3-Methylation alters excited state decay in photoionised uracil?

Physical Chemistry Chemical Physics Pub Date: 2022-09-23 DOI: 10.1039/D2CP03460C

Abstract

UV and VUV-induced processes in DNA/RNA nucleobases are central to understand photo-damaging and photo-protecting mechanisms in our genetic material. Here we model the events following photoionisation and electronic excitation in uracil, methylated in the 1′ and 3′ positions, using the correlated XMS-CASPT2 method. We compare our results against those for uracil and 5-methyl-uracil (thymine) previously published. We find 3-methylation, an epigenetic modification in non-negligible amounts, shows the largest differences in photoionised decay of all three derivatives studied compared to uracil itself. At the S0 minimum, 3-methyl-uracil (3mUra) shows almost degenerate excited cation states. Upon populating the cation manifold, a crossing is predicted featuring different topography compared to other methylated uracil species in this study. We find an effective 3-state conical intersection accessible for 3mUra+, which points towards an additional pathway for radiationless decay. 3-Methylation reduces the potential energy barrier mediating decay to the cation ground state, making it vanish and leading to a pathway that we expect will contribute to the fastest radiationless decay amongst all methylated uracil species studied to date. 1- and 5-methylation, on the other hand, give differences from uracil in detail only: ionisation potentials are slightly red-shifted and the potential energy barrier mediating decay to the cation ground state is small but almost unchanged. By comparing against CASSCF calculations, we establish XMS-CASPT2 is essential to correctly describe conical intersections for 3mUra+. Our calculations show how a chemical modification that seems relatively small electronically can nevertheless have a significant impact on the behaviour of electronic excited states: a single methylation in the 3′ position alters the behaviour of the RNA base uracil and appears to open an additional pathway for radiationless decay following ionisation and electronic excitation.

Graphical abstract: 3-Methylation alters excited state decay in photoionised uracil
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