Canonical nucleobases exhibit excited state lifetimes of sub picoseconds, preventing damage by UV radiation, suggesting photochemical selection of the building blocks of life on a prebiotic earth. The intrinsic photo properties of the nucleobases are sensitive to their environment. For instance, base stacking in solution results in an ultrafast relaxation pathway providing photostability by an exciplex formation channel. To explore the effect of non-covalent interactions in base-stacking, we used laser desorption jet cooling coupled with resonance enhanced multiphoton ionization (REMPI) to study thymine-thymine (T-T) bound to peptide nucleic acids (PNA) as a mimic backbone of DNA. With picosecond pump-probe spectroscopy, we found the excited state lifetime of stacked T-T PNA to be long-lived (of the order of 100 nanoseconds) indicating the absence of excimer decay. In combination with calculations at the M06-2X/6-31G(d) level and IR- UV double resonance spectroscopy, tautomeric assignment of the T-T stacking revealed a di-keto tautomeric form in our molecular beam. We conclude that an excimer decay channel is inaccessible in isolated T-T PNA due to the absence of solvent, suggesting the necessity of solvation for stabilization of stacked bases. we will further explore adenine-adenine homodimers bound to PNA (A-A PNA), as a greater interaction is predicted for this structure.

Excited state dynamics of stacked thymine-thymine bases using peptide nucleic acids

ACS Spring 2022

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