Computing the Ultrafast and Radiationless Electronic Excited State Decay of Cytosine and 5‐methyl‐cytosine Cations: Uncovering the Role of Dynamic Electron Correlation

Abstract

Photoionisation in DNA, i. e. the process of photoinduced electron removal from the chromophoric species -the nucleobases -leading to their cationic form, has been scarcely studied despite being considered to be responsible for significant damaging instances in our genetic material. In this contribution we theoretically characterise the electronic ground and excited state decay pathways of cationic DNA nucleobase cytosine + and its epigenetic derivative 5-methyl-cytosine + , including the effects of dynamic electron correlation on energies and geometries of minima and conical intersections. We do this by comparing the results of XMS-CASPT2 calculations with CASSCF estimates and we find some significant differences between the results of these two methods. In particular, including dynamic electron correlation is found to significantly reduce the barrier to access the ðD 1 =D 0 Þ conical intersection. We find notable similarities in both cytosine and 5methyl-cytosine cations, and accessible conical intersections in the vicinity of the Franck-Condon region are found. This points towards an ultrafast depopulation of their electronic excited states. Moreover, the shape of the ground state potential energy surface strongly directs the decaying excited state population towards the cationic ground state minimum on ultrafast timescales, preventing photo-fragmentation and thus explaining their photostability. To better compare our calculations with the available experimental data we compute the UV (ground and excited state) and IR absorptions.[a] Dr.

Modelling Photoionisations in Tautomeric DNA Nucleobase Derivatives 7H-Adenine and 7H-Guanine: Ultrafast Decay and Photostability

“…The first cationic state in 7H-purine derivatives is characterised by the 2 π + H state, which corresponds to an unpaired electron in the singly occupied molecular orbital (SOMO). This is in line with what has been observed for other DNA/RNA nucleobase systems, where the SOMO is always the one embodying the first (i.e., energetically lowest) electron removal upon ionising radiation exposure [24,[27][28][29][30]. This state is placed at 8.98 and 8.43 eV on average for 7H-adenine and 7H-guanine respectively, and feature similar standard deviations, with respect to the choice of CASPT2 zeroth order Hamiltonian, of 0.19 and 0.22 eV.…”

“…All critical structures found along the cationic excited state decay of 7H-adenine and guanine remain in-plane. This is shared with what we and others have found for pyrimidine nucleobases and derivatives [28][29][30]46], and justifies focusing on evaluating distortions along the different bond lengths affected during the photo-process.…”

“…This makes the choice of the initial state created in our model somewhat ambiguous, as any of the states (or linear combinations of them) are in principle possible. We here assume an electron removal from the highest-lying cationic state considered as done in previous studies [29,30,45]. This enables exploring all critical structures along the potential energy surface until reaching the cationic ground state thus providing an overview of the different pathways potentially triggered in this complex photo-process.…”

“…The reactivity of the ensuing DNA nucleobase cationic species still remains vastly unexplored, despite recent efforts with state-of-the-art instrumentation on this front [25,26]. Whereas pyrimidine nucleobases have been studied in relative detail [27][28][29][30], purines are less explored perhaps due to their more complex electronic structure [31], studies on their tautomeric forms being even scarcer.…”

Solvent‐Dependent Stabilization of a Charge Transfer State is the Key to Ultrafast Triplet State Formation in an Epigenetic DNA Nucleoside

“…This procedure has been profitably used to study the photoactivated behavior of nucleobases in solution, [29,23a] including Cyt derivatives. [5e,f] [30] SOC terms were computed using the pysoc program. [31] See Supporting Information for further details about DFT and CASPT2 calculations.…”