Intramolecular electron transfer from

A Mossbauer study of the protein reacted with benzaldehyde (in parallel with EPR detection of Mo(V) signals) shows partial reduction of the iron—sulfur centers, indicating the involvement of the clusters in the process of substrate oxidation and rapid intramolecular electron transfer from the molybdenum to the iron—sulfur sites. 

Many 1,4-diazabutadiene adducts of dialkylzinc compounds show unusual reactivities (see also Section 2.06.10.7), such as the intramolecular electron transfer from zinc to the chelating ligand and the subsequent dimerization of these radicals.127 In solution, the carbon-carbon coupled dimer [MeZn(ButN=CHCH=NBut)2ZnMe] 71 is in equilibrium with its radical monomers (Scheme 56). Addition of potassium to a THF solution of the dimer produced cleanly 72, the first heteroleptic alkyldiamidozincate. 

In this mechanism, the rate-determining step is the intramolecular electron transfer from the ligand to dioxygen, Eq. (18), via the metal center of the [M(HA)(02)] 1 + complex for which the following structure was proposed ... 

Within this species, an intramolecular electron transfer from Cr(II) to Co(III) must occur, producing Cr(III) and Co(II). The adduct then breaks up and the Cr(III) takes along the chloride as the species CrCP+ ... 

Scheme 60). Griesbeck et al. assume that in a non-polar solvent such as benzene the intramolecular electron transfer from the methionic sulfur group is much faster than the abstraction of hydrogen from the hydroxyl group of the unprotected amino acid. C-Hydrogen abstraction leads to 313, whereas previous lactonization of the zwitterionic biradical 311 yields 314. Since the cis-hydroxy acid is not detected it is conceivable that it cyclizes immediately to the lactone 314. Photolysis of the corresponding methyl ester under the same conditions attains improved yields (84% combined) of two diastereomeric tricyclic products in a ratio of 48 52. 

Chlorides RMe2CCH2Cl [(a) R = Me, R = Ph and (b) R = CH2Ph] reacted with diphenylphosphide ions in liquid ammonia, via a proposed 5rn 1 mechanism and their reactivities were measured. The higher reactivity of (a) has been attributed to efficient intramolecular electron transfer from the phenyl ring to the C—Cl a bond (intra-ET catalysis). The lower reactivity of (b) is ascribed to a decrease in the rate of the intra-ET by elongation of the bridge by one methylene unit. The relative reactivity of (a) versus (b) is proposed to indicate the ratio of the intra-ET rates of the radical anions of both compounds. ... 

Pressure provokes transition of the linear (extended) conformation into the bent (V-like) one. (The V-like form is more compact and occupies a smaller volume.) It is obvious that the V-like form is favorable in respect of intramolecular electron transfer from the donor (the aniline part) to the acceptor (the pyrene part). In the utmost level of the phenomenon, the donor part transforms into the cation-radical moiety, whereas the acceptor part passes into the anion-radical moiety. Such transformation is impossible in the case of the extended conformation because of the large distance between the donor and acceptor moieties. The spectral changes observed reflect this conformational transition at elevated pressures. 

Other examples using pulse radiolysis include (a) studies on cytochrome c-di nitrate reductase from Thio-sphaera pantotropha to provide evidence for a fast intramolecular electron transfer from c-heme to rate constant for the reaction of... 

Ring closure was observed by Johnson and Berchtold, who irradiated cyclic ) -ketosulfide 111 and terf-butyl alcohol with 253.7-nm light. A 2-thietanone derivatives was formed following intramolecular electron transfer from the sulfur atom to the excited carbonyl. An unstable dipolar... 

