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 1 ππ* state is initially populated. The population then branches following two relaxation coordinates in the 1 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 via a conical intersection and fluorescence emission from the 1 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 via 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.
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On the Photostability of Cyanuric Acid and Its Candidature as a Prebiotic Nucleobase
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 S5,6 singlet states, which have ππ* character and large oscillator strengths (0.8). The population reaching the S5,6 states is expected to internally convert to the S1,2 states in an ultrafast time scale. The S1,2 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−1, 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.
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- Award ID(s):
- 1800052
- NSF-PAR ID:
- 10336305
- Date Published:
- Journal Name:
- Molecules
- Volume:
- 27
- Issue:
- 4
- ISSN:
- 1420-3049
- Page Range / eLocation ID:
- 1184
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/molecules27041184
