Tautomer contributions to the near UV spectrum of guanine: towards a refined picture for the spectroscopy of purine molecules
Tautomer contributions to the near UV spectrum of guanine: towards a refined picture for the spectroscopy of purine molecules
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互变异构体对鸟嘌呤近紫外光谱的贡献：对嘌呤分子光谱的精细描述
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发表时间：
2002
期刊：
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通讯作者：
M. Elhanine
中科院分区：
文献类型：
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作者：
Wutharath Chin;M. Mons;I. Dimicoli;F. Piuzzi;B. Tardivel;M. Elhanine
摘要：通过使用去布居激光技术，如IR-UV布居标记和质量选择的R2PI检测，我们证实了鸟嘌呤的近紫外光谱(310-280 nm)是由四个互变异构体负责的：两个烯醇和两个酮，每对都有一个7NH和9NH。除了每个互变异构体的紫外光谱外，还可以从荧光研究中获得有关激发态性质和动力学的额外信息。特别是，超过285 nm的荧光猝灭，背景吸收的存在，以及荧光发射中强红移成分的存在，为激发态中的强电子混合和有效的非辐射过程提供了证据。这些特征的细节被发现是互变构体依赖的。将目前的结果与其他嘌呤分子，如腺嘌呤或9-取代鸟嘌呤的文献数据进行比较，使我们能够对嘌呤分子的光谱和动力学提出新的见解。首先，在嘌呤家族中发现气相中的互变异构体分布有很大的变异性，特别是一个简单的分子变化，就像鸟嘌呤上的9-甲基化一样，可以将互变异构体的分布减少到单一物种(烯醇式)。由于吸收光谱既依赖于互变构体又依赖于取代基，所以互变构体的数目是控制紫外光谱整体形状的主要参数之一。其次，文献中经常提到的激发态模型，包括不同性质的激发态之间的电子耦合，即ππ*和nπ*态，可以解释目前对鸟嘌呤的荧光测量，假设这些体系存在广泛的激发态电子混合。
Abstract:By using depopulation laser techniques, like IR-UV population labeling coupled to mass-selected R2PI detection, we confirm that four tautomers are responsible for the near UV spectroscopy (310-280 nm) of guanine: two enol and two keto forms, each pair having a 7NH and a 9NH form. Besides the UV spectroscopy of each tautomer, additional information on the excited state nature and dynamics is obtained from fluorescence studies. In particular, the quenching of fluorescence beyond 285 nm, the existence of a background absorption, as well as the existence of a strongly red-shifted component in the fluorescence emission provides evidence for a strong electronic mixing in the excited state together with an efficient non-radiative process. The details of these features are found to be tautomer-dependent. Comparison of the present results with literature data on other purine molecules, like adenine or 9-substituted guanines, enables us to propose a new insight on the spectroscopy and dynamics of the purine molecules. First, a large variability of the tautomer distribution in the gas phase is found within the purine family, in particular a molecular change, as simple as a 9-methylation on guanine, can reduce the tautomer distribution to a single species (enol form). Since the absorption spectrum is tautomer-dependent as well as substituent-dependent, it turns out that the tautomer population is one of the major parameters that control the overall shape of the UV spectrum. Second, the excited state model, often evoked in the literature, which involves electronic coupling between excited states of different natures, namely ππ* and nπ* states, might account for the present fluorescence measurements on guanine, providing an extensive excited state electronic mixing is assumed for these systems.