Electronic and Structural Factors Governing the Intersystem Crossing and Internal Conversion Dynamics of Aza-Substituted Nucleobases

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With support of the Chemical Structure Dynamics and Mechanisms-A (CSDM-A) program of the Chemistry Division, Professor Susanne Ullrich and her team at the University of Georgia will study the response of modified nucleobases upon exposure to UV (ultraviolet) light. The canonical nucleobases, which form the building blocks of our DNA, protect themselves against photodamage through internal conversion processes that dissipate harmful UV energy into heat. However, minor changes to the nucleobases, such as substitution of a single ring carbon atom with nitrogen, can profoundly alter their UV photo-response and eliminate their inherent photo-protection. Depending on the position of substitution, internal conversion processes can become inaccessible and instead long-lived, highly reactive triplet excited states are formed. Besides fundamental interest in the unique photophysics of these azabases, modified nucleobases have desirable properties for various uses in biological and pharmacological applications. For example, long-lived reactive triplet states are key to some forms of cancer treatments, antiviral and antimicrobial medicines, and photodynamic therapies. The project will train undergraduate and graduate students as well as summer interns who participate in this research. Outcomes of the project will be broadly disseminated to the public through open house activities and science show performances. Specifically, in this project, Ulrich and her team at the University of Georgia will apply time-resolved photoelectron spectroscopy (TR-PES) and ion yield (TR-IY) measurements to the study of ultrafast internal conversion and intersystem crossing dynamics in a series of azabases. Azabases can be classified into two types: In type 1 azabases, internal conversion is quenched and intersystem crossing into the triplet manifold becomes highly efficient. In contrast, type 2 azabases maintain efficient internal conversion pathways similar to the classic nucleobases. Using gas-phase experiments supplemented by ab initio calculations the molecular-level mechanistic details governing this unique behavior will be derived. A new VUV (vacuum ultraviolet) source, based on a collaborator’s design will be integrated into the existing experimental setup. Through photo-exciting these molecules with UV radiation and probing the evolution of the excited states with VUV, critical information pertaining to their deactivation mechanisms can be gleaned. TR-PES directly observes all relaxation pathways and identifies excited states based on their photoelectron spectra. TR-IY instead provides mass information that helps identify excited states based on characteristic fragmentation patterns. Supplemental theoretical efforts, in collaboration with Professors Barbatti (Aix Marseille) and González (University of Vienna), will go beyond simple static pictures, using dynamics simulations with non-adiabatic and spin-orbit couplings and simulating TR-PES spectra. Comparing a systematic series of azabases and contrasting their type 1 and type 2 behavior has the potential to unravel the electronic and structural factors that control intricate details of their excited state potential energy surfaces such as couplings between excited states, potential energy barriers, and accessibility of conical intersections and crossing points. The intrinsic molecular photoproperties observed here combined with knowledge of solvent effects gleaned through published solution-phase studies, are expected to contribute to a better understanding of azabase photophysics under the widely varying conditions of biological microenvironments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.