Slow electrons mechanism

DR. EPHRAIM BUHKS (University of Delaware) I would like to mention briefly some recent work which demonstrates that quantum-mechanical calculations really can provide a basis for understanding the mechanism of slow electron exchange in systems 3+ 2+... 

The low-spin t2g [Ru(H20)6] is by four orders of magnitude less labile than the t2g [Ru(H20)6] " (83). An la mechanism was attributed from the negative AV. Because the water exchange on both ruthenium complexes is slow, electron exchange could be measured directly by NMR on the [Ru(H20)6] " couple (27). It has been shown that the self-exchange on the... 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

The mechanism of the photoelectron emission is evidently the same as in the hydrocarbons. The loss in sharpness of the maxima and a relative abundance of slow electrons may have an explanation, if the sequence of the molecular orbitals in these pigments is more narrowly spaced than are those of the hydrocarbons. Possibly, there is an excitation of molecular vibrations. 

The reduction mechanism of diphenylacetylene has been variously interpreted. Laitinen and Wawzonek , using aqueous dioxane, proposed protonation via the dianion 1 following a slow electron transfer to the anion radical (Scheme 2). 

The electrochemical redox reaction of a substrate resulting from the heterogeneous electron transfer from the electrode to this substrate (cathodic reduction) or the opposite (anodic oxidation) is said to be electrochemically reversible if it occurs at the Nernstian redox potential without surtension (overpotential). This is the case if the heterogeneous electron transfer is fast, i.e. there must not be a significant structural change in the substrate upon electron transfer. Any structural change slows down the electron transfer. When the rate of heterogeneous electron transfer is within the time scale of the electrochemical experiment, the electrochemical process is fast (reversible). In the opposite case, it appears to be slow (electrochemically irreversible). Structural transformations are accompanied by a slow electron transfer (slow E), except if this transformation occms after electron transfer (EC mechanism). 

In addition to providing a binding site for the intermediate described above, Type 2 copper appears to mediate interactions between the metal components of the enzyme. Thus Type 2 depleted enzyme shows slow electron transfer between the remaining Type 1 and 3 copper species and the reoxidation of the reduced Type 2 depleted enzyme is slow ". Moreover, the redox potential of the Type 3 copper center depends on the redox and ligation state of the Type 2 center and the formation of a Type II copper-OH species apparently inactivates the protein. It appears, therefore, that deeper insight into the function and mechanism of the blue copper oxidases will come through a more fundamental understanding of the role and position of the Type 2 copper. 

We shall now consider in detail the mechanism of energy dissipation by interaction of charged particles with the valence electrons of molecules. The interactions of fast and slow electrons have to be treated separately. Fast electrons have much larger velocities than the valence electrons. This corresponds to energies larger than 100 eV. 

Based on a series of elegant studies on the enzymic and nonenzymic oxidation of amines and substrate analogs, Silverman and co-workers proposed that the mechanism of irreversible inactivation and substrate utilization by MAO is mediated through radical intermediates (95). Precedence for this mechanism is based on the electrochemical oxidation of amines, which is believed to proceed through the radical cation intermediate 22 (Scheme 16) (96-98). Thus, the corresponding mechanism for monoamine oxidation by MAO requires two one-electron transfers from the substrate to flavin (Scheme 17, compounds 23 and 24). Enzymic reaction is initiated by slow electron transfer of an amine non-bonded electron to the flavin cofactor, producing the amine radical cation 23 and the flavin semiquinone radical. Formation of the amine radical cation facilitates loss of the a-proton, thereby avoiding the removal of nonacidic protons that would be necessary in a carbanionic mechanism. Subsequent electron... 

Although in most azurins only a first-order, slow electron transfer is observed for some, the rate constants of this slow reaction phase monitored at both 410 and 625 nm do show a slight dependence on the protein concentrations. This observation is most probably due to a mechanism whereby electron transfer (ET) takes place between RSSR" and Cu(II) in two parallel reactions, one an inter- and the other an intramolecular process. In order to resolve between these two ET processes, the observed rate constants, determined at a given temperature, were plotted as a function of the [Cu(II)]azurin concentrations, and from the straight lines obtained with nonzero intercepts, the rate constants for the unimolecular process were calculated. 

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