黄素体系电子转移过程光谱研究

黄素类物质在生物体内广泛存在，是许多电子转移反应的活性中心，也是电子传递链的重要组成部分。其受到光照激发后引起的电子转移，是许多生命过程的基础与起始步骤。特别地，一种名为隐花色素的黄素蛋白在光激发后经一系列电子转移形成自旋相关自由基对，被认为是最有可能的生物磁敏物质，更使黄素体系电子转移过程的动力学，特别是自旋动力学过程倍受关注。对黄素电子转移过程及相关机理进行研究，有助于厘清多种生命过程的化学机理与影响因素。为此，科学界综合运用了多种仪器与测试手段，其中主要包括紫外-可见光谱，荧光光谱，瞬态吸收光谱，光化学诱导动态核极化（Photo-CIDNP）技术等。通过多年的研究，对黄素在生物体内的作用机理与电子转移过程的认识经历了由浅入深，不断深入的过程。紫外-可见光谱（UV-Vis）主要用于研究黄素系统中的电子激发，自旋动力学和电子转移。结合理论计算，UV-Vis还可以识别电子转移中涉及的基团并进行定量分析。荧光光谱可以识别电子受激发的物质，在反应过程中观察黄素和半醌中间体的产生和消耗，并确定其氧化还原和质子化状态。瞬态吸收光谱适于观测反应过程中出现的短寿命物种，其中飞秒泵浦探测技术的引入大大提高了观测的时间分辨率，并且可以通过光谱特征区分单重态和三重态的自由基对。光化学诱导动态核极化核磁共振（NMR）可以直接观察电子-核自旋动力学过程。磁场依赖性photo-CIDNP NMR揭示了控制单重态与三重态互变的因素，并提出了生物地磁导航可能依赖的化学机制。腔吸收与单分子光谱的运用，从技术上提高了实验装置的灵敏度并降低检测限。主要介绍黄素体系电子转移过程研究所运用的各种光谱手段与取得的成果，并展望其未来。

Spectroscopic Techniques in the Study of Electron Transfer in Flavin Systems

Flavins are widely present in organisms and active centers of many electron-transfer reactions. Therefore, they play an important role in biological electron transport chains. Electron transfer caused by light excitation of flavins is the initial step of many living processes. Cryptochromes containing flavin as a cofactor undergo a series of electron-transfer steps to form spin-correlated radical pairs (SCRP) after light excitation. Cryptochromes are considered the most likely candidate for an avian magnetoreceptor, which initiated research on the dynamics of the electron transfer in the flavin system, especially on their spin dynamics. The study of electron transfer and related processes in flavoproteins will allow one to understand biochemical mechanisms and reveal the influencing factors of various living processes. Therefore, numerous research methods, including UV-Vis spectroscopy, fluorescence spectroscopy, transient absorption spectroscopy, electron paramagnetic resonance, photochemical induced dynamic nuclear polarization (photo-CIDNP) and other spectroscopic techniques. We review studies of domestic and foreign scholars on electron transfer of flavin systems, and discuss the recent progress in various major research methods. UV-Vis spectroscopy is mainly used to study electronic excitation, spin-dynamics, and electron transfer in the flavin systems. UV-Vis spectroscopy might identify the groups involved in electron transfer and perform quantitative analysis combined with theoretical predictions. Fluorescence spectroscopy can identify electronically excited species, observe the rise and decay of, for example, flavin and semiquinone intermediates during the reaction course, and identify their redox and protonation states. Transient optical spectroscopy is suitable for capturing short-lived species that appear in the reaction process. In particular, introducing femtosecond pump-probe technology greatly shortened the time-resolution of observation and can distinguish between singlet- and triplet-born radical pair dynamics. Photo-CIDNP nuclear magnetic resonance (NMR) allows -to observe the electron-nuclear spin dynamics directly. Such direct access to the bio-geomagnetic operational mechanism might pave the way for practical applications. Magnetic field-dependent photo-CIDNP NMR reveals the factors controlling the singlet-to-triplet interconversion and suggests a possible chemical mechanism of bio-geomagnetic navigation. The application of cavity absorption and single-molecule spectroscopy technically improves the sensitivity of the experimental device and reduces the detection limit. This article mainly introduces the various spectroscopic techniques to study the electron-transfer process of flavin systems and their research results. Finally, possible future developments in this field are briefly discussed.