Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA.
Molecules. 2022 Feb 10;27(4):1184. doi: 10.3390/molecules27041184.
Cyanuric acid is a triazine derivative that has been identified from reactions performed under prebiotic conditions and has been proposed as a prospective precursor of ancestral RNA. For cyanuric acid to have played a key role during the prebiotic era, it would have needed to survive the harsh electromagnetic radiation conditions reaching the Earth's surface during prebiotic times (≥200 nm). Therefore, the photostability of cyanuric acid would have been crucial for its accumulation during the prebiotic era. To evaluate the putative photostability of cyanuric acid in water, in this contribution, we employed density functional theory (DFT) and its time-dependent variant (TD-DFT) including implicit and explicit solvent effects. The calculations predict that cyanuric acid has an absorption maximum at ca. 160 nm (7.73 eV), with the lowest-energy absorption band extending to ca. 200 nm in an aqueous solution and exhibiting negligible absorption at longer wavelengths. Excitation of cyanuric acid at 160 nm or longer wavelengths leads to the population of S singlet states, which have ππ* character and large oscillator strengths (0.8). The population reaching the S states is expected to internally convert to the S states in an ultrafast time scale. The S states, which have nπ* character, are predicted to access a conical intersection with the ground state in a nearly barrierless fashion (ca. ≤ 0.13 eV), thus efficiently returning the population to the ground state. Furthermore, based on calculated spin-orbit coupling elements of ca. 6 to 8 cm, the calculations predict that intersystem crossing to the triplet manifold should play a minor role in the electronic relaxation of cyanuric acid. We have also calculated the vertical ionization energy of cyanuric acid at 8.2 eV, which predicts that direct one-photon ionization of cyanuric acid should occur at ca. 150 nm. Collectively, the quantum-chemical calculations predict that cyanuric acid would have been highly photostable under the solar radiation conditions reaching the Earth's surface during the prebiotic era in an aqueous solution. Of relevance to the chemical origin of life and RNA-first theories, these observations lend support to the idea that cyanuric acid could have accumulated in large quantities during the prebiotic era and thus strengthens its candidature as a relevant prebiotic nucleobase.
三聚氰胺酸是一种三嗪衍生物，已从前生物条件下的反应中被鉴定出来，并被提议作为原始 RNA 的潜在前体。为了使三聚氰胺酸在前生物时代发挥关键作用，它需要在到达地球表面的前生物时期的恶劣电磁辐射条件下存活下来（≥200nm）。因此，三聚氰胺酸的光稳定性对于其在前生物时代的积累至关重要。为了评估三聚氰胺酸在水中的假定光稳定性，在本研究中，我们采用了密度泛函理论（DFT）及其时间相关变体（TD-DFT），包括隐式和显式溶剂效应。计算预测，三聚氰胺酸在约 160nm（7.73eV）处具有最大吸收峰，在水溶液中，最低能量吸收带延伸至约 200nm，在较长波长处几乎没有吸收。在 160nm 或更长波长处激发三聚氰胺酸会导致 S 单重态的分布，其具有ππ特征和大的振子强度（0.8）。预计到达 S 态的激发态会在超快时间尺度内内部转化为 S 态。S 态具有 nπ特征，预计以近乎无势垒的方式（约≤0.13eV）进入与基态的锥形交叉，从而有效地将激发态的电子返回到基态。此外，基于约 6 到 8cm 的计算自旋轨道耦合元素，计算预测三重态向三重态的系间窜越应该在三聚氰胺酸的电子弛豫中起次要作用。我们还计算了三聚氰胺酸的垂直电离能为 8.2eV，这表明三聚氰胺酸的直接单光子电离应该发生在约 150nm。总的来说，量子化学计算预测，在水溶液中，三聚氰胺酸在前生物时期到达地球表面的太阳辐射条件下将具有高度的光稳定性。与生命的化学起源和 RNA 优先理论相关，这些观察结果支持了这样一种观点，即在原始 RNA 中，三聚氰胺酸可能在前生物时代大量积累，从而增强了其作为相关前生物碱基的候选地位。
