New insight into the ultrafast excited state deactivation mechanism of guanosine in the gas phase

The deactivation process of guanosine molecule on the first singlet excited state in the gas phase has been explored through the density functional theory (DFT/TDDFT) and the complete active space self-consistent field (CASSCF) methods. To unravel the decay mechanism of this system, the topography of excited potential energy surfaces relevant to the deactivation process were mapped. Two deactivation decay channels characterized by the excited state intramolecular proton transfer process and the puckering motion of ring were explored, and the latter one was proposed to play significant roles in the deactivation process of guanosine for the barrierless energy profile. However, the probability for the excited state intramolecular proton transfer process pathway is lower because of the existence of an energy barrier. This conclusion was further confirmed by the two minimum energy conical intersections (MECIs) located between S-1 and S-0 states, characterized by the ESIPT process and pyramidalization of the C2 atom, respectively. In addition, by comparing the decay mechanism with the previously reported adenosine and pyrimidine nucleosides, it was found that two competitive decay pathways coexist in the purine nucleosides, which is different from the pyrimidine nucleosides. As the next step in the bottom-up investigation of the DNA photostability properties, the new mechanism of the guanosine in gas phase will be conducive for more comprehensive understanding of the photostability of DNA and RNA molecules and provide theoretical guidance for the investigation of the different decay mechanism of the purine nucleosides and pyrimidine nucleosides.