Fractionation oxidation/reduction

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

Indicine IV-oxide (169) (Scheme 36) is a clinically important pyrrolizidine alkaloid being used in the treatment of neoplasms. The compound is an attractive drug candidate because it does not have the acute toxicity observed in other pyrrolizidine alkaloids. Indicine IV-oxide apparently demonstrates increased biological activity and toxicity after reduction to the tertiary amine. Duffel and Gillespie (90) demonstrated that horseradish peroxidase catalyzes the reduction of indicine IV-oxide to indicine in an anaerobic reaction requiring a reduced pyridine nucleotide (either NADH or NADPH) and a flavin coenzyme (FMN or FAD). Rat liver microsomes and the 100,000 x g supernatant fraction also catalyze the reduction of the IV-oxide, and cofactor requirements and inhibition characteristics with these enzyme systems are similar to those exhibited by horseradish peroxidase. Sodium azide inhibited the TV-oxide reduction reaction, while aminotriazole did not. With rat liver microsomes, IV-octylamine decreased... 

Cross, A. R., Harper, A. M., Segal, A. W. (1981). Oxidation-reduction properties of the cytochrome b found in the plasma-membrane fraction of human neutrophils. Biochem. J. 194, 599-606. 

Johnson CM, Roden EE, Welch SA, Beard BL (2004a) Experimental constraints on Ee isotope fractionation during magnetite and Ee carbonate formation coupled to dissimilatory hydrous ferric oxide reduction. Geochim Cosmochim Acta, in press... 

Roden EE, Leonardo MR, Ferris FG (2002) Immobilization of strontium during iron biomineralization coupled to dissimilatory hydrous ferric oxide reduction. Geochim Cosmochim Acta 66 2823-2839 Roe JE, Anbar AD, Barling J (2003) Nonbiological fractionation of Ee isotopes evidence of an equilibrium isotope effect. Chem Geol 195 69-85... 

Element fractionation resulting from oxidation/reduction... 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

AuCl2- or even a higher order complex. While it is possible that the enhanced capacity of Au1 for complexation with soft ligands may account for the disparate distributions of Ag and Au, fractionation of Au and Ag may also be caused by a significant Aum chemistry in seawater. The major species of Au111 in seawater are expected to be Au(OH)3 or Au(OH)3C1 (Baes and Mesmer, 1976). Although the analysis ofTumer etal. (1981) indicated that the field of Aum stability is somewhat outside the oxidation-reduction conditions encountered in seawater, a paucity of direct formation-constant observations for both Aum and Au1 creates substantial uncertainties. Furthermore, with respect to thermodynamic predictions of oxidation-reduction behaviour the ocean is not a system at equilibrium. 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

One of these approaches consists of assuming that the proportion of electrons involved in a particular electrochemical process (w-leclmde) can be related with measurable parameters, assuming that the difference between the cell potential and its oxidation/reduction potential (V,) is the driving force in the distribution of electrons (linear dependence with the overpotentials). Thus, it can be assumed that the fraction of the applied current intensity used in each process depends on the cell potential (AF ork) and on the oxidation (or reduction) potential (AF)) of each process. The fraction can be calculated using (4.25), where AFwork = Fwork — Freference and A Vi = Vi — Freference- In all cases, AFwork must be greater than AFj, otherwise process i cannot develop. 

Chum HL, Ratcliff M, Schroeder HA, Sopher DW (1984)Electrochemistry of biomass-derived materials. I. Characterization, fractionation, and reductive electrolysis of ethanol-extracted explosively depressurized aspen lignin. J Wood Chem Technol 4 505-532 Crozier TE, Johnson DC, Thompson NS (1979) Changes in a southern pine dioxane lignin on oxidation with oxygen in sodium carbonate media. Tappi 62 107-111 Ekman K, Enkvist T (1955) Some determinations of weak and strong acids in various lignin preparations and pulps. Pap Puu 37 369-382... 

Carbon isotopes are fractionated in biological cycles and in inorganic oxidation-reduction reactions. In equilibrium systems of various oxidation states, 13C enrichment up to 500 °K occurs in the following sequence ... 

Although we have considered many reactions so far, we have examined only a tiny fraction of the millions of possible chemical reactions. To make sense of all these reactions, we need some system for grouping reactions into classes. Although there are many different ways to do this, we will use the system most commonly used by practicing chemists. They divide reactions into the following groups precipitation reactions, acid-base reactions, and oxidation-reduction reactions. 

The effects of oxidation - reduction cycles on the activity and selectivity of supported Rh catalysts were investigated using the hydrogenolysis of methylcyclopentane (MCP) as a test reaction. Prom the analysis of catalytic properties and reduction profiles it Is concluded that, on silica-supported catalysts, following the initial oxidation at 400 C successive reduction treatments at increasing temperatures cause a progressive reconstruction of the Rh particles. On y-alumina-supported catalysts the situation is more complex. The interaction of Rh with the support during the Initial oxidation makes a fraction of the Rh Inaccessible to the gas phase. Only after subsequent oxidation - reduction cycles do they behave like the silica-supported catalysts... 

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