Oxidants methylene blue

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

This enzyme, sometimes also called the Schardinger enzyme, occurs in milk. It is capable of " oxidising" acetaldehyde to acetic acid, and also the purine bases xanthine and hypoxanthine to uric acid. The former reaction is not a simple direct oxidation and is assumed to take place as follows. The enzyme activates the hydrated form of the aldehyde so that it readily parts w ith two hydrogen atoms in the presence of a suitable hydrogen acceptor such as methylene-blue the latter being reduced to the colourless leuco-compound. The oxidation of certain substrates will not take place in the absence of such a hydrogen acceptor. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

The catalysis by Mo(VI) of the oxidation of N2H5 to N2 by methylene blue depends on the steps given above, Mo(VI) being regenerated by methylene blue oxidation of the Mo(V) dimer . The latter reaction was studied independently and... 

Photochemical irradiation of dimethyl and diethyl sulphoxides yields the corresponding sulphone in the presence of air and a photosensitizer such as methylene blue in yields up to 99% . Sulphoxides are also oxidized when they act as traps for persulphoxides, the intermediate formed on reaction of a sulphide with photochemically generated singlet oxygen - , equation (9). Isotope studies have shown that such reactions proceed through a linear sulphurane intermediate . Persulphones also react with sulphoxides in a similar manner , equation (10). 

When Methylene Blue is reduced, the yellowish leuco cannot be isolated due to instant air oxidation. Benzoylation of the leuco form provides stabilization. There are also leuco thiazine dyes stable enough to be isolated without the need for aroylation. 

As in the case of the leucos Azure A and B, the exocyclic acyl group is not eliminated on oxidation, resulting in a Methylene Blue-type cationic... 

Fig. 3.12 The dependence on pH of the oxidation-reduction potential for c0x = cRcd (1) 6-dibromphenol indophenol, (2) Lauth s violet, (3) methylene blue, (4) ferricytochrome c/ferrocytochrome c, (5) indigo-carmine... 

In some studies it was shown that (3-carotene decomposes more rapidly than lutein and zeaxan-thin when exposed to oxidants or light in the presence and absence of rose bengal as a photosensitizer (Hurst et al., 2004 Ojima et al., 1993 Siems et al., 1999). However, it is not a rule, as lutein and zeaxanthin are depleted faster than (3-carotene during methylene blue photosensitized oxidation of human plasma (Ojima et al., 1993). 

Bayati, M.R., Golestani-Fard, F., and Moshfegh, A.Z. (2010) Visible photodecomposition of methylene blue over micro arc oxidized WOj-loaded Ti02 nano-porous layers. Applied Catalysis A General, 382 (2), 322-331. 

Jang, Y.J., Simer, C. and Ohm, T. (2006) Comparison of zinc oxide nanoparticles and its nano-crystalline particles on the photocatalytic degradation of methylene blue. Materials Research Bulletin, 41,67-77. 

The photosensitized results are from I.B.C. Matheson and J. Lee 118h It is seen that the quantum yields in photosensitized oxidation depend on the concentrations of luminol and base, and on temperature. At higher temperature (50°) and low luminol concentrations, the quantum yields reached those of hemin-catalyzed hydrogen peroxide oxidation of luminol in aqueous-alkaline solution. Primary products of the photosensitized oxidation are singlet oxygen (1Ag02) or a photoperoxide derived from methylene blue, but neither of these is directly responsible for the luminol chemiluminescence. 

For Aspergillus niger extracellular endo-D-galacturonanase, the role of histidine in the enzyme reaction was investigated by the method of photo-oxidative inactivation, catalyzed by Methylene Blue.140 The inactivation of the enzyme was paralleled by the decomposition of histidine. The similarity of pH profiles, as well as the values of the rate constants of enzyme inactivation (4.0 X 10-2 min-1) and of decomposition of histidine (3.9 X 10-2 min-1), indicate that one of the five histidine residues present in the molecule of the enzyme141 is essential for its activity. 

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