Methyl viologen, reduction with

MV2+ acceptors and SCN electron donors in solution [43], Colloidal semiconductor particles, typically of ca. 10-100 nm diameter, in aqueous sols may be treated as isolated microelectrode systems. Steady-state RRS experiments with c.w. lasers can be used to study phototransients produced at the surfaces of such colloidal semiconductors in flow systems [44], but pulsed laser systems coupled with multichannel detectors are far more versatile. Indeed, a recent TR3S study of methyl viologen reduction on the surface of photoex-cited colloidal CdS crystallites has shown important differences in mechanism between reactions occurring on the nanosecond time scale and those observed with picosecond Raman lasers [45]. Thus, it is apparent that Raman spectroscopy may now be used to study very fast interface kinetics as well as providing sensitive information on chemical structure and bonding in molecular species at electrode surfaces. 

Fig. 3. Time course of methyl viologen reduction at 86° catalyzed by hydrogenase at 250 and 8.5 atm. Reprinted from Miller et al with permission copyright 1989 John Wiley Sons, Inc.

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

Nosaka and Fox determined the quantum yield for the reduction of methyl viologen adsorbed on colloidal CdS particles as a function of incident light intensity. Electron transfer from CdS to MV " competes with electron-hole recombination. They derived a bimolecular rate constant of 9 10 cm s for the latter process. 

Figure 16. Scheme for the photoelectrochemical reduction of C02 at p-InP with formate dehydrogenase (FDH) as the catalyst and methyl viologen (MV2+) as the electron transfer mediator.163... 

Later, an improved system for C02 photofixation was reported by the same authors.164 The new system consisted of 6.5 x 1(T5 M tris(2,2 -bipyridine)ruthenium(II), Ru(bpy)3, as the photosensitive electron donor, methyl viologen (MV2+, 20 mM) as the electron acceptor, and triethanolamine (TEOA, 0.6 M) as a sacrificial electron donor in a C02-saturated aqueous solution (Fig. 18). Under irradiation with a 300-W high-pressure Hg lamp with a CuS04 chemical filter (A > 320 nm), formic acid, which was detected by isotachophoresis, was produced in quantum yields of ca. 0.01%. Recently, however, Kase et al.165 have repeated this experiment using a 13C02 tracer and have claimed that the formic acid obtained was produced not by C02 reduction but rather by oxidative cleavage of TEOA. 

Fig. 18. Electromicrobial reduction of a-keto acids with Proteus vulgaris in the presence of methyl viologen as mediator...

The same authors proposed a complex system for the electrochemically driven enzymatic reduction of carbon dioxide to form methanol. In this case, methyl viologen or the cofactor PQQ were used as mediators for the electroenzymatic reduction of carbon dioxide to formic acid catalyzed by formate dehydrogenase followed by the electrochemically driven enzymatic reduction of formate to methanol catalyzed by a PQQ-dependent alcohol dehydrogenase. With methyl viologen as mediator, the reaction goes through the intermediate formation of formaldehyde while with PQQ, methanol is formed directly [77],... 

Fig, 8. Reduction of oxo acids with Proteus vulgaris, with methyl viologen (MV) as organic mediator. Display of the bench-scale rig... 

Figure 3.4. The infrared bands in the 2120-1900cm spectral region of the soluble NAD-reducing hydrogenase (SH) and the regulatory hydrogenase (RH) from Ralstonia eutropha. A, Aerobic inactive SH as isolated and SH after reduction by hydrogen in the presence of 5 mM methyl viologen (MV) for 60 min at 30°C (Happe et al. 2000). B, Aerobic RH (Nia-S state) and upon reaction with H2 (Nia-C state) (Pierik et al. 1998b).

A second example of a membrane-bound arsenate reductase was isolated from Sulfurospirillum barnesii and was determined to be a aiPiyi-heterotrimic enzyme complex (Newman et al. 1998). The enzyme has a composite molecular mass of 100kDa, and a-, P-, and y-subunits have masses of 65, 31, and 22, respectively. This enzyme couples the reduction of As(V) to As(III) by oxidation of methyl viologen, with an apparent Kra of 0.2 mM. Preliminary compositional analysis suggests that iron-sulfur and molybdenum prosthetic groups are present. Associated with the membrane of S. barnesii is a h-type cytochrome, and the arsenate reductase is proposed to be linked to the electron-transport system of the plasma membrane. 

Physiologic electron acceptors flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) produced similar effects on cathodic hydrogen evolution from mild steel as achieved with methyl viologen (Bryant and Laishley 1990). These experimental results showed that the mild steel rods reacting with phosphate can preferential act as electron donors for the reduction of low-potential electron carriers. All hydrogenases catalyze a reversible reaction for the formation and oxidation of hydrogen, which requires low-potential electron carriers for the enzyme activity (Church et al. 1988 Fauque et al. 1988). 

In the case of two flavoenzyme oxidase systems (glucose oxidase (18) and thiamine oxidase s where both oxidation-reduction potential and semiquinone quantitation values are available, semiquinone formation is viewed to be kinetically rather than thermodynamically stabilized. The respective one-electron redox couples (PFl/PFl- and PFI7PFIH2) are similar in value (from essential equality to a 50 mV differential) which would predict only very low levels of semiquinone (32% when both couples are identical) at equilibrium. However, near quantitative yields (90%) of semiquinone are observed either by photochemical reduction or by titration with dithionite which demonstrates a kinetic barrier for the reduction of the semiquinone to the hydroquinone form. The addition of a low potential one-electron oxidoreductant such as methyl viologen generally acts to circumvent this kinetic barrier and facilitate the rapid reduction of the semiquinone to the hydroquinone form. 

Johansen et al. compared fluorescein, Eosin, Rose Bengal, and Rhodamine B. The system included the electron acceptor, methyl viologen, mv2 + which does not oxidize the dyes (nor are there dye-mv2+ complexes involved) but which reacts with the semireduced radicals formed by reduction of the dyes. The reaction scheme, in the presence of a platinum catalyst, is shown in Eqs. (32)—(35). 

The three-pulse electron spin-echo envelope modulation (ESEEM) technique is particularly sensitive for detecting hyperfine couplings to nuclei with a weak nuclear moment, such as 14N. It has been used to probe the coordination state of nickel in two hydrogenases from M. tkermoautotrophicum, strain AH (56). One of these enzymes contains FAD and catalyzes the reduction of F420 (7,8-dimethyl-8-hydroxy-5-deazaflavin), while the other contains no FAD and has so far only been shown to reduce artificial redox agents such as methyl viologen. 

An interesting aspect of this mechanism is the implicit unreactivity of Z at the electrode, although it reacts with electrogenerated O. This situation has been frequently encountered in the reaction of certain types of biological materials such as heme proteins. An example is the reduction of cytochrome c by electrogenerated methyl viologen cation radical, MV [2] ... 

The [Ru(bpy)3]2+ photosensitized reduction of methyl viologen (MV2+) proceeds rapidly in water-swollen iminodiacetic acid type chelate resin beads which adsorb both [Ru(bpy)3]2+ and MV2+ (RM resin). The reduction takes place with the aid of polymer-bound iminodiacetate as a donor 102). Photosensitized formation of hydrogen peroxide occurs in an aqueous solution containing RM resin and oxygen molecules (Fig. 8)102 The some reaction also occurs using a polystyrene-coated filter paper, onto which both [Ru(bpy)3]2 + and MV2+ were adsorbed 103). 

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