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

Abstract

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 2 p þ , 2 n þ O and 2 n þ N and compare these to the corresponding electronic states in cytosine. Investigating lower-energy decay pathways we find -unexpect-edly -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 Principal Component Analysis of the Ring Deformation in the Nonadiabatic Surface Hopping Dynamics

“…As a tautomer of the cytosine (one of the DNA bases), the photoinduced processes of the keto isocytosine was wildly studied in both experimental and theoretical works. [64][65][66][67][68][69][70][71][72] These previous works indicated that several conical intersections are involved in the nonadiabatic dynamics, which are governed by different ring deformation patterns. 70,71 Therefore, the keto isocytosine is an ideal model to verify the performance of our hierarchical protocol in realistic system.…”

3-Methylation alters excited state decay in photoionised uracil

“…DNA photoionisation has been studied less than photoexcitation, and even less in the context of methyl-substituted and other non-canonical DNA/RNA species. This is partly due to their high ionisation energies, placed in the Vacuum UV (VUV, B8 eV) range, [16][17][18][19][20][21][22][23] which is mostly filtered by the ozone layer and thus not considered as directly relevant to sunlight damage. 24 Two recent advances, however, have rekindled the interest of the community in DNA photoionisation.…”

The principal component analysis of the ring deformation in the nonadiabatic surface hopping dynamics

“…As a tautomer of the cytosine (one of the DNA bases), the photoinduced processes of the keto isocytosine were widely studied in both experimental and theoretical works. 69–77 These previous works indicated that several conical intersections (CIs) are involved in the nonadiabatic dynamics, which are governed by different ring deformation patterns. 75,76 Therefore, the keto isocytosine is an ideal model to verify the performance of our hierarchical protocol in realistic systems.…”