Electron mechanisms, coupled proton

The ISC mechanism is not normally considered to be plausible. Both the RR and the RS mechanism may be further classified according to the sequence of microscopic steps (electron transfer, coupling, protonation and/or cyclization) and with reference to which of the steps is rate determining. For the RS mechanism, the second electron transfer may take place by a reaction in solution (as indicated in Scheme 2) or at the electrode if the coupling step is very fast. 

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

All ECi adsorption coupled mechanisms have been verified by experiments with azobenzene/hydrazobenzene redox couple at a hanging mercury drop electrode [86,128,130]. As mentioned in Sect. 2.5.3, azobenzene undergoes a two-electron and two-proton chemically reversible reduction to hydrazobenzene (reaction 2.202). In an acidic medium, hydrazobenzene rearranges to electrochemically inactive benzidine, through a chemically irreversible follow-up chemical reaction (reaction 2.203). The rate of benzidine rearrangement is controlled by the proton concentration in the electrolyte solution. Both azobenzene and hydrazobenzene, and probably benzidine, adsorb strongly on the mercury electrode surface. 

How is a concentration gradient of protons transformed into ATP We have seen that electron transfer releases, and the proton-motive force conserves, more than enough free energy (about 200 lcJ) per mole of electron pairs to drive the formation of a mole of ATP, which requires about 50 kJ (see Box 13-1). Mitochondrial oxidative phosphorylation therefore poses no thermodynamic problem. But what is the chemical mechanism that couples proton flux with phosphorylation ... 

Cho S-l, Shin S (2000) Comment on the mechanism of proton-coupled electron transfer reactions. J Mol Struct (Theochem) 499 1-12... 

Important classes of chemical reactions in the ground electronic state have equal parity for the in- and out-going channels, e.g., proton transfer and hydride transfer [47, 48], To achieve finite rates, such processes require accessible electronic states with correct parity that play the role of transition structures. These latter acquire here the quality of true molecular species which, due to quantum mechanical couplings with asymptotic channel systems, will be endowed with finite life times. The elementary interconversion step in a chemical reaction is not a nuclear rearrangement associated with a smooth change in electronic structure, it is aFranck-Condon electronic process with timescales in the (sub)femto-second range characteristic of femtochemistry [49],... 

Su Q. Klinman J. P. Probing the mechanism of proton coupled electron transfer to dioxygen the oxidative half-reaction of bovine serum amine oxidase. Biochemistry 1998, 37, 12513-12525. 

In mitochondria there are two types of mechanisms for coupling the electron transport to the movement of protons across the membrane. The first is based on anisotropic reduction and oxidation of a lipid-soluble quinone inside the membrane. The quinone, coenzyme Q, becomes protonated upon reduction and diffuses to an oxidation site on the other side of the membrane where removal of electrons leads to proton release. This is essentially a proton carrier system with the hydroquinone acting as the proton carrier in the lipid phase of the membrane. A further refinement of this system in mitochondria provides for a coenzyme Q redox cycle where the movement of one electron through the chain allows for two protons to cross the... 

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. 

The coupled proton-electron transfer mechanism can also be applied to the molybdenum reductases. For nitrate reductase, a scheme such as Reaction 20 is possible. A Mo (IV)-Mo (VI) couple is used to illustrate this, and while such a couple is viable for some nitrate reductases, the Mo(II)-Mo(IV) or the Mo(III)-Mo(V) couple could also be accommodated... 

This process can be contrasted directly with the oxo transfer scheme (Reaction 16) discussed above. In either case, the cleavage of the N-O bond is assisted by the binding of oxygen to an electrophile (to molybdenum itself in the oxo transfer mechanism or to proton(s) in the coupled proton-electron transfer scheme). Although the coupled proton-electron transfer mechanism would possibly have the advantage of leaving an open site on molybdenum to restart the cycle, there is no strong data to support either of these mechanisms at present. 

The coupling of electron transfer and proton translocation is described by the proton-motive Q-cycle mechanism first proposed by Peter Mitchell [1], In the bc complex, hydroquinone is oxidized at a reaction site which is at the positive side of the membrane,... 

Stiefel, E. L, 1977a, Proposed molecular mechanism of die action of molybdenum in enzymes coupled proton and electron d ansfer, Proc. Nad. Acad. Sci. 70 988fi992. 

A range of chemical analogs of the catalytic centers of Mo and W dithiolene-containing enzymes (pterins) have been prepared. In particular, the rich chemistry of multisulfur transition metal systems allows ligand redox, internal electron transfer, and intermediate redox states. Such redox flexibility may facihtate coupled proton/electron transfer and/or 0x0-transfer mechanisms, which are employed by Mo and W enzymes. 

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