ETPT electron transfer proton

Figure 3.2 Uni-directional (A) and bi-directional (B) PCET. Synonyms for unidirectional PCET are collinear PCET, concerted electron-proton transfer (CEP, ref. 236), electron-proton transfer (EPT, 120 and 237), concerted proton-electron transfer (CPET, 238 and 239) and concerted electron transfer proton transfer (ETPT, ref. 240). Bi-directional PCET is also termed orthogonal PCET, bi-directional concerted electron-proton transfer (CEP, ref. 236) and multisite electron proton transfer (MS-EPT, ref. 237). Adapted from ref. 21.

Proton-coupled electron transfer (pcet) is an important mechanism for charge transfer in biology. In a pcet reaction, the electron and proton may transfer consecutively (et/pt or pt/et) or conceitedly (etpt). These mechanisms are analyzed and expressions for their rates presented. Features that lead to dominance of one mechanism over another are outlined. Dissociative etpt is also discussed, as well as a new mechanism for highly exergonic proton transfer. 

The rate constants, kn, obtained at all pH values studied, were effectively identical. For anions lie and 2ie, the rate of reactions with O2 showed no significant change as the pH was decreased from 2 to 1. These were the first indications that the reaction is zero-order in [H+], namely, pH-independent. Solvent kinetic-isotope effect experiments were carried out in D2O at D+ concentrations corresponding to pH values of 2 and 7.2. The rate remained unchanged when the solvent H2O was replaced by D2O. That provided a second line of evidence that even at pH 2, well below the pAfa = 4.7 of protonated superoxide (HO2 ), proton transfer (PT) occurs after rate-limiting electron transfer to O2 (ETPT mechanism), rather than via concerted proton-electron transfer (CPET) [61-65], in which an electron and proton are transferred simultaneously in a single elementary step—see Sect. 12.3.2. 

In a recent publication [66], we showed that at low pH values in water, the ETPT mechanism discussed in the previous section is accompanied by a parallel pathway concerted proton-electron transfer (CPET Fig. 12.8) [61-65]. 

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