State Key Laboratory of Precision Spectroscopy , East China Normal University , Shanghai , 200062 China.
Collaborative Innovation Center of Extreme Optics , Shanxi University , Taiyuan , Shanxi 030006 , China.
J Phys Chem B. 2018 Jul 19;122(28):7027-7037. doi: 10.1021/acs.jpcb.8b00927. Epub 2018 Jul 9.
Methylated cytosine is proved to have an important role as an epigenetic signal in gene regulation and is often referred to "the fifth base of DNA". A comprehensive understanding of the electronic excited state relaxation in cytosine and its methylated derivatives is crucial for revealing UV-induced photodamage to the biological genome. Because of the existence of multiple closely lying "bright" and "dark" excited states, the decay pathways in these DNA nucleosides are the most complex and the least understood so far. In this study, femtosecond transient absorption with different excitation wavelengths (240-296 nm) was used to study the relaxation of excited electronic states of 5-methylcytosine (5mC) and 2'-deoxy-5-methylcytidine (5mdCyd) in phosphate buffered aqueous solution and in acetonitrile solution. Two distinct nonradiative decay channels were directly observed. The first one is a several picosecond internal conversion channel that involves two bright ππ* states (ππ* and ππ*) when ππ* state is initially populated. The second channel contains the lower energy ππ* state and a so far experimental unidentified long-lived state which exhibits a several nanosecond lifetime. The long-lived state can only be accessed by the initially excited ππ* state. Inspired by this new discovery in 5mC and 5mdCyd, we revisited the decay of excited state of 2'-deoxycytidine (dCyd), revealing very similar decay pathways. Additionally, a well-known dark nπ* state (carbonyl lone pair) with ∼30 ps lifetime is present in both decay channels in dCyd. With our detailed experimental results, we successfully reconcile the long history debate of cytosine excited state relaxation mechanism by pointing out that the reason for the complex dynamics under traditional 266 nm excitation is mixed signals from the above-mentioned two distinct decay pathways. Our findings lead to a dramatically different and new picture of electronic energy relaxation in 5mdCyd/dCyd and could help to understand photostability as well as UV-induced photodamage of these nucleotides and related DNAs.
甲基化胞嘧啶被证明在基因调控中作为一种重要的表观遗传信号，通常被称为“DNA 的第五碱基”。全面了解胞嘧啶及其甲基化衍生物的电子激发态弛豫对于揭示紫外线对生物基因组的光损伤至关重要。由于存在多个紧密相邻的“亮”和“暗”激发态，这些 DNA 核苷的衰减途径是迄今为止最复杂和最不为人知的。在这项研究中，使用不同激发波长（240-296nm）的飞秒瞬态吸收来研究 5-甲基胞嘧啶（5mC）和 2'-脱氧-5-甲基胞嘧啶（5mdCyd）在磷酸盐缓冲水溶液和乙腈溶液中的激发电子态的弛豫。直接观察到两个不同的非辐射衰减通道。第一个是一个几皮秒的内转换通道，当最初占据ππ态时，涉及两个亮ππ态（ππ和ππ）。第二个通道包含较低能量的ππ态和一个迄今为止实验中未识别的长寿命态，该态表现出几纳秒的寿命。长寿命态只能通过最初激发的ππ态进入。受 5mC 和 5mdCyd 中这一新发现的启发，我们重新研究了 2'-脱氧胞嘧啶（dCyd）的激发态衰减，揭示了非常相似的衰减途径。此外，在 dCyd 的两个衰减通道中都存在一个已知的暗 nπ*态（羰基孤对），其寿命约为 30ps。根据我们详细的实验结果，我们成功地调和了关于胞嘧啶激发态弛豫机制的长期争论，指出在传统的 266nm 激发下，复杂动力学的原因是来自上述两个不同衰减途径的混合信号。我们的发现为 5mdCyd/dCyd 中的电子能量弛豫提供了一个截然不同的新图景，并有助于理解这些核苷酸和相关 DNA 的光稳定性以及紫外线诱导的光损伤。
