Rivera Joel J, Trinh Christina, Kim Judy E
Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, United States.
ACS Phys Chem Au. 2022 Nov 29;3(1):63-73. doi: 10.1021/acsphyschemau.2c00042. eCollection 2023 Jan 25.
Tryptophan is one of few residues that participates in biological electron transfer reactions. Upon substitution of the native Cu center with Zn in the blue-copper protein azurin, a long-lived tryptophan neutral radical can be photogenerated. We report the following quantum yield values for Zn-substituted azurin in the presence of the electron acceptor Cu(II)-azurin: formation of the tryptophan neutral radical (Φ), electron transfer (Φ), fluorescence (Φ), and phosphorescence (Φ), as well as the efficiency of proton transfer of the cation radical (Φ). Increasing the concentration of the electron acceptor increased Φ and Φ values and decreased Φ without affecting Φ. At all concentrations of the acceptor, the value of Φ was nearly unity. These observations indicate that the phosphorescent triplet state is the parent state of electron transfer and that nearly all electron transfer events lead to proton loss. Similar results regarding the parent state were obtained with a different electron acceptor, [Co(NH)Cl]; however, Stern-Volmer graphs revealed that [Co(NH)Cl] was a more effective phosphorescence quencher ( = 230 000 M) compared to Cu(II)-azurin ( = 88 000 M). Competition experiments in the presence of both [Co(NH)Cl] and Cu(II)-azurin suggested that [Co(NH)Cl] is the preferred electron acceptor. Implications of these results in terms of quenching mechanisms are discussed.
色氨酸是参与生物电子转移反应的少数残基之一。在蓝色铜蛋白天青蛋白中用锌取代天然铜中心后，可以光生成长寿命的色氨酸中性自由基。我们报告了在电子受体铜（II）-天青蛋白存在下锌取代天青蛋白的以下量子产率值：色氨酸中性自由基的形成（Φ）、电子转移（Φ）、荧光（Φ）和磷光（Φ），以及阳离子自由基的质子转移效率（Φ）。增加电子受体的浓度会增加Φ和Φ值并降低Φ，而不影响Φ。在所有受体浓度下，Φ值几乎为1。这些观察结果表明，磷光三重态是电子转移的母态，并且几乎所有电子转移事件都会导致质子损失。使用不同的电子受体[Co(NH₃)₅Cl]₂也获得了关于母态的类似结果；然而，斯特恩-沃尔默图显示，与铜（II）-天青蛋白（Kₛᵥ = 88000 M⁻¹）相比，[Co(NH₃)₅Cl]₂是一种更有效的磷光猝灭剂（Kₛᵥ = 230000 M⁻¹）。在同时存在[Co(NH₃)₅Cl]₂和铜（II）-天青蛋白的情况下进行的竞争实验表明，[Co(NH₃)₅Cl]₂是首选的电子受体。讨论了这些结果在猝灭机制方面的意义。
