Electron acceptor, phenazine

Complex III - Complex III contains a diversity of electron carrying proteins. They include cytochrome b, iron sulfur centers, and cytochrome cl. Cytochrome b is the first of the heme-carrying proteins (Figure 15.6) involved in electron transport. Passage of electrons from cytochrome b to the iron sulfur centers can be blocked by antimycin A. Also, the artificial electron acceptor phenazine methosulfate can accept electrons from cytochrome b and 2,6-dichlorophenol-indophenol can accept electrons from the iron sulfur proteins (Figure 15.9). The crystal structure of the redox components of complex III from bovine heart mitochondria is shown in Figure 15.16... 

Second only to sulfur-based systems, nitrogen complexes are relatively well represented in the structural literature with 41 complexes reported. Of these, 25 are with I2 as the electron acceptor, 11 are with the interhalogen IC1, three are with Br2, and two are with IBr. As expected, in every case the halogen bond forms between the nitrogen and the softest halogen atom, i.e., iodine, in all of the complexes except those with dibromine. Most N I2 complexes, and all N Br2, N IBr, and N IC1 complexes are simple adducts, mode A. Exceptions for the diiodine complexes include bridging mode (B) observed for diazines, such as pyrazine [86], tetramethylpyrazine [86], phenazine, and quinoxaline [87], and for 9-chloroacridine [89] and the 1 1 complex of diiodine with hexamethylenetetramine [144] and amphoteric bridging mode (BA) observed for 2,2 -bipyridine [85], acridine [89], 9-chloroacridine [89], and 2,3,5,6-tetra-2/-pyridylpyrazine [91]. The occurrence of both B and BA complexes with 9-chloroacridine, and of B and A complexes and an... 

Physical measurements support a molecular weight of approximately 200,000. This value is also in accord with gel exclusion studies on Sephadex G-200. Thus the enzyme contains 1 mole of flavin and 4 g-atoms of nonheme iron per mole.. . . The sedimentation velocity of the beef heart enzyme at 10-15 mg protein/ml is 6.5 S. . . This preparation could oxidize succinate in the presence of ferricyanide or phenazine methosulfate (PMS) as electron acceptor but was unable to transfer elec-to be unable to interact with the respiratory chain. 

D-Lactate cytochrome c reductase can oxidize D-2-hydroxymonocar-boxylic acids, but only D-lactate and D-2-hydroxybutyrate are oxidized at appreciable rates. The enzyme exhibits a similar high specificity for electron acceptors. It reacts with cytochrome c and phenazine methosul-fate as electron acceptors, but not with ferricyanide, methylene blue, 2,6-dichloroindophenol, and menadione 308, 312, 313). With D-lactate as substrate and at Fmax with respect to acceptor, phenazine methosulfate is reduced at 30° eight times as fast as cytochrome c 308). The values at 30° and pH 7.5 are D-lactate, 0.29 mM n-2-hydroxybutyrate,... 

Fig. 12.8. Schematic representation of events occurring during biogenesis of photosystem I reaction center. The subunits are designated as I to VII, the abbreviations are Ferr, ferredoxin P.C., plas-tocyanin A, Aj, Aj and A4, primary, secondary, tertiary and quaternary electron acceptors PMS, phenazine methosulfate DAD, diaminodurine.

Visual localization of electrophoretically separated LDH isoenzymes is accomplished by the reduction of nitro-tetrazolium blue as the electron acceptor (terminal) in a medium containing phenazine methosulfate and NAD. The linked reaction is as follows ... 

The movement of electrons through the electron carrying proteins of the inner mitochondrial membrane is shown in Figure 15.9. Also shown are inhibitors of electron movement at their point of action and the sites where artificial electron acceptors can accept electrons from the electron transport system. Specific inhibitors shown in Figure 15.9 are rotenone, amytal, antimycin A, cyanide, azide, and carbon monoxide. The artificial electron acceptors are methylene blue, phenazine methosulfate, 2,6-indophenol, tetramethyl-p-phenylene diamine, and ferricyanide. 

Ferrocene and its derivatives even mediate the enzymatic redox reactions of PQQ-dependent enzymes. As an example, the mediation of the enzymatic glucose dehydrogenation by PQQ-glucose dehydrogenase has been reported [140] the same enzyme also can use phenazines as electron acceptors [141]. 

The failure of a generation of investigators to obtain soluble succinic dehydrogenase preparations was caused by the unusually fastidious requirements of this enzyme. Of the ordinary electron acceptors, including methylene blue and dichlorophenolindophenol, none supports the oxidation of succinate. Phenazine methosulfate was found to be reduced by solubilized preparations the enzyme is extracted from acetone powders and was undoubtedly present in many extracts, where it was not detected for lack of a suitable oxidant. A preparation from yeast was reported... 

