Proton-coupled electron-transfer activation

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

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... 

Effect of Pressure on Proton-Coupled Electron Transfer Reactions of Seven-Coordinate Iron Complexes in Aqueous Solution It has been shown that seven-coordinate Fe(III) diaqua complexes consisting of a pentaaza macrocyclic ligand possess superoxide dismutase (SOD) activity, and therefore could serve an imitative SOD function.360 Choosing appropriate chemical composition of a chelate system yielded suitable pKa values for the two coordinated water molecules so that the Fe(III) complexes of 2,6-diacetylpyridine-bis(semicarbazone) (dapsox) and 2,6-diacetylpyridine-bis(semioxamazide) (dapsc) (see Scheme 7.12) would be present principally in the highly active aqua-hydroxo form in solution at physiological pH.361... 

Quinones represent an important class of compound that undergo proton-coupled electron transfer reactions [35]. The order and kinetics of the two-electron/two-proton redox reactions of the quinone (Q)/hydroquinone (H2Q) couple continue to be active subjects of investigation. The interconversion of Q to H2Q can involve up to seven different intermediates depending on the pH of the solution and the solvent. However, in low-pH electrolytes electrochemically reversible behavior can be observed despite the significant changes that accompany redox switching. Beyond... 

Hammarstrom L, Styring S. Proton-coupled electron transfer of tyrosines in Photosystem II and model systems for artificial photosynthesis the role of a redox-active link between catalyst and photosensitizer. Energy Environ Sci. 2011 4 2379-88. 

The tris-carbene ligand family with fac geometry points its three wingtip groups downwards around the metal shielding it effectively from the approach of any but small substrates. Its main application is therefore the activation of small molecules, including the activation of dioxygen and proton coupled electron transfer (PCET), a reaction normally performed by certain enzymes [70,71],... 

Most coenzyme Bi2-dependent enzymes lose activity during prolonged reactions with substrates. 5,6-LAM is especially susceptible to substrate-dependent suicide inactivation. Based on extensive biochemical analysis, the chemical course of this process involves proton-coupled electron transfer leading to quenching of the putative substrate radical intermediates. Lysine-dependent suicide inactivation displays the following properties ... 

Proton coupled electron transfer. Titration of 1 with acetic acid results in the reduction of Mn(III) to Mn(II) as shown in Figure 8. Thus, the transfer of an electron will facilitate the addition of a proton and vice versa. Consequently, the activation energy for catalysis by both above mechanisms is lowered. 

Figure 9 ESR spectra of free thyrosyl radical TyrD photogenerated in Mn-depleted photosystem II at 4K (A) and warming to 200 K (B). The spectral change near 0.3362T is attributed to a thermally activated deprotonation of the Yd radical. (Reproduced with permission from Faller P, Goussias C, Rutherford AW, and Un S (2003) Resolving intermediates in biological proton-coupled electron transfer A tyrosyl radical prior to proton movement. Proceedings of the National Academy of Sciences of the USA 100 8732-8735.)...

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