There is increasing evidence that the direct absorption of photons with energies that are lower than the ionization potential of nucleobases may result in oxidative damage to DNA. The present work, which combines nanosecond transient absorption spectroscopy and quantum mechanical calculations, studies this process in alternating adenine–thymine duplexes (AT)_n. We show that the one-photon ionization quantum yield of (AT)₁₀ at 266 nm (4.66 eV) is (1.5 ± 0.3) × 10⁻³. According to our PCM/TD-DFT calculations carried out on model duplexes composed of two base pairs, (AT)₁ and (TA)₁, simultaneous base pairing and stacking does not induce important changes in the absorption spectra of the adenine radical cation and deprotonated radical. The adenine radicals, thus identified in the time-resolved spectra, disappear with a lifetime of 2.5 ms, giving rise to a reaction product that absorbs at 350 nm. In parallel, the fingerprint of reaction intermediates other than radicals, formed directly from singlet excited states and assigned to AT/TA dimers, is detected at shorter wavelengths. PCM/TD-DFT calculations are carried out to map the pathways leading to such species and to characterize their absorption spectra; we find that, in addition to the path leading to the well-known TA* photoproduct, an AT photo-dimerization path may be operative in duplexes.

中文翻译：

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通过直接吸收低能紫外线辐射，在交替的AT双链体中产生腺嘌呤基团

越来越多的证据表明，能量低于核碱基电离电势的光子直接吸收可能会导致DNA的氧化损伤。目前的工作结合了纳秒瞬态吸收光谱和量子力学计算，在交替的腺嘌呤-胸腺嘧啶双链体（AT）_n中研究了该过程。我们显示（AT）₁₀在266 nm（4.66 eV）处的单光子电离量子产率为（1.5±0.3）×10 ^-3。根据我们对由两个碱基对（AT）₁和（TA）₁组成的模型双工进行的PCM / TD-DFT计算，同时碱基配对和堆积不会在腺嘌呤自由基阳离子和去质子化自由基的吸收光谱中引起重要变化。因此，在时间分辨光谱中鉴定出的腺嘌呤自由基以2.5 ms的寿命消失，从而产生了在350 nm处吸收的反应产物。并行地，在短波长下检测直接由单重态激发态形成并分配给AT / TA二聚体的除自由基以外的反应中间体的指纹。进行PCM / TD-DFT计算以绘制导致此类物质的途径并表征其吸收光谱。我们发现，除了导致众所周知的TA *光产物的途径外，AT光二聚途径也可能在双链体中起作用。