Electron transfer sequential

We note that the reaction coordinate is coupled to an infinite bath of harmonic oscillators, which represent the bulk protein, and to a protein promoting vibration. For each mathematical implementation, we here choose the zero of promoting vibration coupling to be in the well rather than at the barrier top, but this is arbitrary. We point out that we can tune this model to allow for both sequential and concerted hydrogen-electron transfer. Sequential transfer is found with a very high transfer rate, and concerted with a lower one. 

The sequential electron-proton-electron transfer mechanism is in agreement with the experimental observation by Ohno et al. [141]. The mechanism was confirmed by Selvaraju and Ramamurthy [142] from photophysical and photochemical study of a NADH model compound, 1,8-acridinedione dyes in micelles. 

Scheme 30 Sequential electron-proton-electron transfer...

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

Moreover, the efficiency of catalytic autoxidation in Scheme 26 is also attributed to the short lifetime of the hydroquinone cation radical (t < 10 10 s-1)254, which renders the sequential electron-transfer/proton-trans-fer cycles extremely efficient. 

The use of short (fs) laser pulses allows even highly transient ion-radical pairs with lifetimes of t 10 12 s to be detected, and their subsequent (dark) decay to products is temporally monitored through the sequential spectral changes. As such, time-resolved (ps) spectroscopy provides the technique of choice for establishing the viability of the electron-transfer paradigm. This photochemical (ET) mechanism has been demonstrated for a variety of donor-acceptor interactions, as presented in the foregoing section. 

Berlin YA, Ratner MA (2005) Intra-molecular electron transfer and electric conductance via sequential hopping unified theoretical description. Radiat Phys Chem 74 124—131... 

Figure 6.25 Sequential electron transfer processes between protein-bound groups...

The lifetime of D+.C.A produced by the sequential electron transfer increases as the distance between D+ and A increases. Porphyrin-imide-fullerene triads have been synthesised (Figure 12.11), with the three components separated by nonreactive spacer molecules. 

As an example, Figure 43 shows the cyclic voltammetric profiles of a species which undergoes sequential electron transfers. 

This scheme includes ET rate constants only for the d - d electron-transfer processes, in which the system conformation is conserved, and conformational and ET steps only occur sequentially. Intuitively, it might be expected that the kinetic scheme must include ET that is synchronous with a conformational change in the medium coordinate. However, we showed [10a] that it is not necessary to include the diagonal processes (e.g., A Ig) when considering stable substates. 

The fact that ET and conformational reactions thus are sequential (Scheme III), and not concerted, is an important factor in efforts to disentangle eonforma-tional and electron-transfer influences, because standard detection methods monitor only the ET event, and not conformational changes within one electronic state. In many, if not most, instances the measured time course of a single gated ET reaction is likely to be indistinguishable from a reaction without gating. 

Figure 29. Comparison of quantum path-integral results (thick tines) and ZPE-corrected mapping results (thin lines) for the diabatic electronic populations of a three-state electron transfer model describing (a) sequential and (b) superexchange electron transfer.

The following description of the electron transfer-proton transport scheme is illustrated in Figure 7.26. First, an electron is transferred from doubly reduced dihydroplastoquinone (PQFI2) to a high potential electron transfer chain that consists of the Reiske iron-sulfur protein and the cytochrome protein containing heme f. Rappaport,Lavergne and co-workers have reported a midpoint potential at pH 7.0 of +355 mV for heme f. These two centers reside on the electropositive (lumen or p) side of the membrane, exterior to the membrane. As a result, two protons are transferred to the aqueous lumen phase. A second electron is transferred from PQH2 sequentially to heme bp. 

In the fully reduced model, four electrons are transferred to dioxygen through sequential one-electron oxidations of heme as s iron ion, the Cub ion, the heme a iron ion, and one of the bimetallic center s Cua ions. The sequence of electron transferal differs in the mixed valence model, and a tyrosine radical (tyr) is generated. The proposed formation of a tyrosine radical during catalytic turnover arises from the known post-translational modification in most CcO s in which a covalent bond is formed between the his240 ligand of Cub... 

Three oxidations states are potentially available in a binuclear iron center. Enzymes with octahedral fi-o o bridged iron clusters can be isolated in each of the three states the diferric and diferrous states appear to be the functional terminal oxidation states for most of the enzymes, while the mixed valence state may be an important intermediate or transition state for some reactions (Que and True, 1991). In these enzymes the cluster participates primarily as a two-electron partner in the redox of substrates, perhaps using sequential one-electron steps. Without additional coupled redox steps the enzyme is in a new oxidation state after one turnover. In contrast only the diferric and mixed valence oxidation states have been found for 2Fe 2S clusters. The diferrous state may not be obtainable because of the high negative charge on [2Fe 2S(4RS)] versus -1 or 0 net charge for the diferrous octahedral (i.e., non-Fe S) clusters. The 2Fe 2S proteins either are one-electron donor/acceptors or serve as transient electron transfer intermediates. 

As exemplified in Figure 2, Type 1 mechanism, electron transfer from L to sens yields two radicals, the substrate radical, L", and the sensitizer radical anion (sens ). In the next step, the lipid radical may induce a chain peroxidation cascade involving propagation reactions -The sensitizer radical anion may also start a sequential one-electron reduction of 2 generating HO in the presence of reduced transition metals. As a result, this may lead to abstraction of a lipid allylic hydrogen with subsequent generation of a carbon-centered lipid radical, L, that is rapidly oxidized to a peroxyl radical (vide supra). 

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