NADH和FAD在醇-水溶液中量子产率的测定：辐射和非辐射弛豫途径的分析
Determination of quantum yield of NADH and FAD in alcohol-water solutions: the analysis of radiative and nonradiative relaxation pathways
摘要：
本文用TCSPC方法研究了辅酶NADH和FAD在水-甲醇、水-乙醇和水-丙二醇混合溶剂中的荧光量子产率以及在450和355 nm激发下的时间分辨荧光。乙醇和甲醇中量子产率与乙醇浓度的关系相似，而丙二醇中量子产率与乙醇浓度的关系不同。发现FAD的量子产率行为几乎与醇的类型无关，并显示出随醇浓度增加5- 6倍的戏剧性增加。基于量子力学理论，建立了皮秒和纳秒时域内分子激发态弛豫动力学模型，并对实验结果进行了分析。特别是，该模型提供了一个新的洞察衰变相关光谱（DAS）现象的性质。对NADH中皮秒和纳秒淬灭作用的分析表明，甲醇和乙醇中的皮秒衰变可能不是通过烟酰胺和腺嘌呤部分的堆叠构型中的电子转移发生的，而是通过其他机制发生的。然而在丙二醇中，NADH的皮秒猝灭强烈依赖于丙二醇浓度，并且所测量的量子产率的急剧增加与皮秒和纳秒猝灭机制的贡献相关联。发现FAD的皮秒猝灭显著依赖于任何醇浓度，这表明它对测量的量子产率随醇浓度的上升做出了深刻的贡献。这一结论支持了FAD中异咯嗪和腺嘌呤部分之间通过$π$ -堆叠构象中的电子转移反应实现荧光猝灭的既定机制。
英文摘要：
Combined studies on fluorescence quantum yield in coenzymes NADH and FAD in water-methanol, water-ethanol and water-propylene glycol mixtures and on time-resolved fluorescence of the same molecules under excitation at 450 and 355~nm by means of the TCSPC method have been carried out. The dependence of quantum yield in NADH on alcohol concentration was found to be similar in methanol and ethanol and different from that in propylene glycol. The behavior of quantum yield in FAD was found to be almost independent of the type of alcohol and exhibited a dramatic 5--6 times increase with alcohol concentration. A model describing molecular excited state relaxation dynamics in pico- and nanosecond time domain was developed on the basis on the quantum mechanical theory and used for analysis of the experimental results. In particular, the model provided a new insight into the nature of the Decay Associated Spectra (DAS) phenomenon. The analysis of the role of pico- and nanosecond quenching in NADH suggested that the picosecond decay in methanol and ethanol likely does not occur through electron transfer in the stacking configuration of the nicotinamide and adenine moieties, but through other mechanisms. However in propylene glycol the picosecond quenching in NADH depended strongly on propylene glycol concentration and sharp increase of the measured quantum yield was associated with the contribution of both picosecond and nanosecond quenching mechanisms. The picosecond quenching to FAD was found to depend significantly on the any alcohol concentration suggesting that it gives a profound contribution to the rise of the measured quantum yield with alcohol concentration. This conclusion supports the established mechanism of the fluorescence quenching in FAD through electron transfer reaction in the $π$-stacked conformation between isoalloxazine and adenine moieties.
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