The Inner-Sphere Barrier

X is thus related to the entire nuclear reorganization of an electron-transfer reaction. These nuclear barriers, which precede the actual electron transfer, involve bond-length changes within the reactants, and reorientation of the surrounding solvent dipoles. 

At the transition state, electron transfer takes place rapidly. During this brief moment, the nuclear geometry of the transition state remains fixed (the Franck-Condon principle). Following electron transfer, nuclear relaxation to the equilibrated successor state takes place. The products then separate from the successor state into the bulk of the solution. If the products are charged species, then the work, Wj, involved as the ions separate is w = — wc, and can be estimated from Eq. (28). 

Let us now take a closer examination of the expressions used to estimate the total reorganizational energy. 

The reorganization energy due to bond-length changes, Le., bond stretching, compression, bending, etc., can be estimated from 

Electron transfers in which vibrational (bond) changes are predominantly rate determining are classified as inner-sphere , i.e., X, Xs. These are to be distinguished from outer-sphere electron transfers where solvent motion plays the major role, i.e., Xs Xy. 

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Ruthenium complexes are excellent reagents for protein modification and electron-transfer studies. Ru +-aquo complexes readily react with surface His residues on proteins to form stable derivatives [20, 21]. Low-spin pseudo-octahedral Ru-complexes exhibit small structural changes upon redox cycling between the Ru + and Ru + formal oxidation states [3, 22]. Hence, the inner-sphere barriers to electron transfer (Ai) are small. With the appropriate choice of ligand, the Ru + + reduction potential can be varied from <0.0 to >1.5 V versus NHE [23]. Ru-bpy complexes bound to Lys and Cys residues have been employed to great advantage in studies of protein-protein ET reactions. The kinetics of electron transfer in cytochrome 65/cytochrome c [24], cytochrome c/cytochrome c peroxidase [12], and cytochrome c/cytochrome c oxidase [25] complexes have been measured with the aid of laser-initiated ET from a Ru-bpy label. 

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