Proton-coupled electron-transfer redox couples

The cis-[RuVI(tet-Me6)(0)2]2+ complex has also been reported to display proton-coupled electron-transfer redox couples in aqueous medium (134,136). The following electrode reactions for the couples have been observed (pH 1.0) ... 

In biochemical systems, acid-base and redox reactions are essential. Electron transfer plays an obvious, crucial role in photosynthesis, and redox reactions are central to the response to oxidative stress, and to the innate immune system and inflammatory response. Acid-base and proton transfer reactions are a part of most enzyme mechanisms, and are also closely linked to protein folding and stability. Proton and electron transfer are often coupled, as in almost all the steps of the mitochondrial respiratory chain. 

An essential feature of reactions catalyzed by metal-sulfur oxidoreductases is the coupling of proton- and electron-transfer processes. In this context, an important question is how primary protonation of metal-sulfur sites influences the metal-sulfur cores, small molecules bound to them, and the subsequent transfer of electrons. In order to shed light upon this question, protonations, isoelectronic alkylations, and redox reactions of [M(L) (S )] complexes were investigated (M = Fe, Ru, Mo L = CO, NO S = Sj, US24 ). The CO and NO ligands served as infrared (IR) probe for the electron density at the metal centers. Resulting complexes were characterized as far as possible by X-ray crystallography. Scheme 23 shows examples of such complexes. 

The formation of the same iron-oxygen covalent bonds from either (1) oxidized iron plus oxy anions via electron-transfer (redox) reactions or (2) radical-radical coupling reactions is summarized in Table 3-11. The valence-electron hybridization for the iron center is included as well as the spin state and estimated covalent bond-formation free energy (AGbf)- A similar set of reactions and data for iron-porphyrin compounds is presented in Table 3-12. Section a emphasizes that, just as the combination of a proton with a hydroxide ion yields a covalent H-OH bond (Table 3-11), (1) the combination of protons and porphyrin dianion (Por -) yields covalent porphine (H2Por), and (2) the addition of Lewis acids (Zn2+ or Fe2+) to porphine (H Por) oxidatively displaces protons to give covalent-bonded ZnilPor and Fe iPor. 

The surface CV of anthraquinone- or its derivative-adsorbed electrode shows a reversible 2-electron-transfer redox couple. For example. Figure 7.9(A) shows the surface CV of anthraquinone-carboxlyic-allyl ester (ACAE) adsorbed on a basal plane graphite electrode surface. The pH dependent of the peak potentials gives a slope of 57 mV pH , suggesting that the redox process involves two electrons and two protons. The surface reaction reactions on the electrode surface can be assigned as Reaction (7-VI) ... 

Many important chemical and biological reactions involve transfer of both electrons and protons. This is illustrated, for instance, by Pourbaix s extensive 1963 Atlas of Electrochemical Equilibria. These have come to be called proton-coupled electron transfer (PCET) reactions. Due to the widespread interest in this topic, the term PCET is being used by many authors in a variety of dilferent contexts and with different eonnotations. As a result, a very broad definition of PCET has taken hold, eneompassing any redox proeess whose rate or energetics are affected by one or more protons. This includes processes in which protons and electrons transfer among one or more reactants, regardless of mechanism, and processes in which protons modulate ET processes even if they do not transfer. ... 

A plausible connection between proton motions and electron transfer reactions is simply the changed electrostatic field in different redox states of the protein. The calculation of electrostatic potentials throughout a protein has been attempted with increasing success in recent years (see references 13-15 for reviews). It has been shown to be possible to calculate electrochemical midpointsand pKa s. The work presented here demonstrates that classical electrostatics can also provide insight into the coupling between proton and electron transfers. 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

For aminophenols, one-electron oxidation and the proton elimination can run together in one stage. This leads to a cation-radical containing O and +NH3 fragments within one and the same molecular carcass (Rhile et al. 2006). Such concerted reactions are classified as proton-coupled electron transfer (Mayer 2004). Proton-coupled electron transfer differs from conventional one-electron redox reaction in the sense that proton motion affects electron transfer. Because the transfers of a proton and an electron proceed in a single step, we can say about the hydrogen-atom transference, (H+ -I- e)=H. It is the fundamental feature of proton-coupled electron-transfer reactions that the proton and electron are transferred simultaneously, but from different places (see Tanko 2006). 

---