UV-induced long-lived decays in solvated pyrimidine nucleosides resolved at the MS-CASPT2/MM level

Abstract

The most relevant 'dark' electronic excited states in DNA/RNA pyrimidine nucleosides are mapped in water employing hybrid MS-CASPT2/MM optimisations with explicit solvation and including the sugar. Conical intersections (CIs) between initially accessed bright ππ* and the lowest energy darknπ* excited states, involving the lone pair localised on the oxygen and/or nitrogen atoms are characterised. They are found in the vicinities of the Franck-Condon (FC) region and are shown to facilitate non-adiabatic population transfer. The excited state population of the nπ* state, localised in the carbonyl moiety on all pyrimidine nucleosides, is predicted to rapidly evolve to its minimum, displaying non-negligible potential energy barriers along its non-radiative decay, and accounting for the ps signal registered in pump-probe experiments as well as for an efficient population of the triplet state. Cytidine displays an additional nπ* state localised in the N3 atom and that leads to its excited state minimum displaying large potential energy barriers in the pathway connecting to the CI with the ground state. Sugar-to-base hydrogen/proton transfer processes are assessed in solution for the first time, displaying a sizable barrier along its decay and thus being competitive with other slow decay channels in the ps and ns timescales. A unified deactivation scheme for the long-lived channels of pyrimidine nucleosides is delivered, where the nπ* state is found to mediate the long-lived decay in the singlet manifold and act as the doorway for triplet population and thus accounting for the recorded phosphorescence and, more generally, for the transient/photoelectron spectral signals registered up to the ns timescale.

Photodynamics of alternative DNA base isoguanine

“…Geometrically similar S 1 (n O p*) minima were also reported for the keto tautomers of C, G and isoC. 24,[45][46][47]50,70 However, surface-hopping nonadiabatic molecular dynamics simulations executed for G showed that this local minimum is sampled by sparse trajectories and the prevalent photodeactivation channel drives keto-G towards the pp*/S 0 conical intersection. 70 Our direct optimization of the S 1 minimum of keto-G initiated from the S 0 geometry also did not stabilize the n O p* state, whereas the same optimization procedure returned the S 1 (n O p*) minima for both of the studied keto tautomers of isoG.…”

“…We also performed surface-hopping nonadiabatic molecular dynamics simulations 44 for two of the enol tautomers of isoG using the ADC(2) method for electronic structure calculations. Our results indicate that keto tautomers of isoG exhibit excited state properties similar to the keto form of cytosine, which are determined by a long lived np* excitation, [45][46][47][48][49][50] while the enol tautomers are characterized by more efficient photorelaxation mechanisms dominated by the lowest-lying singlet pp* states.…”

Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation

“…The characterised cationic ground (D 0 ) and excited (D 1 ) state minima, as well as the different low-lying conical intersections (CIs) were also optimised at the CASPT2 level of theory to establish the role of dynamic electron correlation on the geometrical parameters in these cations, as it has been shown to significantly impact the singlet manifold. [42][43][44][45] CASPT2 minima and CI optimisations (using the projection method of Bearpark et al 46 ) were also carried out with analytical gradients [47][48][49] and couplings 50 as implemented in BAGEL 51 to ensure that those obtained using numerical differentiation are correct. Linear interpolations in internal coordinates were performed between the key structures optimised (minima and conical intersections) in order to rule out the presence of potential energy barriers.…”

Triplet‐Induced Lesion Formation at CpT and TpC Sites in DNA

“…Additionally,t he photophysics of individual thymine and cytosine derivatives are still the topic of current research (e.g.,s ee refs. [26][27][28][29][30]), building the foundation for ad etailed understanding of photoreactions in the corresponding dipyrimidines.…”