Vayá Ignacio, Brazard Johanna, Huix-Rotllant Miquel, Thazhathveetil Arun K, Lewis Frederick D, Gustavsson Thomas, Burghardt Irene, Improta Roberto, Markovitsi Dimitra
LIDYL, CEA, CNRS, Université Paris Saclay, 91191, Gif-sur-Yvette, France.
Institut für Physikalische und Theoretische Chemie, Goethe-Universität, Max-von-Laue-Str. 7, 60438, Frankfurt am Main, Germany.
Chemistry. 2016 Mar 24;22(14):4904-14. doi: 10.1002/chem.201504007. Epub 2016 Mar 1.
The electronic excited states populated upon absorption of UV photons by DNA are extensively studied in relation to the UV-induced damage to the genetic code. Here, we report a new unexpected relaxation pathway in adenine-thymine double-stranded structures (AT)n . Fluorescence measurements on (AT)n hairpins (six and ten base pairs) and duplexes (20 and 2000 base pairs) reveal the existence of an emission band peaking at approximately 320 nm and decaying on the nanosecond time scale. Time-dependent (TD)-DFT calculations, performed for two base pairs and exploring various relaxation pathways, allow the assignment of this emission band to excited states resulting from mixing between Frenkel excitons and adenine-to-thymine charge-transfer states. Emission from such high-energy long-lived mixed (HELM) states is in agreement with their fluorescence anisotropy (0.03), which is lower than that expected for π-π* states (≥0.1). An increase in the size of the system quenches π-π* fluorescence while enhancing HELM fluorescence. The latter process varies linearly with the hypochromism of the absorption spectra, both depending on the coupling between π-π* and charge-transfer states. Subsequently, we identify the common features between the HELM states of (AT)n structures with those reported previously for alternating (GC)n : high emission energy, low fluorescence anisotropy, nanosecond lifetimes, and sensitivity to conformational disorder. These features are also detected for calf thymus DNA in which HELM states could evolve toward reactive π-π* states, giving rise to delayed fluorescence.
关于紫外线对遗传密码的损伤，人们对DNA吸收紫外线光子后所产生的电子激发态进行了广泛研究。在此，我们报道了腺嘌呤 - 胸腺嘧啶双链结构(AT)n中一种新的意外弛豫途径。对(AT)n发夹结构（六个和十个碱基对）以及双链结构（20和2000个碱基对）进行的荧光测量揭示了存在一个发射带，其峰值约为320 nm，并在纳秒时间尺度上衰减。针对两个碱基对进行的含时密度泛函理论(TD)-DFT计算，并探索了各种弛豫途径，使得该发射带可归因于由弗伦克尔激子与腺嘌呤到胸腺嘧啶电荷转移态之间混合产生的激发态。来自这种高能长寿命混合(HELM)态的发射与其荧光各向异性(0.03)一致，该荧光各向异性低于π - π态预期的值(≥0.1)。系统尺寸的增加会淬灭π - π荧光，同时增强HELM荧光。后一过程与吸收光谱的减色效应呈线性变化，两者均取决于π - π与电荷转移态之间的耦合。随后，我们确定了(AT)n结构的HELM态与先前报道的交替(GC)n结构的HELM态之间的共同特征：高发射能量、低荧光各向异性、纳秒级寿命以及对构象无序的敏感性。在小牛胸腺DNA中也检测到了这些特征，其中HELM态可能会演变为反应性π - π态，从而产生延迟荧光。
