Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid
Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid
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DOI：
10.1021/acs.jpca.9b02023
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发表时间：
2019-05-23
影响因子：
2.9
通讯作者：
Bieske, Evan J.
中科院分区：
文献类型：
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作者：
Bull, James N.;da Silva, Gabriel;Bieske, Evan J.
去质子化的对香豆酸通常被认为是光活性黄色蛋白中生色团的模型，它在吸收蓝光后发生E-GT；Z异构化。用串联离子迁移质谱与激光激发相结合的方法研究了去质子化对香豆酸的光化学，结果表明，苯氧基和羧酸形式的E异构体在430 nm和360 nm处的作用光谱分别与最大值有明显的光化学响应。酚氧阴离子的E异构体在其最低的pi pi*跃迁激发下发生有效的自脱。对于羧酸去异构体的E异构体，单向光引发的质子转移通过一种可能涉及激发态烯醇酮互变异构化的机制产生苯氧基去异构体，随后发生一系列基态重排，包括第二次质子转移。这一机理得到了相关中间体酮基异构体制备和光谱探测的实验支持，并通过对可能的基态异构化过程的主方程模拟得到了支持。羧酸脱异构体的Z异构体表现出弱的Z->；E光异构化反应，与光破坏竞争(可能是电子分离)，表明E和Z异构体在激发态经历了不同的过程。这项研究强调了异构体选择光谱在表征具有多个去质子化中心的孤立阴离子的光化学方面的应用。
Deprotonated para-coumaric acid is commonly considered as a model for the chromophore in photoactive yellow protein, which undergoes E -> Z isomerization following absorption of blue light. Here, tandem ion mobility mass spectrometry is coupled with laser excitation to study the photochemistry of deprotonated para-coumaric acid, to show that the E isomers of the phenoxide and carboxylate forms have distinct photochemical responses with maxima in their action spectra at 430 and 360 nm, respectively. The E isomer of the phenoxide anion undergoes efficient autodetachment upon excitation of its lowest pi pi* transition. For the E isomer of the carboxylate deprotomer, a one-way photoinitiated proton transfer generates the phenoxide deprotomer through a mechanism postulated to involve an excited-state enol-keto tautomerism followed by a series of ground-state rearrangements including a second proton transfer. This mechanism is supported by experiments in which the relevant intermediate keto isomer is prepared and spectroscopically probed and through master equation modeling of possible ground-state isomerization processes. The Z isomer of the carboxylate deprotomer shows a weak Z -> E photoisomerization response that occurs in competition with photodestruction (presumably electron detachment), demonstrating that the E and Z isomers undergo different processes in their excited states. The study highlights the utility of isomer-selective spectroscopy for characterizing the photochemistry of isolated anions possessing multiple deprotonation sites.