Theoretical Insights on the Excited State Deactivation Mechanism in Protonated Adenosine.

We present herein our computational exploration of the conformational landscape and photophysical properties of protonated adenosine (AdoH). Several different protonated isomers and conformers have been considered and their relevant photophysical properties have been addressed. From our quantum computational results, an S/S conical intersection (CI) has been located for all considered conformers, providing a significant route for the ultrafast deactivation mechanism of the S excited state of AdoH. Our results are also supported by nonadiabatic dynamics (NAD) simulation results indicating the S excited state lifetime of 240-300 fs for the two most stable conformers of AdoH (i.e., the most stable - and -N protonated tautomers), which is comparable with protonated adenine, reported in the literature. The results confirm the ultrafast deactivation mechanism as well as photostability in nucleosides in protonated form.