Spanning Four Mechanistic Regions of Intramolecular Proton-Coupled Electron Transfer in a Ru(bpy)3 2+–Tyrosine Complex

• Published in: Journal of the American Chemical Society Vol. 134; no. 39; pp. 16247 - 16254
• Main Authors: Irebo, Tania, Zhang, Ming-Tian, Markle, Todd F, Scott, Amy M, Hammarström, Leif
• Format: Journal Article
• Language: English
• Published: American Chemical Society 03.10.2012
• Subjects: artificial photosynthesis; Fysikalisk kemi; phenol oxidation; Physical Chemistry; proton-coupled electron transfer
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja3053859

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 RuIII/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 RuIII/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 (H2O, OH–) as primary proton acceptor.