Our models of light-driven electron removal in 7H-Guanine and Adenine show differences in reactivity despite their structural resemblance
Following interest in understanding photoionisation of DNA nucleobases,[1,2] together with Prof. Mike Bearpark and MRes student Sara Nouri (Imperial College) we looked at the electron removal processes of DNA purine derivatives in 7H-adenine and 7H-guanine.[3]
This study served to benchmark the accuracy of the CASPT2 method we often employ against both experiment and other models, showing it to be a good compromise between cost and accuracy, and adds to the open discussion of whether gas-phase guanine is more stable in its 7H- or 9H- form.[4] This is particularly important to rationalise which species is the one recorded in gas phase experiments, and therefore compared against our models; our results lean towards the 9H tautomer on the basis of the more clear-cut differences found in the modelled photoelectron signals and their comparison with experiment.
Regarding their photophysics, cation excited state decay mechanisms of 7H-adenine and 7H-guanine show similar barrierless deactivations to those previously observed in other DNA systems.[1,2] This might be related to their ability to release the extra energy accrued upon radiation absorption harmlessly (i.e. their photostability), which is connected with their lowest-lying sloped and single-path[3] conical intersection that facilitates efficient population transfer to the ground state.
Another interesting aspect found was the very small energy differences observed between the first excited and ground cation states for 7H-adenine as shown above; these are the smallest differences we've observed to date and may trigger interesting electron dynamics, which we are currently studying.
This completes work on their biologically relevant 9H tautomers which has also been studied and will soon be published elsewhere.
References
[1] J. Segarra-Martí, T. Tran and M. J. Bearpark, "Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation", Phys. Chem. Chem. Phys. 2019, 21, 14322-14330.
[2] J. Segarra-Martí, T. Tran and M. J. Bearpark, "Computing the ultrafast and radiationless electronic excited state decay of cytosine and 5-methyl-cytosine cations: uncovering the role of dynamic electron correlation", ChemPhotoChem 2019, 3, 856-865.
[3] J. Segarra-Martí, S. M. Nouri and M. J. Bearpark, "Simulating the photoionisation of tautomeric purine DNA nucleobase derivatives 7H-adenine and 7H-guanine: ultrafast decay and photostability", Photochem 2021, 1, 287-301. [4] C. M. Marian, "The Guanine Tautomer Puzzle: Quantum Chemical Investigation of Ground and Excited States", J. Phys. Chem. A 2007, 111, 1545–1553.