Electron Coupled Proton Transfers

Rosenthal J, Nocera DG. 2007. Role of proton-coupled electron transfer in 0-0 bond activation. Acc Chem Res 40 543. 

Belevich I, Verkhovsky MI, Wikstrom M (2006) Proton-coupled electron transfer drives the proton pump of cytochrome c oxidase. Nature 440 829-832. 

Keywords Excited-state intramolecular proton transfer Fluorescence dye Photoinduced electron transfer Proton coupled electron transfer Relaxation dynamics... 

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

From the ab initio quantum chemistry coupled with a molecular dynamics treatment of the surrounding media, Zhao and Cuckier have reported a study of a proton coupled electron transfer reaction [115],... 

Zhao, X. G. and Cuckier, R. I. Molecular dynamics and quantum chemistry study of a proton-coupled electron transfer reaction, J.Phys.Chem., 99 (1995), 945-954... 

Protons are in general indispensable for the dismutation of superoxide (Eq. (4)). Also in the case of its dismutation catalyzed by a metal center, two protons are needed for the dissociation of the product (H2O2) from the metal center (Scheme 9). Therefore, a complex which can accept two protons upon reduction and release them upon oxidation is an excellent candidate for SOD activity. The studies on proton-coupled electron transfer in Fe- and Mn-SODs 48), demonstrated that the active site of MnSOD consists of more than one proton acceptor (Scheme 10). Since the assignment of species involved in proton transfer is extremely difficult in the case of enzymatic systems, relevant investigations on adequate model complexes could be of vast importance. H2dapsox coordinates to Fe(II) in its neutral form, whereas in the case of Fe(III) it coordinates in the dapsox form. Thus, oxidation and reduction of its iron complex is a proton-coupled electron transfer process 46), which as an energetically favorable... 

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). 

Shafirovich V, Geacintov NE (2004) Proton-Coupled Electron Transfer Reactions at a Distance in DNA Duplexes. 237 129-157... 

The electrochemistry of dioxoosmium(VI) complexes has also been extensively studied. The tra 5-dioxoosmium(VI) complexes of polypyridyl and macrocyclic tertiary amine ligands display very similar proton-coupled electron transfer couples. In aqueous solutions at pH < 5-7 the cyclic voltammograms of n-a i-[0s (0)2(bpy)2] show a remarkable reversible three-electron couple and a one-electron Os coimle. In the Pourbaix diagram two break points are observed in the pH dependence of the Os couple, which correspond to the pAa values of Os —OH2 and Os —(OHXOH2) (Figure 10). The redox reactions are shown in Equations (41)-(43). At pH >8 the 3e Os wave splits into a pH-independent le Os wave and a 2e/2H" Os wave (Equations (44) and (45)). 

The species truw5-[0s NN(CH2)40 (tpy)(Cl)2] can be protonated to give the hydrazido(l-) species truw5-[0s N(H)N(CH2)40 (tpy)(Cl)2]. The protonated species undergoes rapid oxidation by benzoquinone, Q, to give tra i-[0s NN(CH2)40 (tpy)(Cl)2] and H2Q. This reaction occurs with a remarkable N— H/N—kinetic isotope effect of 41.4, consistent with a proton-coupled electron transfer mechanism. 

Os -NP(H)Et2 and Os -NPEt2 by Q. A remarkable P— H/P—kinetic isotope of 178 is observed, consistent with a proton-coupled electron transfer mechanism." ... 

The electrochemical behavior of ruthenium(IV) 0x0 complexes has been extensively studied. They exhibit interesting electrochemical behavior, particularly in aqueous solutions, where proton-coupled electron transfer processes occur. In general, two couples are observed in the cyclic voltammetry of a Ru =0 complex in aqueous solutions (Equations (84) and (85)) ... 

Under the advisement of PhD mentor Professor Joseph T. Hupp, the PI successfully used spectroelectrochemical quartz crystal microgravimetry to elucidate the mechanism of charge transport... for both aqueous and nonaqueous sytems. This was the first demonstration of proton-coupled electron transfer at oxide semiconductor interfaces. These findings were then successfully applied to a new interpretation of photoinduced electron transfer at similar interfaces, which are of importance in the field of solar energy conversion. ... 

For oxidation of G in duplex DNA, Steenken concluded that the proton on N-1 of G shifts spontaneously to N-3 of the cytosine in the normal Watson-Crick base pair to generate [C+(H)/G ]. Consistent with this proposal, calculations indicate that charge transfer in oxidized DNA is coupled with proton transfer from G to Experiments carried out in D2O also reveal a kinetic isotope effect for G oxidation, implicating a concerted proton-coupled electron transfer mechanism. However, density functional theory (DFT) calculations in the gas phase predict that the structure with a proton on G N-1 [C/HG ] is more stable than [C (H)/G ] by 1.4kcal/mol. " ... 

In order to understand the effect of temperature we consider the energetics involved for the stepwise formation of T(C6)H in DNA. The proton-coupled electron transfer from cytosine to thymine in DNA is depicted in reaction 1 and protonation of the thymine anion radical in reaction 2. 

Proton-Coupled Electron Transfer Reactions at a Distance in DNA Duplexes... 

Keywords DNA Oxidative damage Proton-coupled electron transfer Hole transfer Laser flash photolysis... 

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