Tuning proton coupled electron transfer from tyrosine: A competition between concerted and step-wise mechanisms

The intra-molecular, proton-coupled electron transfer from a tyrosine residue to covalently linked tris-bipyridine ruthenium(III) complexes in aqueous solution (Ru^III-TyrOH → Ru^II-TyrO^. + H⁺) is studied in two complexes. The Ru^III-TyrOH state is generated by laser flash-induced photo-oxidation in the presence of the electron acceptor methyl viologen. The reaction is shown to follow either a concerted electron transfer-deprotonation (CEP) mechanism or a step-wise mechanism with electron transfer followed by deprotonation (ETPT). The CEP is characterised by a pH-dependent rate constant, a large reorganisation energy (λ = 1.4 eV at pH = 7) and a significant kinetic isotope effect: k_H/k_D = 1.5-3. We can explain the pH-dependence and the high λ by the pH-dependent ΔG°′ for proton release to bulk water, and by the additional reorganisation energy associated with the proton transfer coordinate (both internal and solvent), respectively. In the calculation of λ from the temperature dependent rate constant, correction is made for the large entropy increase of the reaction (TΔS_rxn ≈ 0.41 eV at pH = 7 and T = 298 K). The step-wise ETPT mechanism on the other hand shows a pH-independent rate, a lower reorganisation energy and no kinetic isotope effect. We propose that our complexes can be used as models to understand proton-coupled electron transfer in radical proteins. We show that the mechanism can be switched between CEP and ETPT by tuning the reaction pH and the electrochemical potential of the Ru^III/II oxidant. With a low driving force for the overall reaction the "energy conservative" CEP mechanism may dominate, in spite of the higher reorganisation energy as compared to ETPT.