Carbohydrates oxidative reactions

Apart from these academic model catalysts a more practical field of application for nanostructured metal colloids in heterogeneous catalysis resides in the manufacture of fine chemicals. ] Bimetallic, even multimetallic precursors on various supports, eventually promoted by dopants, have been successfully tested as highly active, selective catalysts of remarkable durability. For example, surfactant-stabilized Pd-Pt-charcoal catalysts, promoted by bismuth have been shown to be superior catalysts for the carbohydrate oxidation reaction shown in Eq. (1) ... 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

The substrates of catabolism—proteins, carbohydrates, and lipids—are good sources of chemical energy because the carbon atoms in these molecules are in a relatively reduced state (Figure 18.9). In the oxidative reactions of catabolism, reducing equivalents are released from these substrates, often in the form of hydride ions (a proton coupled with two electrons, H ). These hydride ions are transferred in enzymatic dehydrogenase reactions from the substrates... 

A reaction of practical importance is the oxidation of a carbohydrate aldehyde group to a carboxyl group. This is the basis for a process converting glucose to calcium gluconate, a substance of pharmaceutical interest. The oxidation reaction occurs at graphite electrodes in the presence of the Brj/Br" redox system. Calcium salt is added to the solution to prevent further oxidation of free gluconic acid. 

Dissolve a glycoprotein to be oxidized in 0.1 M sodium acetate, pH 5.5 (oxidation buffer), at a concentration of 2-10mg/ml. PBS at physiological pH may be used for this reaction, as well. The use of cold buffers for the oxidation step will limit the extent of carbohydrate oxidation and the potential for protein oxidation. 

The 1,3,2-dioxastannolanes are important in organic synthesis because they can readily be derived from dialkyltin oxide and 1,2-diols, as in carbohydrates the reaction can be carried out in toluene in a few minutes under microwave irradiation.387 The dioxastannolanes can then be subjected to regioselective reaction with an electrophile such as an acyl chloride (Equation (140)) or sulfonyl chloride, or an isocyanate. The acylation or sulfonation can be carried out with catalytic amounts of the dialkyltin oxide, including the recoverable (C6F13CH2CH2)2Sn0.388... 

The first suggestion that substrates in carbohydrate oxidation might exert catalytic effects on the oxidation of other intermediates (cf.earlier demonstration of such action in the urea cycle by Krebs and Henseleit, 1932 see Chapter 6) arose from the work of Szent-Gyorgi (1936). He demonstrated that succinate and its 4C oxidation products catalytically stimulated the rate of respiration by muscle tissues. He also observed that reactions between the 4C intermediates were reversible and that if muscle was incubated with oxaloacetate, fumarate and malate made up 50-75% of the products, 2-oxoglutarate 10-25% and, significantly, 1-2% of the C was converted to citrate. These observations were... 

FAD is a coenzyme for a large number of oxidation reactions, largely of carbohydrates. Correspondingly, FADH2 is a coenzyme for a number of reduction reactions. Certain of the reactions of FAD and FADH2 are involved in the electron transport chain in mitochondria, associated with the synthesis of ATP. We shall see examples in chapter 17. 

H. W. Arnold and W, L. Bvans, The mechanism of carbohydrate oxidation. Fait XXB. The preparation and reactions of glyceraldehyde diethyl mereaptal, J. Am. Chan. Soc. 58 1950... 

For the release of an unaltered base, the sugar moiety must be damaged. In principle, the base could already be released from a radical site at the sugar moiety, i.e. on the time-scale of the lifetime of the DNA radicals. The observation of 2-dRL incorporated into DNA as a product formed upon OH attack shows that a damage at C(l ) contributes to the release of an unaltered base. In the carbohydrate series, hydrolytic scission at the glycosidic linkage when this site contains a free-radical is a well-documented phenomenon, and it has been estimated that the rate of reaction must be faster than 35 s 1 (von Sonntag and Schuchmann 2001). As it stands, it cannot be excluded, that under certain conditions the base release from the C(l ) radical [reaction (38)] occurs in competition to its oxidation [reaction (2)]. In a cellular environment, there is also the reduction of DNA... 

Preparations of HRP may vary in their carbohydrate content and this can affect the oxidation reaction. Free carbohydrate can be removed by gel filtration. Increasing the sodium periodate concentration to 0.2 M can also help, but further increases lead to inactivation of the peroxidase. 

H. S. Isbell, Enolization and oxidation reactions of reducing sugars, in R. F. Gould (Ed.), Carbohydrates in solution, Advances in Chemistry, ACS, Washington, 117 (1973) 70-87. 

A. G. Fadnis, Metal-ion oxidation reactions of monosaccharides a kinetic study, Carbohydr. Res., 146 (1986) 97-105. 

Pulsed amperometric detection is used for the analysis of carbohydrates and other nonchromophoric molecules such as alcohols, aldehydes, and amines. In the DC mode the products of the oxidation reaction of those compounds poison the surface of the working electrode, and further analyte oxidation is inhibited. This results in peaks that decrease rapidly in height. To maintain a stable and active electrode surface, alternatively positive and negative potentials are repeatedly pulsed. 

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