Influence of Proton Acceptors on the Proton-Coupled Electron Transfer Reaction Kinetics of a Ruthenium–Tyrosine Complex,The Journal of Physical Chemistry B

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Influence of Proton Acceptors on the Proton-Coupled Electron Transfer Reaction Kinetics of a Ruthenium–Tyrosine Complex
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2017-11-13 00:00:00 , DOI: 10.1021/acs.jpcb.7b06443
J. Christian Lennox ₁ , Jillian L. Dempsey ₁

Affiliation

A polypyridyl ruthenium complex with fluorinated bipyridine ligands and a covalently bound tyrosine moiety was synthesized, and its photo-induced proton-coupled electron transfer (PCET) reactivity in acetonitrile was investigated with transient absorption spectroscopy. Using flash–quench methodology with methyl viologen as an oxidative quencher, a Ru³⁺ species is generated that is capable of initiating the intramolecular PCET oxidation of the tyrosine moiety. Using a series of substituted pyridine bases, the reaction kinetics were found to vary as a function of proton acceptor concentration and identity, with no significant H/D kinetic isotope effect. Through analysis of the kinetics traces and comparison to a control complex without the tyrosine moiety, PCET reactivity was found to proceed through an equilibrium electron transfer followed by proton transfer (ET-PT) pathway in which irreversible deprotonation of the tyrosine radical cation shifts the ET equilibrium, conferring a base dependence on the reaction. Comprehensive kinetics modeling allowed for deconvolution of complex kinetics and determination of rate constants for each elementary step. Across the five pyridine bases explored, spanning a range of 4.2 pK_a units, a linear free-energy relationship was found for the proton transfer rate constant with a slope of 0.32. These findings highlight the influence that proton transfer driving force exerts on PCET reaction kinetics.

中文翻译：

质子受体对钌-酪氨酸配合物质子耦合电子转移反应动力学的影响

合成了具有氟化联吡啶配体和共价键合的酪氨酸部分的聚吡啶钌复合物，并通过瞬态吸收光谱研究了其在乙腈中的光诱导质子偶联电子转移（PCET）反应性。使用快速淬灭法，以甲基紫精作为氧化猝灭剂，Ru ³⁺产生了能够引发酪氨酸部分的分子内PCET氧化的分子物种。使用一系列取代的吡啶碱，发现反应动力学随质子受体浓度和特性的变化而变化，没有明显的H / D动力学同位素效应。通过动力学痕量分析并与不含酪氨酸部分的对照复合物进行比较，发现PCET反应性通过平衡电子转移，然后通过质子转移（ET-PT）途径进行，其中酪氨酸自由基阳离子的不可逆去质子化改变了ET平衡，赋予了对反应的基本依赖性。全面的动力学建模可以对复杂的动力学进行反卷积，并可以确定每个基本步骤的速率常数。在探索的五个吡啶碱基中，以Ka_为单位，发现质子传递速率常数的线性自由能关系具有0.32的斜率。这些发现突出了质子传递驱动力对PCET反应动力学的影响。

更新日期：2017-11-13

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