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 d þ H 1 ) 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 (l ¼ 1.4 eV at pH ¼ 7) and a significant kinetic isotope effect: k H /k D ¼ 1.5-3. We can explain the pH-dependence and the high l by the pH-dependent DG1 0 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 l from the temperature dependent rate constant, correction is made for the large entropy increase of the reaction (TDS rxn E0.41 eV at pH ¼ 7 and T ¼ 298 K). The step-wise ETPT mechanism on the other hand shows a pHindependent 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.

No abstract

We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO(2). The complex mimics the function of the tyrosine(Z)() and chlorophyll unit P(680) in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO(2) via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO(2). The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 micros with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO(2) to the photogenerated Ru(III).