Proton mechanisms, coupled

The results of Hirota and Weissman are also remarkable in that hyperfine coupling to y-protons was clearly resolved (Hirota and Weissman, 1960). This is one of the very few instances in which this coupling has been detected, and the very small coupling of 0-12 G confirms the conclusion that such protons should couple very weakly indeed with the unpaired electron (Symons, 1959). The mechanism of this coupling has been discussed in terms of C—C hyperconjugation, and is thought to be linked to the 13C coupling for the y-carbon atoms (Symons, 1962). 

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],... 

Oxidative phosphorylation is the name given to the synthesis of ATP (phosphorylation) that occurs when NADH and FADH2 are oxidized (hence oxidative) by electron transport through the respiratory chain. Unlike substrate level phosphorylation (see Topics J3 and LI), it does not involve phosphorylated chemical intermediates. Rather, a very different mechanism was proposed by Peter Mitchell in 1961, the chemiosmotic hypothesis. This proposes that energy liberated by electron transport is used to create a proton gradient across the mitochondrial inner membrane and that it is this that is used to drive ATP synthesis. Thus the proton gradient couples electron transport and ATP synthesis, not a chemical intermediate. The evidence is overwhelming that this is indeed the way that oxidative phosphorylation works. The actual synthesis of ATP is carried out by an enzyme called ATP synthase located in the inner mitochondrial membrane (Fig. 3). 

Figure 34.32. Proton Transport-Coupled Rotation of the Flagellum. (A) MotA-MotB may form a structure having two half-channels. (B) One model for the mechanism of coupling rotation to a proton gradient requires protons to be taken up into the outer half-channel and transferred to the MS ring. The MS ring rotates in a counterclockwise direction, and the protons are released into the inner half-channel. The flagellum is linked to the MS ring and so the flagellum rotates as well.

Fig. 3.6 implies that cytochrome oxidase generates pmf by two different mechanisms coupled in series. The first is the pure proton-translocating function, which may specifically involve haem a (see below). The second and thermodynamically equivalent function is the annihilation of electrical charges when electrons deriving from cytochrome c on the C side of the membrane meet with protons deriving from the M side, in the reduction of Oj to water at the haem Uj centre. 

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