Another redox switchable system is based on dyad 21 in which 2-chloro-1,4-naphthoquinone is covalently attached to 5-dimethyl-aminonaphthalene via a non-conjugated spacer. The intrinsic fluorescence of the dansyl excited state in dyad 21 is strongly quenched, due to the intramolecular electron transfer from the excited dansyl to the adjacent quinone acceptor. However, the fluorescence can be switched on by addition of a reducing agent. Apart from chemical switching, the fluorescence of dyad 21 can also be switched electrochemically. This can be realized using a photoelec -trochemical cell, and the solution starts to fluoresce upon application of a reductive potential.31... 

Upon deprotonation, [(trpy)(bpy)OsIII(4,4 -bpy)RuII(OH2)(bpy)2]5+ and [(trpy)(bpy)Osin(4,4 -bpy)RuUI(OH)(bpy)2]5+ undergo rapid intramolecular electron transfer to form [ (trpy)(bpy)Osn(4,4 -bpy)-RunI(OH)(bpy)2]4+ and [(trpy)(bpy)0sn(4,4 -bpy)RuIV(0)(bpy)2]4+, respectively, but no details have been given about the rates 228). Fast intramolecular electron transfer from the excited states of Os-(trpy)2]2+ complexes with donor or acceptor substituents have also been observed 229). 

The experimental approaches developed for the direct study of intramolecular electron transfer, as well as the chemical systems themselves, have found application in some very interesting studies. For example, reduction of the Rum site to Ru11 in the Rum—Co111 species (5) is followed by intramolecular electron transfer from Ru11 to Co111.101... 

The kinetics of intramolecular electron transfer from Ru(II) to Fe(III) in ruthenium-modified cytochrome c has been studied [77-80]. In these studies electron transfer from electron-excited Ru(II) (bpy)3, which was added to the protein solution, to ruthenium-modified horse heart cytochrome c, (NH3)5Ru(III) (His-33)cyt(Fe(III)), was found to produce (NH3)5Ru(II) (His-33)cyt (Fe(III)) in fivefold excess to (NH3)5Ru(III) (His-33)cyt(Fe(II)). As in refs. 72 and 73, in the presence of EDTA the (NH3)5Ru(II)(His-33)cyt(Fe(III)) decays mainly by intramolecular electron transfer to (NH3)5Ru(III)(His-33)cyt(Fe(II)). The rate constant k — 30 3s 1 at 296 K and does not vary substantially over the temperature range 273-353 K. Above 353 K intramolecular Ru(II) - Fe(III) electron transfer was not observed owing to the displacement of methionine-80 from the iron coordination sphere. The distance of intramolecular electron transfer in this case is also equal to 11.8 A (see Fig. 19). 

Intramolecular electron transfer from Ru(II) to Fe(III) in (NH3)3Ru(II) (His-33)cyt(Fe(III)) induced by pulse-radiolysis reduction of Ru(III) in the (NH3)5Ru(III) (His-33)cyt(Fe(III)) complex were investigated [84]. The results obtained differ from those of refs. 77-80 where flash photolysis was used to study the similar electron transfer reaction. It was found [84] that, over the temperature range 276-317 K the rate of electron transfer from Ru(II) to Fe(III) is weakly temperature dependent with EA 3.3 kcal mol 1. At 298 K the value of kt = 53 2 s"1. The small differences in the temperature dependence of the electron tunneling rate in ruthenium-modified cytochrome c reported in refs. 77-80 and 84 was explained [84] by the different experimental conditions used in these two studies. 

The charge photoseparation in porphyrin-quinone compounds with a rigid bicyclo[2.2.2]octyl bridge, ensuring a distance between the centres of P and Q of about 16 A, has been studied [57]. The rate constant of intramolecular electron transfer from P to Q was found to depend on the dielectric properties of the medium and reached 3.3 x 107s 1 for a solution of P-L-Q in propionitrile. 

Long range electron-transfer has also been demonstrated within the complex between zinc-substituted cytochrome c peroxidase and cyt c 59). The kinetics of intramolecular electron-transfer from Ru(II) to Fe(III) in ruthenium modified cyt c has also been investigated 58). 

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