Consiglio Nazionale delle Ricerche, Istituto di Biostrutture e Bioimmagini (IBB-CNR), Via De Amicis 95, I-80145 Napoli, Italy.
Int J Mol Sci. 2023 Aug 9;24(16):12614. doi: 10.3390/ijms241612614.
I-motifs are non-canonical DNA structures formed by intercalated hemiprotonated (CH·C)+ pairs, i.e., formed by a cytosine (C) and a protonated cytosine (CH+), which are currently drawing great attention due to their biological relevance and promising nanotechnological properties. It is important to characterize the processes occurring in I-motifs following irradiation by UV light because they can lead to harmful consequences for genetic code and because optical spectroscopies are the most-used tools to characterize I-motifs. By using time-dependent DFT calculations, we here provide the first comprehensive picture of the photoactivated behavior of the (CH·C)+ core of I-motifs, from absorption to emission, while also considering the possible photochemical reactions. We reproduce and assign their spectral signatures, i.e., infrared, absorption, fluorescence and circular dichroism spectra, disentangling the underlying chemical-physical effects. We show that the main photophysical paths involve C and CH+ bases on adjacent steps and, using this basis, interpret the available time-resolved spectra. We propose that a photodimerization reaction can occur on an excited state with strong C→CH+ charge transfer character and examine some of the possible photoproducts. Based on the results reported, some future perspectives for the study of I-motifs are discussed.
I 型发夹结构是由交错半质子化（CH·C）+对形成的非规范 DNA 结构，即由胞嘧啶（C）和质子化胞嘧啶（CH+）形成，由于其生物学相关性和有前途的纳米技术特性，目前引起了极大的关注。重要的是要描述 DNA 中的 I 型发夹结构在受到紫外线照射后发生的过程，因为它们可能对遗传密码产生有害影响，并且因为光光谱学是表征 I 型发夹结构的最常用工具。通过使用时间依赖的 DFT 计算，我们在这里提供了 I 型发夹结构（CH·C）+核心从吸收到发射的光激活行为的第一幅综合图景，同时还考虑了可能的光化学反应。我们复制并分配了它们的光谱特征，即红外、吸收、荧光和圆二色性光谱，阐明了潜在的化学物理效应。我们表明，主要的光物理途径涉及相邻步骤上的 C 和 CH+碱基，并在此基础上解释了现有的时间分辨光谱。我们提出，具有强 C→CH+电荷转移特征的激发态上可能发生光二聚化反应，并检查了一些可能的光产物。基于报告的结果，讨论了研究 I 型发夹结构的一些未来展望。
