Electron accepting site

Another well-studied electron transfer reaction is the oxidation of aqueous benzidine in the presence of various clays (2, 40, 43, 50, 51). An electron from the colorless benzidine molecule is abstracted by the clay with formation of a blue monovalent radical cation. Upon drying of the blue clay-benzidine system, a yellow color is produced. There is disagreement in the literature with respect to the chemical identity of the yellow product (2, 40, 52) however, in the case of hectorite, the yellow product has been suggested to be the protonated form of the radical cation (divalent radical cation) (2, 52). There is also disagreement about whether the electron-accepting sites of the clay are ferric iron at the planar surfaces, aluminum ions at the edges, or exchangeable cations <2, I). 

Cu(ll)phthalocyanine--type structure (electron-accepting site) 55... 

In the polymerization system, the catalyst reacts with acetaldehyde to form the acetaldehyde-catalyst complex. The equilibrium existing between monomeric [XVIII] and dimeric forms [XIX] of the complex is conceivably shifted far to the monomeric form in the polymerizing system than in benzene solution referred to earlier, due to the large difference in dielectric constants. The electron accepting site in the monomeric complex molecule is an aluminum atom and the electron... 

Cu(II) is intramolecular. The effect of fluoride on the reduction rate is consistent with both a direct involvement of type 2 Cu(II) in the reduction or an indirect effect mediated via a change in conformation or in redox potential of the type 1 Cu(II). The type 2 copper ion could be the primary electron-accepting site of the laccase molecule, as has been proposed for the reduction of the enzyme by hydroquinone (36), the first-order process observed being therefore the electron transfer from type 2 Cu to type 1 Cu(II). The particaption of type 3 Cu(II) instead of type 2 Cu(II) is not excluded, but no associated change of its absorption band at 330 nm could be observed during the redox cycle described for the 614-nm band. 

Nitrite reductases catalyze both of the reactions below the physiological electron donors are either c-type cytochromes or small blue-copper proteins (eqnations 1 and 2). h28 xhe Type 1 center acts as an electron-accepting site, which then transfers the electron to the Type 2 copper where snbstrate binding and rednction occur. 

Table 3 summarizes some of the properties of PS I reaction center and specific functions of its individual subunits. The purified preparation contains about 100 Chi a molecules per P-700 [9,10]. However, this number can be decreased to about 40 while the order of the Chi a molecules increases [70]. Washing out more of the Chi molecules caused a decrease in the dichroic ratio, indicating that those 40 Chls are the highly oriented primary light-harvesting antenna of the reaction center. It was also shown that the S-carotene is in very close proximity to P-700 and it is highly oriented with respect to the latter [70]. P-700 as well as the primary electron acceptor (Aj) may be composed of specialized Chi a molecules [80]. There are at least three more electron acceptors which are part of the reaction center and their function is to slow down the rate of the reaction and thereby stabilize the redox potential difference [72]. It was not until Malkin and Bearden [81] discovered the bound ferredoxins that this part of PS I started to be understood. Today it appears that at least four different clusters are involved in the electron-accepting site of the... 

It is very important that the new bands are present not only in the emission but also in the absorption spectra. This fact is a direct evidence that the electron transfer and CTC formation take place in the ground-state of adsorbed naphthalene as a result of its interaction with the electron-accepting sites of the zeolite. So, in our case, we deal with the electron donor - acceptor CTC, not with the exciplexes or excimers. 

The second significant feature of the spectra is that the new bands position is practically the same for the different cationic and decationated forms of zeolites. According to the simple Mulliken s CTC theory [4] this observation allows us to suggest that the electron-accepting sites are the same for of all forms of the zeolites under investigation. Therefore the second conclusion is that the exchangeable alkali and alkali-earth cations cannot be considered as electron-acceptor sites. 

The close similarity of CTC of naphthalene adsorbed on the decationated zeolite, porous silica and alumina allows us to make an assumption that the acid sites of Bronsted and Lewis type related to the coordinatively unsaturated silicon and aluminium ions are the real electron-accepting sites. 

Color Acid electron-accepting sites 5-line donating sites... 

Among metalloids, the toxic As(III) has been proven to be oxidized to the less toxic As(V) by Mn oxides. The ability of Mn oxides to oxidize As(III) varies with their structural and surface properties. The extent of masking of the electron-accepting sites on the Mn oxides for oxidation of As(III) to As(V) substantially varies with the kinds and levels of coatings. Reactive Mn oxides may be added to some environments, that have been contaminated with As(III), to alleviate the toxicity of As(ni) through converting As(III) to the less toxic and mobile As(V). 

We note however that the reactivity summarized in Table 17.1 for transition metal acceptors fundamentally differs from their organic counterparts in one important aspect - the proton and electron accepting sites for the reactions in Table 17.1 are distinct. It has been well documented that transition metal complexes that are capable of abstracting hydrogen atoms from substrates do not need to have unpaired spin density at the abstracting atom [52]. With the unpaired spin residing mainly at the transition metal center, upon completion of a PCET event, the electron is transferred to the metal M") while the proton comes to... 

Copper- and heme-containing NiRs are both key enzymes in denitrification. They are both homooligomers and their subunits contain two distinct redox-active metal centers, an electron accepting site and a catalytic electron delivery center where the single electron reduction of nitrite to NO takes place. Thus, PR studies providing comparison of the two enzyme families are helping to resolve the different mechanisms of control of intramolecular ET reactivity. Internal electron transfer could be a rate-determining step in the catalytic cycle of both enzymes. 

Lewis Acidity. Lewis acidity was generated on the present sample by dehydroxylation, as shown by interaction with ammonia to give characteristic infrared bands of ammonia coordinated to electron-accepting sites (at 3375, 3275, and 1615 cm1). Interaction of ammonia adsorbed on these Lewis sites was comparatively weak and could be removed by evacuation at 300°C. 

The EPR, UV-visible investigations as well as Raman scattering results provide informative clues about the formation and nature of long-lived electron-hole pairs through spontaneous biphenyl ionization upon sorption in the void space of activated ZSM-5 zeolites. The transferred electron is trapped within the framework by electron accepting site, while biphenyl radical cation captures one electron from electron donating sites of zeolite framework to restore BP ground state and causes an electron deficient hole. 

Anaerobic redox titrations [61) have established the presence of approximately four electron accepting sites in the molecule, these being divided among two one-electron acceptors and one two-electron acceptor [62) there are no other functional redox centers in the molecule. The two-electron center has been associated with the 330 nm absorption band [62). The redox potentials of the individual sites, as determined by a combination of various potentiometric and spectrophoto-metric measurements, are found in Table 2. 

Redox Potentials of the Electron Accepting Sites in Ceruloplasmin... 

Fungal laccase is a copper-containing oxidase which catalyses the oxidation of p-diphenols and related substances (notably aryl amines) by molecular oxygen. It seems that laccase contains two electron-accepting sites other than Type 1 (blue) and Type 2 (non-blue) copper(ii), and a mechanism involving the co-operation of several electron-accepting sites has been proposed which is consistent with recent kinetic results. 

Two mechanisms of formation of sulfoniumions are possible (1) by approaches to the catalyst s electron-accepting sites, (2) by abstraction of hydrides by methyl cations. ... 

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