometry was employed to observe the biosensor responses during enzymatic hydrolysis of propionaldehyde and also to construct inhibition curves with maneb and zineb fungicides. Best results were found with the following conditions: [NAD+] = 0.25 mmol L-1; [propionaldehyde] = 80 µmol L-1; enzyme loading = 0.8 U per electrode; response time = 10 min, and inhibition time = 10 min. Current intensities around 103 ± 13 nA with the sensors and good stability was obtained for both immobilization procedures. Detection limits, calculated using 10% inhibition were 31.5 µg L-1 and 35 µg L-1 for maneb and zineb, respectively. Results obtained with other MBRS-modified electrodes consisting of mono and bi-enzymic sensors were compared. The ability to catalyze NADH oxidation by MB was also highlighted.

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The contextualized understanding of concepts in Chemistry by students from other areas is a challenging task. In this experiment, the synthesis of biodiesel is done by base catalyzed transesterification of refined soy oil with methanol at room temperature and common glassware found in any chemistry laboratory. The proposal permits introducing several concepts, such as that of emulsion, viscosity and catalysis to illustrate an activity based on an actual problem. In this didactic approach, some common problems of biodiesel production, such as soap formation and phase separation, are introduced into the procedure in order to raise questions and motivate the students to participate in the experimental work and stimulate reflections about critical aspects of biodiesel production. This experiment was carried out in the first semester of 2006, in experimental general chemistry taken by physics and agricultural, civil and chemical engineering students of UNICAMP.

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The development of the palladium catalyzed cross-coupling reactions employing organosilicon compounds is described. Important synthetic methods utilized to prepare organosilicons and different types of cross-coupling reactions involving these compounds are presented. Mechanistic aspects are also discussed.

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In this work we describe both a chromatographic purification procedure and a spot test for the enzyme peroxidase (POD: EC 1.11.1.7). The enzyme was obtained from crude extracts of sweet potatoes and the chromatographic enzyme purification procedure resulted in several fractions. Therefore a simple, fast and economic spot test for monitoring peroxidase during the purification procedure was developed. The spot test is based on the reaction of hydrogen peroxide and guaiacol, which is catalyzed by the presence of peroxidase yielding the colored tetraguaiacol.

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Monoglycerides (MAG) are non-ionic surfactants, widely used in the pharmaceutical, food and cosmetic industries. Although MAGs are manufactured on an industrial scale by chemical glycerolysis of oils and fats, new developments in lipase catalyzed synthesis have been studied as an alternative to the classical method seeking to use clean technology and green chemistry. In this work, different methods such as glycerolysis, selective hydrolysis of fats and oils, and esterification of fatty acids or transesterification of esters with glycerol are presented. The properties and applications of the monoglycerides are also included in this review.

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The kinetic study of decomposition of hydrogen peroxide catalyzed by activated carbon was carried out. The effect of concentrations of reactants and temperature were experimentally studied. Kinetic data were evaluated using differential method of initial rates of reaction. When a typical kinetic law for reactions in homogeneous phase is used, first order of reaction is obtained for hydrogen peroxide and activated carbon, and activation energy of 27 kJ mol-1 for the reaction was estimated. Experimentally was observed that surface of activated carbon is chemically modified during decomposition of hydrogen peroxide, based on this result a scheme of reaction was proposed and evaluated. Experimental data fits very well to a Langmuir- Hinshelwood kinetic model and activation energy of 40 kJ mol-1 was estimated for reaction in heterogeneous phase.