Photoionisation mechanisms in isocytosine, an isomer of DNA nucleobase cytosine, are studied to assess whether they might have contributed to the selection of the latter over the former as part of our DNA during prebiotic times
Our genetic material is quite resistant to incident high-energy radiation, which makes it photostable. This self-protecting property might have been important in the selection of the specific monomeric units the DNA/RNA nucleobases that compose our genetic lexicon: nucleobases had to be resilient and withstand considerable UV radiation exposure during prebiotic times to avoid photo-degradation, and this might have been important in their selection as the building blocks in which to encode our hereditary material. This hypothesis is used to explain why nucleobases (uracil, thymine, cytosine, adenine and guanine) release harmlessly (and very quickly) the excess energy gained upon absorbing radiation similarly, despite presenting different molecular (nuclear and electronic) structures.[1]
We've looked into the photoionisation mechanisms of non-canonical DNA base derivative isocytosine in both keto and enol tautomeric forms, modelling nuclear relaxation of the cations created when exposed to high-energy UV radiation.[2] Recent theoretical work showed isocytosine UV-photoinduced mechanisms are similar to cytosine's, which suggests their photostability to be comparable.[3] We found differences when considering photoionisation (i.e. in the cationic manifold): while cytosine shows cationic excited-state barrierless deactivation profiles in line with what is normally considered to be photostable, keto-isocytosine has sizeable barriers hampering excited-state relaxation. This suggests substantially longer lifetimes might be associated to its deactivation, making it in principle less photostable. Enol-isocytosine, on the other hand, behaves like cytosine despite being structurally more different, and points at complex structure–reactivity relationships in cationic systems that I will further explore in future work.
This builds upon previous work in exploring photostability beyond UV radiation[4] as a potential additional prerequisite when selecting the most resilient building blocks for encoding our hereditary material during prebiotic times.
This work was recently featured in the Front Cover of ChemPhysChem! A brief cover profile can also be found here.
References
[1] S. Boldissar and M. S. de Vries, "How nature covers its bases", Phys. Chem. Chem. Phys. 2018, 20, 9701-9716. [2] J. Segarra-Martí and M. J. Bearpark,"Modelling photoionisation in isocytosine: potential formation of longer-lived excited state cations in its keto form", ChemPhysChem, DOI:10.1002/cphc.202100402 (2021).
[3] R. Szabla, R. W. Góra, J. Sponer, "Ultrafast excited-state dynamics of isocytosine", Phys. Chem. Chem. Phys. 2016, 18, 20208–20218.
[4] J. Segarra-Martí, T. Tran and M. J. Bearpark, "Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation", Phys. Chem. Chem. Phys. 2019, 21, 14322-14330.