Theoretical Insights\non the Excited State Deactivation\nMechanism in Protonated Adenosine

We present herein our computational exploration of the\nconformational\nlandscape and photophysical properties of protonated adenosine (AdoH<sup>+</sup>). Several different protonated isomers and conformers have\nbeen considered and their relevant photophysical properties have been\naddressed. From our <i>ab initio</i> quantum computational\nresults, an S<sub>1</sub>/S<sub>0</sub> conical intersection (CI)\nhas been located for all considered conformers, providing a significant\nroute for the ultrafast deactivation mechanism of the S<sub>1</sub> excited state of AdoH<sup>+</sup>. Our results are also supported\nby nonadiabatic dynamics (NAD) simulation results indicating the S<sub>1</sub> excited state lifetime of 240–300 fs for the two most\nstable conformers of AdoH<sup>+</sup> (i.e., the most stable <i>syn</i>- and <i>anti</i>-N<sub>3</sub> protonated\ntautomers), which is comparable with protonated adenine, reported\nin the literature. The results confirm the ultrafast deactivation\nmechanism as well as photostability in nucleosides in protonated form.