Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA.
Phys Chem Chem Phys. 2022 Feb 16;24(7):4204-4211. doi: 10.1039/d1cp05269a.
Ultraviolet radiation (UVR) from the sun is essential for the prebiotic syntheses of nucleotides, but it can also induce photolesions such as the cyclobutane pyrimidine dimers (CPDs) to RNA or DNA oligonucleotide in prebiotic Earth. 2,6-Diaminopurine (26DAP) has been proposed to repair CPDs in high yield under prebiotic conditions and be a key component in enhancing the photostability of higher-order prebiotic DNA structures. However, its electronic relaxation pathways have not been studied, which is necessary to know whether 26DAP could have survived the intense UV fluxes of the prebiotic Earth. We investigate the electronic relaxation mechanism of both 26DAP and its 2'-deoxyribonucleoside (26DAP-d) in aqueous solution using steady-state and femtosecond transient absorption measurements that are complemented with electronic-structure calculations. The results demonstrate that both purine derivatives are significantly photostable to UVR. It is shown that upon excitation at 287 nm, the lowest energy ππ* state is initially populated. The population then branches following two relaxation coordinates in the ππ* potential energy surface, which are identified as the C2- and C6-relaxation coordinates. The population following the C6-coordinate internally converts to the ground state nonradiatively through a nearly barrierless conical intersection within 0.7 ps in 26DAP or within 1.1 ps in 26DAP-d. The population that follows the C2-relaxation coordinate decays back to the ground state by a combination of nonradiative internal conversion a conical intersection and fluorescence emission from the ππ* minimum in 43 ps and 1.8 ns for the N9 and N7 tautomers of 26DAP, respectively, or in 70 ps for 26DAP-d. Fluorescence quantum yields of 0.037 and 0.008 are determined for 26DAP and 26DAP-d, respectively. Collectively, it is demonstrated that most of the excited state population in 26DAP and 26DAP-d decays back to the ground state both nonradiative and radiative relaxation pathways. This result lends support to the idea that 26DAP could have accumulated in large enough quantities during the prebiotic era to participate in the formation of prebiotic RNA or DNA oligomers and act as a key component in the protection of the prebiotic genetic alphabet.
太阳的紫外线辐射 (UVR) 对核苷酸的前生物合成至关重要，但它也会诱导嘧啶二聚体 (CPD) 等光损伤形成于前生物地球的 RNA 或 DNA 寡核苷酸中。2,6-二氨基嘌呤 (26DAP) 已被提议在前生物条件下高产率修复 CPD，并成为增强更高阶前生物 DNA 结构光稳定性的关键组成部分。然而，其电子弛豫途径尚未被研究，这对于了解 26DAP 是否能够在前生物地球的强烈紫外线通量下存活下来是必要的。我们使用稳态和飞秒瞬态吸收测量以及电子结构计算来研究水溶液中 26DAP 和其 2'-脱氧核苷 (26DAP-d) 的电子弛豫机制。结果表明，这两种嘌呤衍生物对 UVR 具有显著的光稳定性。结果表明，在 287nm 激发时，最初占据最低能量的 ππ* 态。然后，种群沿着 ππ* 势能面中的两个弛豫坐标分支，这两个坐标被确定为 C2-和 C6-弛豫坐标。在 26DAP 中，通过内部转换在 0.7ps 内或在 26DAP-d 中在 1.1ps 内，沿 C6-坐标分支的种群通过近乎无势垒的锥形交叉非辐射地回到基态。沿 C2-弛豫坐标分支的种群通过非辐射内部转换和从 ππ* 最小的荧光发射组合回到基态在 43ps 和 1.8ns 内对于 26DAP 的 N9 和 N7 互变异构体，或在 70ps 内对于 26DAP-d。确定 26DAP 和 26DAP-d 的荧光量子产率分别为 0.037 和 0.008。总的来说，结果表明 26DAP 和 26DAP-d 中的大部分激发态种群通过非辐射和辐射弛豫途径回到基态。这一结果支持了 26DAP 在前生物时代可能积累到足够数量以参与前生物 RNA 或 DNA 寡聚物的形成并作为保护前生物遗传字母的关键组成部分的观点。
