Photochemistry and Molecular Science, Department of Chemistry, Ångström Laboratory, Uppsala University, Box 532, SE-751 20 Uppsala, Sweden.
J Am Chem Soc. 2012 Oct 3;134(39):16247-54. doi: 10.1021/ja3053859. Epub 2012 Sep 24.
Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to a covalently linked Ru(bpy)(3) 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.
质子偶联电子转移（PCET）从酪氨酸（TyrOH）到共价连接的[Ru（bpy）（3）]（2+）光敏剂在水介质中已被激光闪光猝灭动力学系统地重新研究，作为自由基酶和光化学能量转换中 PCET 的模型系统。以前对 Ru-TyrOH 分子的动力学研究（Sjödin 等人，J. Am. Chem. Soc. 2000，122，3932;Irebo 等人，J. Am. Chem. Soc. 2007，129，15462）已经建立了两种机制。当 pH < pK（a）（TyrOH）时，观察到协同电子-质子（CEP）转移，该转移是 pH 依赖性的，但与 [OH（-）] 不成一级关系，并且当缓冲浓度足够低（小于约 5 mM）时，也与缓冲浓度无关。此外，在 pH > 10 时报道了从酪氨酸酚盐（TyrO（-））到电子转移的 pH 独立速率常数。在这里，我们比较了四个 Ru-TyrOH 分子的 PCET 速率和动力学同位素效应（k（H）/k（D）），这些分子的 Ru（III/II）氧化剂强度在 pH 为 1-12.5 的范围内变化。基于这些数据，通过分析动力学竞争和动力学同位素效应（KIE），观察到并确定了另外两种机制：（i）在低 pH 下占主导地位的机制，分配给逐步电子优先的 PCET，（ii）以 OH（-）为质子受体的逐步质子优先的 PCET，在 pH = 10 左右占主导地位。讨论了溶液 pH 和 Ru（III/II）氧化剂的电化学势对不同机制之间竞争的影响。所研究的系统可以作为一般 PCET 反应在水（H（2）O，OH（-））作为主要质子受体的机制多样性的模型。
