Proton coupled electron transfer PCET

Comproportionation between cA-RuIV(bpy)2(py)02 + and cis- Run(bpy)2(py)(H20)2+ takes place by proton-coupled electron transfer (PCET) and exhibits a KIE of 16.1. Other PCET reactions of these and related ruthenium and osmium complexes also feature large KIEs. For example, oxidations of H202 by RuIV(bpy)2 (py)O2 + and by Ruin(bpy)2(py)OH2 + have KIEs of22.1 and 16.7, respectively. Oxidation of benzyl... 

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],... 

The original reaction is between a MnOH species and a tyrosine radical forming a MnO moiety. The process is known as a proton coupled electron transfer (PCET) and this reaction step is modelled by the process depicted in Figure 3.131. 

In 2001, Itoh and coworkers reexplored the mechanism of ortAo-phenol hydroxylations using [ Cu°(L ) 2(02)] " (7, Figure 10). This complex contains a deutero-benzyl-amine moiety, and can undergo hgand auto-oxidation, forming benzaldehyde, through a proton-coupled electron transfer (PCET) reaction with the peroxo ligand. 

RNRs catalyze the reduction of ribonucleotides to deoxyribonucleotides, which represents the first committed step in DNA biosynthesis and repair.These enzymes are therefore required for all known life forms. Three classes of RNRs have been identified, all of which turn out to be metalloenzymes. The so-called class I RNRs contain a diiron site (see Cobalt Bn Enzymes Coenzymes and Iron-Sulfur Proteins for the other two types of RNRs). As diagrammed in Figure 5, these enzymes generate first a tyrosyl radical proximal to the diiron site in the protein subunit labeled R2, and then a thiyl radical in an adjacent subunit (Rl) that ultimately abstracts a hydrogen atom from the ribonucleotide substrate. This controlled tyrosine/thiol radical transfer must occur over an estimated distance of 35 A, and a highly choreographed proton-coupled electron transfer (PCET) mechanism across intervening aromatic residues has been proposed. Perhaps, even more remarkably,... 

Proton-coupled electron transfer (PCET) reactions play a vital role in a wide range of chemical and biological processes. For example, PCET is required for the conversion of energy in photosynthesis [1] and respiration [2], In particular, the coupling between proton motion and electron transfer is involved in the pumping of protons across biological membranes in photosynthetic reaction centers [1] and in the conduction of electrons in cytochrome c [3]. In addition to biological processes, PCET is also important in electrochemical processes [4, 5] and in solid state materials [6]. 

Figure 7. Proposed proton-coupled electron transfer (PCET) pathway between the R2 and R1 subunits of the E. coli ribonucleotide reductase (RNR) complex. The conserved amino acids are shown schematically E. coli numbering).

Hammes-Schiffer expounds in Ch. 16 her group s theoretical formulation for proton-coupled electron transfer (PCET) mechanism and rates, pointing out the similarities with the separate spedal limits of electron transfer and (tunneling) proton transfer, and emphasizing the new features of PCET. The latter include the... 

Aminyl radicals have also been detected indirectly during the reaction of hydroxyl radicals (HO ) or their conjugated base ( 0 ) with the free amino group of amino acids (Reactions (3.9) and (3.10)) [40-43], and identified by time-resolved EPR experiments [44]. Similar reactions may be expected for peptides. While Reactions (3.9) and (3.10) show a net hydrogen transfer, they likely proceed via a stepwise electron-transfer and proton-transfer (Reaction (3.11)), a reaction commonly referred to as proton-coupled electron transfer (PCET). Proton transfer from the ami-nium radical cation to the base (OH ) will likely occur within the solvent cage. 

Oxidative cleavage can occur with some radicals, such as when H02 is oxidized to 02 + H +. Oxidative cleavage of H02 is actually a form of proton-coupled electron transfer (PCET), discussed below. It occurs in the reaction of H02 with Cu2 +, 32 Ce4 +, Am4+, various Ni(III) complexes, and [Ru(bpy)3]3 +, 19... 

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. 

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 . 

Meyer TJ, Huynh MHV, Thorp HH (2007) The possible role of proton-coupled electron transfer (PCET) in water oxidation by Photosystem n. Angew Chem Int Ed 46 5284-5304... 

---