Kinetics of intramolecular electron transfer

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). 

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). 

Many clever methods have been developed for generating the thermodynamically unfavored form of the one-electron reduced protein. Details of these methods, which are based on a variety of photochemical and pulse radiolysis techniques, may be found in the original references. The kinetics of intramolecular electron transfer may then be followed by monitoring the rate of approach to the thermodynamically favored state. 

Despite the clear implication of the involvement of intramolecular electron transfer in the chemiluminescence of certain dioxetanes, there have been no clear examples of intermolecular electron exchange luminescence processes with dioxetanes. In a recent note, however, Wilson (1979) reports the observation of catalysis of the chemiluminescence of tetramethoxy-1,2-dioxetane by rubrene and, most surprisingly, by 9,10-dicyanoanthracene. While catalysis by the added fluorescers was not kinetically discernible, a lowering of the activation energy for chemiluminescence was observed. These results were interpreted not in terms of an actual electron transfer with the formation of radical ions, but rather in terms of charge transfer interactions between fluorescer and dioxetane in the collision complex. In any event, these results certainly emphasize the need for caution in considering the fluorescer as a passive energy acceptor in dioxetane chemiluminescence. 

Machonkin, T. E. and Solomon, E. T. (2000) The thermodynamics, kinetics, end molecular mechanism of intramolecular electron transfer in human ceruloplasmin, J. Am. Chem. Soc. 122,12547-12560. 

The midpoint redox potentials for flavin reduction in PDR are E ox/sq = nl74mV for the first couple and E sq/red = n274mV for the second couple (Gassner et al., 1995). The potential for the 2Fe-2S center is E p = nl74 mV. Thus, the thermodynamic driving force favors reduction of the 2Fe-2S center, especially by the first electron transfer. The rate of intramolecular electron transfer for the reduced flavin to the oxidized 2Fe-2S center has been estimated at >200sec based on simulation of stopped flow kinetic data (Correll et ah, 1992). 

Ealzon, L., and Davidson,V. L., 1996, Kinetic model for the regulation hy substrate of intramolecular electron transfer in trimethylamine dehydrogenase. Biochemistry 35 244592452. 

Hexacy am ferrates (III). These are of particular interest as pigments and in relation to kinetics and mechanisms of intramolecular electron transfer. The structure of Bi[Fe(CN)6]-41120 has been elucidated, and Ce[Fe(CN)6]-5H20 and Gd[Fe(CN)6]-4H20 studied by X-ray crystallography and Mossbauer spectroscopy. ... 

Electron Transfer Reactions and Exciolexes - Photoinduced electron transfer is one of the most important areas of research. A review of photoinduced electron transfer and electron acceptor complexes usefully surveys the subject . Details of the mechanisms can be obtained by very short time resolution spectroscopy. Dynamic solvent effects on intramolecular electron-transfer involve solvent fluctuations. Time resolved ps emission spectroscopy has been used to examine the kinetics of intramolecular charge transfer in bis(4-aminophenyl)sulphone in ethanol as a function of temperature in this respect 2. it has... 

T -C5Me5) Ir (CO) (n -NCCgH4Cl) in hydrocarbon solvents results in activation of the solvent C-H bonds,and a study of the photoactivation of methane matrices by complexes of the type (M(n -C5Rs)(CO)2l (M Ir or Rh, R H or Me) has been reported.Photoinduced charge separation and recombination kinetics have been measured for the dimeric Ir(I) complexes [ Ir ( i-pz ) (CO) (PPh2 o(CH2)2R) ) I2 IP 3,5-dime thy Ipyrazolyl R pyridine (py), 4-phenylpyridine]. The results suggest that these complexes are well adapted to detailed studies of Intramolecular electron-transfer reactions. 

All three classes of intramolecular electron transfer are examples of scheme I ( 12.2.2.2.1) kinetics in which the precursor complex is sufficiently stable to observe under some condition. These classes differ in the components that make up the precursor complex. In one, two metal ions with similar coordination spheres are involved. The classic example is [(NH3)jRu(pyz)Ru(NH3)5] where pyz is pyrazine there are other examples with Ru Cu(l) and Cu(II), ferrocene and cyanide complexes of Fe. This field began with Prussian Blue (1704 est.). In these species the phenomenon of intervalence transfer is exhibited. Species of this sort have a low extinction coeffi-... 

The overall kinetics of bimolecular electron transfer reactions are complicated by the reactant association and product dissociation steps. To eliminate the contributions of these events to the reaction kinetics, many recent studies have concentrated on intramolecular electron transfer processes, where both the donor and acceptor groups are present in the same molecule. These systems provide the opportunity to measure directly the rate of electron transfer in the absence of competing bimolecular processes. For this reason, the examples in this section are concerned only with intramolecular processes. 

In order to measure intramolecular rate constants directly, a number of approaches have been taken which rely on judicious applications of synthetic chemistry or by taking advantage of the substitutional and/or binding properties of the reactants involved. Reaction (7) is an example of a system in which a direct measurement of intramolecular electron transfer has been made. The keys to the success of the experiment were the preparation of the initial Ru —Co species, the kinetically preferred reduction at the Ru site to give Ru -Co, and the irreversibility of the net reaction once formed, the Co moiety is rapidly lost by aquation. An example of a more straightforward case experimentally is shown in equation (75), where the combination of relatively facile substitution at the reducing agent and a low intramolecular electron transfer rate constant leads to the direct observation of intramolecular electron transfer by conventional spectroscopic techniques. ... 

Winkler JR, Nocera DG, Yocom KM, Bordignon E, Gray HB (1982) Electron-transfer kinetics of pentaamminemthenium(in)(histidine-33)-ferricytochrome c. Measurement of the rate of intramolecular electron transfer between redox centers separated by 15.ANG. in a protein. J Am Chem Soc 104 5798-5800... 

The resting to pulsed state kinetic transition involves an increase in the rate of intramolecular electron transfer through the oxidase (i.e., from Cu to Cu,-Heme Oj) [63], Unlike the conformations involved with proton pumping, the resting and pulsed conformational states persist over minutes. Observation of the resting to pulsed state kinetic transition indicates that electfon ttansfer through the... 

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