Ruthenium electron-transfer protein models

The pathway model makes a nmnber of key predictions, including (a) a substantial role for hydrogen bond mediation of tunnelling, (b) a difference in mediation characteristics as a function of secondary and tertiary structure, (c) an intrinsically nonexponential decay of rate with distance, and (d) pathway specific hot and cold spots for electron transfer. These predictions have been tested extensively. The most systematic and critical tests are provided with ruthenium-modilied proteins, where a synthetic ET active group can be attached to the protein and the rate of ET via a specific medium structure can be probed (figure C3.2.5). 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

Lockwood and colleagues performed an analysis of decoherence in a ruthenium-modified blue copper protein similar to amicyanin. They found a short characteristic decoherence time of 2.4 fs, which they attributed on one hand to the diverging motion of the protein nuclei and on the other hand to the solvent molecules. Their conclusion was that both solvent and protein dynamics can affect both the rate and mechanism of electron transfer which is different to our conclusions on solvated TTQ where the solvent does not seem to play any role in decoherence. More precisely, solvent molecules start to play a role once decoherence has already occurred due to the intramolecular motions within the TTQ. Lockwood et al. used a classical force field for all the atoms, including those of the copper and ruthenium complexes, and a rigid SPC water model. In addition they did not carry out large ensembles of diverging... 

---