Long-lived triplet states are critical intermediates of thiobases for their applications in photodynamic therapy and as photoprobes for DNA/RNA–protein interactions, where thiobases are embedded in DNA/RNA and exist as thionucleosides. However, sugar moieties accelerate triplet decay rates, which is a common issue that must be resolved for thionucleosides. Here, we explore whether protonation of 2-thiocytosine (2tCyt) and 2-thiocytidine (2tCyd) under acidic pH can alter their triplet decays. Femtosecond spectroscopy demonstrates that 2tCytH⁺ and 2tCydH⁺ exhibit similar triplet lifetimes (3.1 ns), suggesting protonation diminishes the influence of ribose on triplet decay. Notably, the triplet lifetime of 2tCydH⁺ is 2-fold prolonged compared to 2tCyd. Further calculations reveal protonation effectively weakens the influence of ribose by altering electronic structures of triplet states and inhibiting the photoisomerization leading to rapid decay. These findings point to a new avenue of using protonation to modulate triplet decay dynamics of thionucleosides and develop pH-responsive strategy in photochemical applications.

中文翻译：

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质子化减弱了核糖对 2-硫胞苷三联体衰变的影响


长寿命三重态是硫代碱基的关键中间体，用于光动力疗法和作为 DNA/RNA-蛋白质相互作用的光探针，其中硫代碱基嵌入 DNA/RNA 并以硫核苷的形式存在。然而，糖基团会加速三联体衰变速率，这是硫核苷必须解决的常见问题。在这里，我们探讨了在酸性 pH 值下 2-硫胞嘧啶 （2tCyt） 和 2-硫胞苷 （2tCyd） 的质子化是否可以改变它们的三联体衰变。飞秒光谱表明，2tCytH⁺ 和 2tCydH⁺ 表现出相似的三重态寿命 （3.1 ns），表明质子化减少了核糖对三重态衰变的影响。值得注意的是，与 2tCyd 相比，2tCydH ⁺ 的三重体寿命延长了 2 倍。进一步的计算表明，质子化通过改变三重态的电子结构和抑制光异构化导致快速衰变，有效地削弱了核糖的影响。这些发现指出了一条使用质子化来调节硫核苷的三重态衰变动力学并在光化学应用中开发 pH 响应策略的新途径。