Sequential transfer process

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

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. 

F ure 5.17 Sequential steps for a nonradiative energy transfer process (see the text). 

The TTF-porphyrin dyad 3 was described by the group of Odense.11 The fluorescence of 3 is significantly quenched by the photoinduced electron transfer process. Notably, the fluorescence intensity of dyad 3 increases largely after addition of Fe3 + that oxidizes TTF into TTF" +. Successive reduction of TTF" + is not reported. Nevertheless, it is anticipated that the fluorescence of dyad 3 can be reversibly modulated by redox reactions. In fact, the fluorescence of the supramolecule 4, formed between Zn-tetraphenylporphyrin and a pyridine-substituted TTF (TTF- ), can be reversibly tuned by sequential oxidation and reduction of the TTF moiety in 4.12 It should be noted in this context that the synthetically challenging system associating a porphyrin ring fused to four TTFs (5) was also reported.13... 

The heme iron in the peroxidase is oxidized by the peroxide from III+ to V4- in compound I. The compound I is reduced by two sequential one-electron transfer processes giving rise to the original enzyme. A substrate-free radical is in turn generated. This may have toxicological implications. Thus the myeloperoxidase in the bone marrow may catalyze the metabolic activation of phenol or other metabolites of benzene. This may underlie the toxicity of benzene to the bone marrow, which causes aplastic anemia (see below and chap. 6). The myeloperoxidase found in neutrophils and monocytes may be involved in the metabolism and activation of a number of drugs such as isoniazid, clozapine, procainamide, and hydralazine (see below). In in vitro systems, the products of the activation were found to be cytotoxic in vitro. 

In the presence of two equivalents of silver fluoride, N-protected bis[(trimethylsi-lyl)methyl]amines lead also to azomethine ylids which can be trapped by dipolarophiles. The mechanism of the cycloaddition reaction involves sequential electron-Me3Si+-electron transfer process from the amine to silver fluoride, which forms silver metal, ruling out a fluoride-induced desilylation process. Although silver is recovered at the end of the reaction, a cheaper oxidizing reagent is still lacking.448,449... 

The first step of the reaction involves the dimerization of MPG via single electron transfer process by Ti(III) and the sequential Ti(IV)-catalyzed intramolecular heterolytic cleavage of the dimer, regenerating M PG and the nucleophilic radical (Equations 14.25 and 14.26). 

Pandey, G., Hajra, S., Ghorai, M.K., and Kumar, K.R. (1997) Designing photosystems for harvesting photons into electrons by sequential electron-transfer processes reversing the reactivity profiles of a,p-unsaturated ketones as carbon radical precursor by one electron reductive p-activation. Journal of the American Chemical Society, 119, 8777-8787. 

N-Phthalimide-linked peptides that contain C-terminal a-amidotrimethylsilyl groups undergo macrocyclization reactions via sequential single-electron transfer processes [11, 12]. Thus, electron transfer from the neighboring amide to the excited phthalimide chromophore leads to an amide radical cation. Hole migration to the ot-amidotrimethylsilane center is followed by desilylation to form an 1, co-biradical intermediate, which finally cydizes to the macrocydic peptide (Scheme 9.9). 

Figure 9(a) illustrates how a combination of OAT (117) and CEPT (118) describes the substrate reaction catalyzed by SO and accounts for observed intermediates. Note that the regeneration of the oxidized Mo(VI) state by a CEPT process on the left side of Fig. 9(a) occurs in two discrete one-electron steps via a Mo(V) state. Evidence to support participation of a Mo(V) intermediate in SO, as well as in other enzymes, comes from EPR spectroscopy (102, 103). In the case of SO, the electrons lost in the CEPT process are sequentially transferred to a Fe(III) center of heme b located within the protein and subsequently to cytochrome c (61). 

The last step in the photosynthetic algal H2 production process occurs with the sequential transfer of two negative charges from reduced ferredoxin to a soluble [FeFe] hydrogenase,98,216 which is almost isoenergetic (A/hi = 0.020)11 with ferre... 

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