Spanning Four Mechanistic Regions of Intramolecular Proton-Coupled Electron Transfer in a Ru(bpy)32+-Tyrosine Complex
2012 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 134, no 39, 16247-16254 p.Article in journal (Refereed) Published
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 (Sjodin 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 (H2O, OH-) as primary proton acceptor.
Place, publisher, year, edition, pages
2012. Vol. 134, no 39, 16247-16254 p.
proton-coupled electron transfer, phenol oxidation, artificial photosynthesis
Research subject Physical Chemistry
IdentifiersURN: urn:nbn:se:uu:diva-112056DOI: 10.1021/ja3053859ISI: 000309335000029OAI: oai:DiVA.org:uu-112056DiVA: diva2:284606
De 2 första författarna delar förstaförfattarskapet.2010-01-072010-01-072012-10-26Bibliographically approved