Today’s genetic composition is the result of continual refinement processes on primordial heterocycles present in prebiotic Earth and at least partially regulated by ultraviolet radiation. Femtosecond transient absorption spectroscopy and state-of-the-art ab initio calculations are combined to unravel the electronic relaxation mechanism of pyrimidine, the common chromophore of the nucleobases. The excitation of pyrimidine at 268 nm populates the S₁(nπ*) state directly. A fraction of the population intersystem crosses to the triplet manifold within 7.8 ps, partially decaying within 1.5 ns, while another fraction recovers the ground state in >3 ns. The pyrimidine chromophore is not responsible for the photostability of the nucleobases. Instead, C2 and C4 amino and/or carbonyl functionalization is essential for shaping the topography of pyrimidine’s potential energy surfaces and results in accessible conical intersections between the initially populated electronic excited state and the ground state.

中文翻译：

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DNA和RNA单体光稳定性的起源：嘧啶发色团的激发态弛豫机理。

今天的遗传成分是对益生元地球中存在的并且至少部分受紫外线辐射调控的原始杂环不断精制过程的结果。飞秒瞬态吸收光谱法和最新的从头算相结合，以揭示嘧啶（核碱基的常见发色团）的电子弛豫机理。嘧啶在268 nm激发激发了S ₁（nπ*）状态直接。一部分系统间系统在7.8 ps内与三重态歧管交叉，在1.5 ns内部分衰减，而另一部分在> 3 ns内恢复基态。嘧啶发色团不负责核碱基的光稳定性。取而代之的是，C2和C4的氨基和/或羰基官能化对于塑造嘧啶势能表面的形貌是必不可少的，并且会导致初始填充的电子激发态和基态之间可及的圆锥形交点。