Methyl viologen electron acceptor

Oxygen uptake by a digitonin-derived PSI submembrane fraction (8) was monitored at 22 C with methyl viologen as acceptor and DCIP/ascor-bate as donor. Samples were preincubated for 5 min at various temperatures. In the PSI preparations, oxygen uptake increased with incubation temperature similarly to the case in whole thylakoid membranes (Fig. 1, ref. 3, 5-6). The occurrence of heat stress stimulation in a PSI submembrane fraction confirms the conclusion by Thomas et al. (6) to the effect that the enhanced PSI electron flow is not due to an increased PSI antenna size following thermal dissociation of the peripheral PSII antenna complex. 

Gratzel and co-workers found in their first flash experiments that the luminescence of CdS decayed with a lifetime of 0.3 ns When methyl viologen was present, the signal of the half-reduced electron acceptor, MV, was present immediately after the laser flash. With increasing concentration, the amount of MV formed... 

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. 

Donor/acceptor association and the electron-transfer paradigm form the unifying theme for the C—C bond cleavage of various benzpinacols and diary-lethane-like donors in the presence of different electron acceptors (such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), triphenylpyrylium (TPP+), methyl viologen, nitrosonium cation, etc.). Scheme 13 reminds us how this is achieved by either CT photolysis of the D/A pair or via diffusional quenching of the excited electron acceptor A by the electron donor D. 

The oxazinones 74 and 79, already described as chiral glycine templates in Section 11.11.6.3, have been prepared by the PET cyclisation of 252 by irradiation in the presence of 1,4-dicyanonaphthalene as the electron acceptor and methyl viologen as electron-transfer mediator. When the reaction was carried out under strictly anhydrous conditions, compound 79 was isolated, whereas when the reaction was carried out in wet MeCN, compound 74 was the exclusive product (Scheme 33). In any case, the products were obtained with high stereoselectivity, which is the condition required to use them as chiral auxiliaries <2000EJ0657>. 

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). 

H20 served as the electron donor and methyl viologen as the electron acceptor. 0 consumption was measured with a Clark-type electrode and phosphorylation was measured colorimetrically. Data are presented as averaged I values SD obtained with three Isolations of thylakoids. Average specific activities were o9 3 ymoles 0, consumed and 171 16 ymoles Pi esterified/mg Chi h for the coupled reactions, and 223 i 5 ymoles 0 consumed/mg Chi h for the uncoupled reaction. 

Figure 1. Representative polarographic traces that depict inhibition by energy transfer inhibitors and allelochemicals of ADP-stimulated electron transport in isolated spinach thylakoids and circumvention of the inhibition by an uncoupler (FCCP, 2 pM). Trace A chlorotributyltin (TBT, 1 pM) trace B phlorizin (400 pM) trace C DCCD (20 pM) trace D quercetin (200 pM) trace E naringenin (1 mM). Water served as electron donor and methyl viologen as electron acceptor. Rates of oxygen utilization, that resulted from the autooxidation of methyl viologen, expressed as pmol 0 consumed/mg Chi h, are indicated parenthet ically.

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

Other acceptors which have reduction potentials below the conduction band edge of the illuminated semiconductor can also function in this role. The single electron reduction product derived from methyl viologen, for example, can be detected spectroscopically when a colloidal suspension of titanium dioxide is flashed in the presence of this electron acceptor, Eq. (2)... 

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). 

There is no evidence for ground state charge-transfer complex formation between stilbenes and neutral amines. Amine cations and dications are powerful electron acceptors and can form ground state complexes in which t-1 serves as the electron donor. Complex formation between t-1 and the organic dication methyl viologen is responsible for quenching of the fluorescence of surfactant stilbenes in organized assemblies (112). 

Fig. 14. Schematic representation of light-driven (2e + 2H+) symport across a membrane via the quinone carrier molecule vitamin Kj and its hydroquinone form proflavine (PF)-sen-sitized photoreduction of methyl-viologen MV2+ in the RED phase, yields the reducing species MV+, with simultaneous oxidative decomposition of EDTA used as electron donor the OX phase contains ferricyanide as electron acceptor [6.49].

Mn "(TMPyP)5 + Mn (TMPyP)4+ Mn,v(TMPyP)6+ Solvent water with adjusted pH X, = visible light Mn" formed in the presence of electron donors (EDTA, TEA, nicotine), Mn,v in the presence of electron acceptors (e.g. methyl viologen) [129]... 

Figure 27. Schematic representation of the reconstituted Mb electron acceptor composite via electrostatic interaction. The photoinduced electron transfer occurs from the reconstituted zinc Mb to electron acceptor such as methyl viologen, anthraquinone disulfonate, cytochrome c within the stable complex.

Dynamics in a microemulsion must differentiate those reactions occurring between donor-acceptor pairs co-included within the core and those in which one redox component is inside and one outside the microemulsion. With duroquinone and diphenylamine, the first situation applies here electron transfer was found to occur in two steps — a subnanosecond reaction from the singlet state and a slower reaction (requiring microseconds) from the triplet manifold [73]. With methyl viologen and N-methylphenothiazine, the second situation applies, and while two... 

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