Succinoxidase. Washed particles from heart muscle oxidize succinate to fumarate with oxygen as electron acceptor, This is the so-called succinoxidase preparation. The initial step in the oxidation of succinate has recently been found to be catalyzed by an iron-flavoprotein. When this is isolated from other components of succinoxidase, it fails to react with other enzymes and reduces efficiently only phenazines of the several oxidants that are effective with more complex preparations. Partially fragmented particles have been obtained by differential centrifugation of isobutanol-treated preparations of larger particles from mitochondria or by digestion with trypsin in the presence of cholate. These treatments yield preparations that can reduce cytochrome c. ... 

Comparison of the electron-transfer efficiency provided by different mediators in the presence of the same redox enzyme allows the definition of the important parameters of MET. The reaction of glucose oxidase (GOx) has been extensively studied with a number of artificial electron acceptors including organic dyes such as phenazine methosul-fate, 2,6-dichlorophenolindophenol, and N, N, N, A -tetramethyl-4-phenylenedia-mine [26]. However, these mediators have a number of limitations such as poor stability and the pH dependence of their redox potentials. Simple inorganic redox species such as hexacyanofer-rate [27], hexacyanoruthenate, and pen-taamine pyridine ruthenium [28] do not suffer from these problems. These... 

The spectrophotometric methods described earlier were used to determine the optimum pH for the isoenzymes (Schabort et al., 1971). Cytochrome c [together with a small amount of phenazine methosulfate (PMS) as intermediate electron acceptor] was particularly useful for determinations below pH 6.4, because the absorbance of DCIP at 600 nm decreases rapidly below this value. The five isoenzymes showed the same optimum pH of 6.8 for both the dehydrogenation and the total conversion of jSCA into aCA. The temperature stability of the five isoenzymes was essentially the same. They lost all their activity after heat treatment for 10 min at 75.5°C, but retained 70% of their activity after 10 min at 55" C. 

Electronic spectroscopic and electrochemical data for [Ru(bpy)2(98)] " ((98) = dipyrido [3,2-a 2, 3 -c]phenazine) are consistent with the presence of two electronically separate units, one behaving like [Ru(bpy)3] + and the other resembling a phenazine-like acceptor. " Spectroscopic... 

A poly(phenylquinoxaline) was prepared for electroluminescence applications <1996SM(76)105>. Crystallization of solution donor-acceptor complexes of 2,3-dimethylquinoxaline 1,4-dioxide or phenazine 5,10-dioxide with TCNE afforded two-component solids containing weakly bound 1-D donor-acceptor arrays <1997TL7665>. A pyrazine ladder polymer was constructed from two different pyrazine units, as an optical device <1999JA8783>. The new electron-deficient macrocycle tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine was prepared from l,2-di(2-pyridyl)ethanedione and 2,3-diaminomaleonitrile for a study of its electrochemical properties <2004IC8626>. 

The activity of complex II (succinate dehydrogenase) is measured as the succinate-dependent reduction of decylubiquinone, which is in turn recorded spectro-photometrically through the reduction of dichlorophenol indophenol at 600 nm (e 19,100-M -cm Fig. 3.8.5). In order to ensure a linear rate for the activity, the medium is added with rotenone, ATP, and a high concentration of succinate. As noticed previously for complex I, decylubiquinone is not a perfect acceptor for electrons from the membrane-inserted complex II [70]. Malonate, a competitive inhibitor of the enzyme, is used to inhibit it. Rather than decylubiquinone, phenazine methosulfate can be utilized, which diverts the electrons from the complex before they are conveyed through subunits C and D, therefore allowing measurement of the activity of subunits A and B. 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

The electrochemistry of both NAD+ and NADH at clean electrodes occurs at high overvoltages (of the order of 1V) and hence causes imwanted side reactions, which tend to foul the electrode [142-144]. One way around this problem is to use mediators (two electron-proton acceptor/donors) such as o-quinone [145-147] or p-phenylenediamine derivatives [148], arylnitro derivative such as 2-nitro-9-fluorenone [149], PQQ with Ca " [150] and polyaromatic dye molecules, i.e., phenazine, phenoxazine and phenothiazine derivatives [11] that substantially lower the voltage for needed NADH oxidation. 

This review summarizes the preparation, structural chemistry, and selected physical properties of organic molecular solids in which substituted phenazines act as tt-electron donors (D). The parent phenazine (P) is a weak rc-acceptor (A). 

QDHs are independent from classical coenzymes like NAD(P)+. The substrate electrons are preferentially transferred to organic acceptors (quinones) and nonnative redox mediators such as phenazine derivatives, DCPIP, Wursters blue11711, ferrocene11011, ferricyanide1 72- 173], osmium complexes11741, or direct contact to an electrode11751. 

The authors conclude that the sensitized polymerization is initiated by the phenazine (Ph) acting as an electron and proton acceptor in its excited state. PhH or PhHj radicals are created, as well as acid radicals and anions, which could act as precursor states of the decarboxylation (R = CHj—(CH ) — with m = 11, 13, 15) ... 

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