Proton-Coupled Electron Transfers: pH-Dependent Driving Forces? Fundamentals and Artifacts
Proton-Coupled Electron Transfers: pH-Dependent Driving Forces? Fundamentals and Artifacts
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DOI：
10.1021/ja406712c
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发表时间：
2013-09-25
影响因子：
15
通讯作者：
Saveant, Jean-Michel
中科院分区：
文献类型：
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作者：
Bonin, Julien;Costentin, Cyrille;Saveant, Jean-Michel
除了其本身的兴趣之外，光生钌配合物对色氨酸的氧化也是利用协同质子电子转移（CPET）到水作为质子受体（具有 pH 依赖性驱动力）来解释数据的几个例子之一。由于这个概念违背了化学物理学的基本原理，因此尝试用易于接近的基质来揭示这种矛盾的根源是很有趣的。仔细检查带有 NH3+/NH2 基团的色氨酸（乙酯衍生物）的氧化情况表明，没有发现这种具有 pH 依赖性驱动力的非常规 H2O-CPET 的痕迹。反应机制简单地包括色氨酸衍生物的 NH3+ 酸和 NH2 碱性形式，先是决定速率的电子转移步骤，然后是去质子化步骤。色氨酸的乙酯-甲基酰胺衍生物也是如此，由于分子不带有酸碱基团，其行为更加简单。苯酚（另一种易于接触的基材）没有发现这种非常规的 H2O-CPET。因此可以推断，这同样适用于电子转移发生在分子内的不太容易获得的系统。这些观察结果有助于消除此类人为障碍，并改进现有的 H2O-CPET 反应模型，这是理解水链在自然系统中的作用的里程碑。
Besides its own interest, tryptophan oxidation by photogenerated Ru complexes is one of the several examples where concerted proton-electron transfer (CPET) to water as proton acceptor endowed with a pH-dependent driving force has been invoked to explain the data. Since this notion is contrary to the very basic principles of chemical physics, it was interesting to attempt uncovering the source of this contradiction with an easily accessible substrate. Careful examination of the oxidation of the tryptophan (ethyl ester derivative) bearing a NH3+/NH2 group showed that there is no trace of such an unconventional H2O-CPET with a pH-dependent driving force. The reaction Mechanism simply consists, with both the NH3+ acid and NH2 basic forms of the tryptophan derivative, in a rate-determining electron-transfer step followed by deprotonation steps. The same is true with the ethyl ester-methyl amide derivative of tryptophan, whose behavior is even simpler since the molecule does not bear an acid-base group. No such unconventional H2O-CPET was found with phenol, another easily accessible substrate. It may thus be inferred that the same applies to less easily available systems in which electron transfer occurs intramolecularly. These observations help to rid the road of such artificial obstacles and improve present models of H2O-CPET reactions, a landmark towards the understanding of the role of water chains in natural systems.