Modelling Photoionisation in Isocytosine: Potential Formation of Longer‐Lived Excited State Cations in its Keto Form

Studying the effects of UV and VUV radiation on non‐canonical DNA/RNA nucleobases allows us to compare how they release excess energy following absorption with respect to their canonical counterparts. This has attracted much research attention in recent years because of its likely influence on the origin of our genetic lexicon in prebiotic times. Here we present a CASSCF and XMS‐CASPT2 theoretical study of the photoionisation of non‐canonical pyrimidine nucleobase isocytosine in both its keto and enol tautomeric forms. We analyse their lowest energy cationic excited states including 2π+ , 2nO+ and 2nN+ and compare these to the corresponding electronic states in cytosine. Investigating lower‐energy decay pathways we find – unexpectedly ‐ that keto‐isocytosine+ presents a sizeable energy barrier potentially inhibiting decay to its cationic ground state, whereas enol‐isocytosine+ features a barrierless and consequently ultrafast pathway analogous to the one previously found for the canonical (keto) form of cytosine+. Dynamic electron correlation reduces the energy barrier in the keto form substantially (by ∼1 eV) but it is nevertheless still present. We additionally compute the UV/Vis absorption signals of the structures encountered along these decay channels to provide spectroscopic fingerprints to assist future experiments in monitoring these intricate photo‐processes.
The photostability of isocytosine upon ionising radiation exposure is assessed theoretically in both keto and enol tautomeric forms. This non‐canonical base, which features analogous UV excited state reactivity to DNA nucleobase cytosine, unexpectedly displays sizeable energy barriers along the cationic excited state in its keto form that hamper the decay. Simulations suggest isocytosine is less photostable against photoionisation, a factor that might contribute to selecting the more resilient cytosine under prebiotic VUV radiation exposure.