Proton-electron mechanism

Advances were still being made in the 1990s concerning the mechanism of these reactions. In the first reaction (Stage A), the mechanism has been called the proton-electron mechanism and is portrayed in Fig. 13.42. The oxide is a mixture of Mn4+, Mn3+, O2", and OH" ", and the proton hops along between O2- and OH", whereas the electron leaps onto Mn4+ from Mn3+. This picture is a simplification and more details have been elucidated in terms of the Mn02 structure, but our scope will be limited to the above. 

On the electron-proton-electron mechanism for l-benzyl-1,4-dihydronicotinamide oxidations. 

Proton—Electron Mechanism of Discharge of the Positive Active Mass... 

The reduction of oxygen in Complex IV is accompanied by transport of protons across the inner mitochondrial membrane. Transfer of four electrons through this complex drives the transport of approximately four protons. The mechanism of proton transport is unknown but is thought to involve the steps from state P to state O (Figure 21.20). Four protons are taken up on the matrix side for every two protons transported to the cytoplasm (see Figure 21.17). 

The sequential electron-proton-electron transfer mechanism is in agreement with the experimental observation by Ohno et al. [141]. The mechanism was confirmed by Selvaraju and Ramamurthy [142] from photophysical and photochemical study of a NADH model compound, 1,8-acridinedione dyes in micelles. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

In the charge, or electrolysis mode, the process splits water into hydrogen and oxygen and can produce hydrogen directly without mechanical compression. Water enters the cell and is split at the surface of the membrane to form protons, electrons and gaseous oxygen. 

Like atomic orbitals (AOs), molecular orbitals (MOs) are conveniently described by quantum mechanics theory. Nevertheless, the approach is more complex, because the interaction involves not simply one proton and one electron, as in the case of AOs, but several protons and electrons. For instance, in the simple case of two hydrogen atoms combined in a diatomic molecule, the bulk coulombic energy generated by the various interactions is given by four attractive effects (proton-electron) and two repulsive effects (proton-proton and electron-electron cf figure 1.20) ... 

This raises the burning question starting out from a simple substance (not to say elementary) made up of photons, electrons, neutrinos, neutrons and protons, what mechanisms exist for synthesising the many and varied nuclei to be found in nature This in turn raises the question where and when did these processes take place, and how do they fit together chronologically as the Universe has evolved ... 

The study of chemical reactions requires the definition of simple concepts associated with the properties ofthe system. Topological approaches of bonding, based on the analysis of the gradient field of well-defined local functions, evaluated from any quantum mechanical method are close to chemists intuition and experience and provide method-independent techniques [4-7]. In this work, we have used the concepts developed in the Bonding Evolution Theory [8] (BET, see Appendix B), applied to the Electron Localization Function (ELF, see Appendix A) [9]. This method has been applied successfully to proton transfer mechanism [10,11] as well as isomerization reaction [12]. The latter approach focuses on the evolution of chemical properties by assuming an isomorphism between chemical structures and the molecular graph defined in Appendix C. 

The mechanism for intramolecular hydrocyclization of enecarboxylates was originally thought to involve nucleophilic addition of the enecarboxylate radical-anion onto the ketone function [26], A more recent suggestion is that a sequence of electron, proton, electron additions leads to the P-carbanion ... 

This auto-catalytic reaction stops after about 30% decomposition. This is usually called low temperature decomposition (LTD) and it is believed that the electron transfer mechanism is operative in this region. At temperatures above 350 °C, high temperature decomposition (HTD) takes place usually by proton transfer mechanism and finally AP decomposes into NH3 and 4 (Equation 4.23) ... 

Similar studies have been carried out in the 1,4-dihydropyridine series. Nevertheless, the detailed mechanism remains questionable and evidence in support of both possible reaction pathways, direct hydride transfer and electron-proton-electron transfer, were presented. 

The photo-induced electron transfer of l,4-bis(methylene)cyclohexane in acetonitrile-methanol solution with 1,4-dicyanobenzene (DCB) affords two products, both consistent with nucleophilic attack on the radical cation followed by reduction and protonation or by combination with DCB ).63 In the absence of a nucleophile, the product mixture is highly complex, as is the case under electro-oxidative conditions. Under UV irradiation, /nmv-stilbene undergoes dimerization and oxygenation (to benzaldehyde) by a single-electron mechanism in the presence of a sensitizer such as 2,4,6-triphenylpyrilium tetrafluoroborate (TPT).64 This reaction was found to yield a similar product mixture with the sulfur analogue of TPT and their relative merits as well as electrochemical and photophysical properties are discussed. 

Such structure of enzyme catalytic sites causes a stereotype of chemical mechanisms of the substrate enzymatic transformations, which is associated with the unique individuality of their adsorption sites in relation to substrate fixation . It is common knowledge that donors and acceptors of catalytic groups exchanging protons, electrons and hydride ions with the substrates have different origins in different enzymes. 

In the second mechanism, the electron transfer from the nucleophile cluster into the aromatic ring should be facilitated by the decrease of the ionization potential (IP) of the solvent clusters as n increases. This mechanism is convincing for the ammonia or methanol clusters which show relatively low IPs when cluster size is increasing however, for water clusters, the IPs of n > 3 clusters are not known. The IPs of water and its dimer are 12.6 and 11.2 eV, respectively (Ng et al. 1977). However, these IPs are certainly higher than the one of PDFB (9.2 eV), which is not in favor of a sequential electron transfer followed by a proton transfer mechanism. This mechanism is more likely possible if one assumes, in agreement with Brutschy and coworkers, that the barrier to the reaction is lowered by a concerted electron transfer/proton transfer mechanism (Brutschy 1989, 1990 Brutschy et al. 1988, 1991, 1992, in press). 

The second proton transfer mechanism involves protonation of carboxyl or histidyl groups associated with electron carriers in the membrane and release of protons from these sites through proposed channels when the electron carrier is oxidized. This is essentially a proton channel system with movement through the channel gated by the oxidation-reduction state of the prosthetic group on the electron transport protein. The classical example of this is seen in cytochrome c oxidase (Figure 3). 

Krab, K. Wikstrom, M. (1987). Principles of coupling between electron transfer and proton translocation with special reference to proton-translocation mechanisms in cytochrome oxidase. Biochim. Biophys. Acta 895,25-39. 

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