Abstract
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 <pKa(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 (kH/kD) 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.
| Original language | English |
|---|---|
| Pages (from-to) | 16247-16254 |
| Number of pages | 8 |
| Journal | Journal of the American Chemical Society |
| Volume | 134 |
| Issue number | 39 |
| DOIs | |
| Publication status | Published - Oct 3 2012 |
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ASJC Scopus subject areas
- Chemistry(all)
- Catalysis
- Biochemistry
- Colloid and Surface Chemistry
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Spanning four mechanistic regions of intramolecular proton-coupled electron transfer in aRu(bpy)3 2+-Tyrosine Complex. / Irebo, Tania; Zhang, Ming Tian; Markle, Todd F.; Scott, Amy M.; Hammarström, Leif.
In: Journal of the American Chemical Society, Vol. 134, No. 39, 03.10.2012, p. 16247-16254.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Spanning four mechanistic regions of intramolecular proton-coupled electron transfer in aRu(bpy)3 2+-Tyrosine Complex
AU - Irebo, Tania
AU - Zhang, Ming Tian
AU - Markle, Todd F.
AU - Scott, Amy M.
AU - Hammarström, Leif
PY - 2012/10/3
Y1 - 2012/10/3
N2 - 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 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 (kH/kD) 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.
AB - 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 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 (kH/kD) 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.
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U2 - 10.1021/ja3053859
DO - 10.1021/ja3053859
M3 - Article
VL - 134
SP - 16247
EP - 16254
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 39
ER -