Dismutation reactions

Olefin Metathesis. The olefin metathesis (dismutation) reaction (30), discovered by Eleuterio (31), converts olefins to lower and higher molecular weight olefins. For example, propylene is converted into ethylene and butene... 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Under conditions where the dismutation reaction is slow the exchange between Au(III) and Au(I) has been shown to proceed at a measurable rate at 0 °C in 0.09 M HCl, an exchange half-time of about 2 min was observed. The isotopic method ( Au) and a separation method based on the precipitation of dipyridine -chloroaurate(III) was used to obtain data. An acceleration in the exchange rate was observed as the HCl concentration was increased. ... 

Wolfgang and Dodson suggest that the exchange process in the absence of cyanide is controlled by a rapid dismutation reaction... 

Vaz ADN, S Chakraborty, V Massey (1995) Old yellow enzyme aromatization of cyclic enones and the mechanism of a novel dismutation reaction. Biochemistry 34 4246-4256. 

Initiated by a dismutation reaction, yielding formic acid and methanol as products, if the microorganism has a formaldehyde dismutase enzyme... 

Aryl derivatives of the heavier group 6 derivatives can also serve for the synthesis of arylmercury compounds.52 For instance, treatment of the diaryltellurium complex 30 with HgCl2 leads to the formation of a coordination complex which undergoes a slow dismutation reaction to afford the arylmercury chloride derivative 31 and the corresponding tellurium species (Scheme 3).53... 

Due to the rapidity of the spontaneous dismutation reactions, the steady-state concentrations of 02 achieved by chemical or by enzymatic reactions are usually quite low. The physical methods for detecting 02, although direct and unequivocal, are restricted to measurements of steady-state concentrations and are thus often found to lack of sensitivity. For distince, due to the reason mentioned above, when the EPR method was employed for studying the 02 production by xanthine oxidase, it was necessary to use a high concentration of the reactants and to work at elevated pH so... 

An alternative preparative route to R2QER acid esters (Q = P, As, Sb, Bi E = S, Se, Te) of interest due to its generally applicability to all the heavier group 15 and 16 elements, is the dismutation reaction between diorganodichalco-genides and tetraorgano-diphosphines, -diarsines, -distibines or -dibismuthines (Equation 17).2... 

Then, the couple for acetate ion can be written as a dismutation reaction,... 

This is a dismutation reaction because one molecule of O2 is reduced whilst the other is oxidised ... 

Some sulfate-reducing bacteria have been shown to contain SODs and catalases, but other species do not demonstrate the expected activities for these enzymes (Hatchikian et al. 1977 van Niel and Gottschal 1998 Dos Santos et al. 2000). The apparent absence of these enzymes can be rationalized on the basis that their catalytic dismutation reactions (Eqs. 10.1 and 10.2) generate dioxygen, which may be disadvantageous for strict anaerobes but raises the question of how these organisms protect themselves against transient air exposure. 

It is believed that MnSOD plays a pivotal role in many diseases. There have been many reviews of the biochemistry of MnSOD " and focusing on the structural aspects of the enzyme. Four different types of SOD are known, a Cu/Zn-containing SOD, a FeSOD, a NiSOD, and MnSOD. MnSODs, which are structurally related to the FeSODs, have a of 23,000 ( 200 amino acids) and function as a dimer or as a tetramer. MnSOD catalyzes the dismutation reaction by cycling between the - -2 and +3 oxidation states. One proton is taken up by the system in each step (Equation (2)) ... 

The dismutation reaction of a mixture of flavoquinone and flavohydroquinone in DMF was studied kinetically in the presence and absence of redox-inactive metal ions This system shows a very complex kinetic behaviour with the formation... 

The enzyme complex that catalyses steps d to/of Fig. 25-20 has an unusual composition. An a3 trimer of 23.5-kDa subunits is contained within an icosahe-dral shell of 60 16-kDa (3 subunits, similar to the protein coats of the icosahedral viruses (Chapter 7). The (3 subunits catalyze the formation of dimethylribityllu-mazine (steps d, e), while the a3 trimer catalyzes the dismutation reaction of step/, the final step in riboflavin formation.365 A separate bifunctional bacterial ATP-dependent synthetase phosphorylates riboflavin and adds the adenylyl group to form FAD.366 Two separate mammalian enzymes are required.367 Synthesis of deazaflavins of methanogens (Fig. 15-22) follows pathways similar to those of riboflavin. However, the phenolic ring of the deazaflavin originates from the shikimate pathway.368... 

An individual step may be qualified as electrochemical if it involves the transfer of an electron through the interface in order to change the oxidation state of a species Y + e Z. In addition, one can think of chemical steps, in which a species is converted into a more reactive form (or the reverse) with the same oxidation state, e.g. Y Y. A third possibility is that an intermediate species undergoes some kind of a dismutation reaction, e.g. 2Y= 0 + R. It is convenient to encode a mechanism by means of the letters E, C and D, placed in the order of the subsequent steps, e.g. EE, CEE, CED, etc. Note, however, that all the steps are considered here as heterogeneous reactions, i.e. taking place only at the interface. 

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