Relaxation parameters, electron-transfer

Fig. 12.2. Free energy data for electron transfer between the protein cytochrome c and the small acceptor microperoxidase-8 (MP8), from recent simulations [47]. Top Gibbs free energy derivative versus the coupling parameter A. The data correspond to solvated cytochrome c the MP8 contribution is not shown (adapted from [47]) Bottom the Marcus diabatic free energy curves. The simulation data correspond to cyt c and MP8, infinitely separated in aqueous solution. The curves intersect at 77 = 0, as they should. The reaction free energy is decomposed into a static and relaxation component, using the two steps shown by arrows a static, vertical step, then relaxation into the product state. All free energies in kcalmol-1. Adapted with permission from reference [88]...

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

When the quencher contains heavy atoms nonradialive relaxation of the exciple occurs via the triplet state (heavy atom perturbation). A second mode of exciplex dissociation is through electron transfer between the excited molecule and the quencher. Ionization potential of the donor, electron affinity of the acceptor and solvent dielectric constant are important parameters in such cases. 

Dynamic solvent effect — is a phenomenon typical for adiabatic -> electron transfer and -> proton transfer reactions. This effect is responsible for a dependence of the reaction rate on solvent relaxation parameters. The initial search for a dynamic solvent effect (conventionally assumed to be a feature of reaction adiabatic-ity) consisted in checking the viscosity effect. However, this approach can lead to controversial conclusions because the viscosity cannot be varied without changing the -> permittivity, i.e. a dynamic solvent effect cannot be unambiguously separated from a -> static solvent effect [i]. Typically a slower solvent relaxation goes along with a higher permittivity, and the interplay of the two solvents effects can easily look as if either of them is weaker. The problems of theoretical treatment of the dynamic solvent effect of solvents having several relaxation times are considered in refs, [ii-iii]. 

The first explanation offered for the phenomenon of dispersive kinetics is that it is caused by a distribution of rates of primary electron transfer, and that the islowi P lifetimes originate from a minority of reaction centres from the islowi tail of this distribution. The energetic basis for this distribution could be an inhomogeneous distribution of a rate-determining parameter such as X, AG or Vda (or any combination of these) (Figure lOA). The principal alternative explanation is that the multiple lifetimes represent a time-dependent energetic relaxation of the P Ha intermediate due... 

The frequency associated with electron transfer, Vet, is estimated by quantum mechanics and depends on the degree of reaction adiabacity as measured by the coupling parameters, and the reorganization energy. Ex. In the case of adiabatic reactions it also depends on the longitudinal relaxation time of the solvent, Tl. A general expression for Vgt is... 

A second aspect of the cytochrome c/cytochrome a reaction relates to the electron transfer distance. When the cytochrome c binding sites are occupied by the Fe(III) species, the EPR parameters of neither nor are perturbed this indicates that the distance between the paramagnets is greater than lOA. Both fluorescence and magnetic relaxation measurements support this conclusion. This observation makes difficult a simple outer-sphere electron transfer explanation for the high rates observed in the cy-... 

In Section I we briefly discuss the relationship between the theoretical parameters and experimental observables in these experiments in terms of the spectroscopy of electrons in liquids. Experimental techniques are considered in more detail in Section II, while the data from electron solvation in pure liquids are reviewed in Section III in the context of the molecular dynamics of the host liquid. Section IV presents current results on electron trapping in very dilute polar systems and leads to speculation on mechanisms of electron localization. In Section V the first direct observations of a photoselective, laser-induced electron-transfer process are presented, following which we summarize as yet unresolved issues and speculate on future directions in the laser spectroscopy of electron-relaxation processes. 

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