Spanning four mechanistic regions of intramolecular proton-coupled electron transfer in a Ru(bpy)3(2+)-tyrosine complex.
Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to a covalently linked [Ru(bpy)(3)](2+) photosensitizer in aqueous media has been systematically reinvestigated by laser flash-quench kinetics as a model system for PCET in radical enzymes and in photochemical energy conversion. Previous kinetic studies on Ru-TyrOH molecules (Sjödin et al. J. Am. Chem. Soc. 2000, 122, 3932; Irebo et al. J. Am. Chem. Soc. 2007, 129, 15462) have established two mechanisms. Concerted electron-proton (CEP) transfer has been observed when pH < pK(a)(TyrOH), which is pH-dependent but not first-order in [OH(-)] and not dependent on the buffer concentration when it is sufficiently low (less than ca. 5 mM). In addition, the pH-independent rate constant for electron transfer from tyrosine phenolate (TyrO(-)) was reported at pH >10. Here we compare the PCET rates and kinetic isotope effects (k(H)/k(D)) of four Ru-TyrOH molecules with varying Ru(III/II) oxidant strengths over a pH range of 1-12.5. On the basis of these data, two additional mechanistic regimes were observed and identified through analysis of kinetic competition and kinetic isotope effects (KIE): (i) a mechanism dominating at low pH assigned to a stepwise electron-first PCET and (ii) a stepwise proton-first PCET with OH(-) as proton acceptor that dominates around pH = 10. The effect of solution pH and electrochemical potential of the Ru(III/II) oxidant on the competition between the different mechanisms is discussed. The systems investigated may serve as models for the mechanistic diversity of PCET reactions in general with water (H(2)O, OH(-)) as primary proton acceptor.
Photochemistry and Molecular Science, Department of Chemistry, Ångström Laboratory, Uppsala University, Box 532, SE-751 20 Uppsala, Sweden.
Tania
T
Irebo
Ming-Tian
MT
Zhang
Todd F
TF
Markle
Amy M
AM
Scott
Leif
L
Hammarström
D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives* / Q000737:chemistry; D056831:Coordination Complexes; D004579:Electron Transport; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D016877:Oxidants / Q000737:chemistry; D010084:Oxidation-Reduction; D011522:Protons; D014443:Tyrosine / Q000737:chemistry*; D014867:Water / Q000737:chemistry; D017975:Ruthenium Compounds
D015082:2,2'-Dipyridyl; D056831:Coordination Complexes; D016877:Oxidants; D011522:Protons; D014443:Tyrosine; D014867:Water; C547232:tris(2,2-bipyridine)-ruthenium(II); D017975:Ruthenium Compounds