Proton transport mechanisms oxygen ions

As shown by DFTB and CPMD simulations, the principal features of the transport mechanism are rotational diffusion of the protonic defect and proton transfer toward a neighboring oxide ion. That is, only the proton shows long-range diffusion, whereas the oxygens reside in their crystallographic positions. Both experiments " " and quantum-MD simulations, have revealed that rotational diffu-... 

These studies demonstrate the general mechanism of synchronization of biochemical systems, which I expect to be operative in even more complex systems, such as the mitochondrial respiration or the periodic activity of the slime mold Dictyostelium discoideum. As shown in a number of laboratories under suitable conditions mitochondrial respiration can break into self-sustained oscillations of ATP and ADP, NADH, cytochromes, and oxygen uptake as well as various ion transport and proton transport functions. It is important to note that mitochondrial respiration and oxidative phosphorylation under conditions of oscillations is open for the source, namely, oxygen, as well as with respect to a number of sink reactions producing water, carbon dioxide, and heat. 

The predominating transport mechanism for such protons is by free proton jumps (Grotthus mechanism) between neighboring oxygen ions although, statistically, a concentration of oxygen vacancies will enable some protons to move as OH ions when the host oxygen ions jump to vacancies. 

The free transport is the principal mode of transport of protons in oxides, and in this mechanism protons jump from one oxygen ion to a neighbouring one. After each jump the proton in the hydroxide rotates such that the proton reorients in the electron cloud and becomes aligned for the next jump. This is illustrated schematically in Fig.5.11. The rotation and reorientation is believed to involve a small activation energy and the jump itself is considered to be the ratedetermining step. 

In the vehicle mechanism the proton is transported as a passenger on an oxide ion. Thus this mechanism may be considered to constitute transport of hydroxide ions. The hydroxide ion may in principle move by an oxygen vacancy mechanism or as an interstitial hydroxide ion. It may be noted that the hydroxide... 

Similar considerations may be applied to free transport of protons (cf. Fig.5.11). For dilute solutions of protons in an oxide essentially all nearest neighbour oxygen ions are available, and thus in this case Nd is unity. However, the specification of Z, s and co is not straightforward in this case. The dynamics of free proton diffusion in oxides are complicated by 1) the multistep process (jump+rotation), 2) the dependency on the dynamics of the oxygen ion sublattice, and 3) the quantum mechanical behaviour of a light particle such as the proton. 

Complex IV, or cytochrome c oxidase, was the first of the mitochondrial electron transport complexes to have its molecular stmcture and the internal path of electron transfer revealed by X-ray crystallography. The catalytic core of the complex consists of two subunits. Subunit II contains a binuclear copper center (Cua) that is directly responsible for the oxidation of cytochrome c. From there electrons are passed to haem a and then to the adjacent binuclear center that consists of haem 03 and another copper ion (Cub), which are all held within subunit I (Fig. 13.1.4). Oxygen is bound and reduced between Cub and the iron of haem 03, and access paths for protons from the inside of the membrane and for oxygen from within the membrane have been defined from several crystal stmctures available for bovine and bacterial enzymes. In addition to the protons taken up for the reduction of oxygen, translocation of further protons across the membrane is coupled to electron transfer by a mechanism that is not yet understood (reviewed in Refs. [71, 72]). 

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