Proton-coupled electron transfer defined

An H transfer, or hydrogen atom transfer (HAT), reaction has been defined by Mayer as the concerted movement of a proton and an electron. .. in a single kinetic step where both. .. originate from the same reactant and travel to the same product. [18] Mayer considers HAT to be one type of the broad class of proton-coupled electron transfer (PCET) reactions, which also includes reactions where the proton and electron are separated. The distinction is a matter of ongoing discussion [19, 20], and other acronyms have been proposed [19, 21], but all the reactions to be considered in this chapter can be satisfactorily described as H transfer . 

Water plays multiple roles in biological electron transfer (ET) energy bath, polarizable medium that defines the reaction coordinate, electronic coupling bridge, and intimate participant in molecular recognition. This article explores these many faces of water in ET. Links are drawn to reactions in photosynthesis, oxidative phosphorylation, proton-coupled ET, and DNA damage and repair. 

Many chemical reactions in wine are characterized by electron transfers, leading to the oxidation and reduction phenomena. These reactions occur simultaneously and continue until an oxidation-reduction equilibrium is reached. The oxidation-reduction potential of a wine is an observation of the oxidation and reduction levels of the medium at a certain equilibrium. This value is quite comparable to pH as a measurement of a wine s acidity. Its value is linked to the quantity of dissolved oxygen, just as pH depends on the quantity of (H+) protons. Furthermore, it is possible to define the normal potential Eq of a given oxidiz-ing/reducing couple when half the component is oxidized and half is reduced. This characterizes the wine s oxidation capacity in the same way as p indicates the strength of an acid. 

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]). 

The range of pH-dependent behavior for a redox couple will therefore be defined by the pAa values of the complex for the two oxidation states involved in that couple, e.g., the generalized M LH couple will be pH-dependent between pAa and pAa . As Figure 2 graphically illustrates, it is equivalent to say that the pH-dependent range is defined by A1/2 and Ai/2, the respective potentials for the protonated and deprotonated couples, since A1/2 —Ai/2 = AAi/2 = (59mV)ApAa for a one-electron/one-proton couple, where ApAa = pAa pAa . Therefore, the extent of this reciprocal and mutual influence between electron transfer and proton transfer is reflected equally in the differences between the individual pAa values and redox potentials. The relationship between pAa and oxidation state of the complex will depend on a number of factors,... 

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