Reaction electron self-exchange

Sato et have used paramagnetic proton NMR to measure electron selfexchange rates in five different plastocyanins. This work attempts to understand the relationship between the self-exchange rate and the structures of acidic and hydrophobic residues on the electron transfer pathway. 

Hansen et alP describe a proton NMR method for detecting transient interactions between protein molecules in solution. The method relies on the intermolecular paramagnetic contribution to the NMR relaxation of protons at or close to the interaction surface. The method was apphed to the soluble electron transfer protein, plastocyanin, and used to locate surface patches that may be involved in electron transfer. 

---

Esr spectroscopy has also been used to study pure solvent dynamics in electron self-exchange reactions (Grampp et al., 1990a Grampp and Jaenicke, 1984a,b). When the systems are not linked by a spacer (i.e. TCNQ- /TCNQ (TCNQ = tetracyanoquinodimethane), the homogeneous bimolecular rate constants /chom are given by (10), with fcA the association constant and kET... 

The Marcus therory provides an appropriate formalism for calculating the rate constant of an outer-sphere redox reaction from a set of nonkinetic parameters1139"1425. The simplest possible process is a self-exchange reaction, where AG = 0. In an outer-sphere electron self-exchange reaction the electron is transferred within the precursor complex (Eq. 10.4). 

Fig. 13.5 Arrhenius plots of electron self-exchange reaction between ZnT(t-Bu)PP+ and ZnT(t-Bu)PP in different solvents [16].

Ionic conductivity also exists in redox conductors (see - solid-state electrochemistry), when ion movement accompanies (is coupled to) the electron self-exchange reaction. In those systems, the redox centers are immobilized, but there exists a relatively large population of mobile ions (counter ions) to assure the local electroneutrality. 

Figure 22 Effect of electron self-exchange reaction (Fe /Fe ") on oxidized heme methyl resonance line widths of Pseudomonas aeruginosa ferricytochrome C551, 25 °C. (a) 100% Fe , (b) 98% Fe L This ESE reaction is fast on the NMR timescale ( 10 s )...

Electron self-exchange reactions in macrobicyclic cobalt complexes have intensively been investigated. The rate constant of such reactions obtained for a variety of complexes, listed in Table 52, differ by several orders of magnitude (from 0.011 and 0.02 for the [CoCdiMesAMHsar)] and [Co(diAMHsar)]° cations to 2.8x10 for the hexathioether macrobicyclic [Co(diMEsar-S6)] + cation). The available data allow one to determine certain rules for the variation in the rate of electron self-exchange in macrobicyclic cobalt complexes. 

It is called the cross-relation because it is algebraically derived from expressions for the two related electron self-exchange reactions shown inEquations 1.21 and 1.22.. Associated with these reactions are two self-exchange rate constants k and k22 and reorganization energies Xu and 22-... 

The simplest reactions in solution chemistry are electron self-exchange reactions (Equation 6.10), in which the reactants and products are the same (the asterisk is used to identify a specific isotope). The only... 

On the other hand, [Cu(I)L ]" (10,24), a stronger reducing agent than Cu" (aq), affects the rate constants of the reduction reactions in a more complex way. The rate constant of the outer-sphere reduction is nearly not affected, that is, the decrease in the rate constant of the electron self-exchange reaction compensates the increase in the redox potential. For the inner-sphere reactions, slows down the reaction with ClsCCOO" probably due to steric hindrance. On the other hand, accelerates considerably the reduction of NO2 probably by stabilizing the transient complex L Cu(I)N02 and has only a minor effect on the rate of reduction of [Co(III)(NH3)5Cl]. ... 

A mechanism involving the polarization of the ascorbate ligand by a Cu(II) central ion was proposed (138), though the involvement of Cu(I) cannot be ruled out (139). All these reactions proceed via the inner-sphere mechanism however, the copper-catalyzed reduction of superoxide boimd to a binuclear cobalt(III) complex by 2-aminoethanethiol proceeds via the outer-sphere mechanism (140). This is attributed to the effect of 2-aminoethanethiol as a hgand on the rate constant of the Cu(ll/1) electron self-exchange reaction which is suggested to proceed via the gated mechanism. 

Table 2.2. Electron Self-Exchange Reactions of Transition Metal Complex Couples at 25 °C ... 

White, B.A. and R.W. Murray (1987). Kinetics of electron self-exchange reactions between metaUoporphyrin sites in submicrometer polymeric films on electrodes. J. Am. Chem. Soc. 109, 2576-2581. 

---