Theoretical Analysis of the Excited State Properties of Wybutine: A Natural Probe for Transfer RNA Dynamics

We have theoretically characterized the ground state and the excited state properties of wybutine, a naturally fluorescent modified base occuring in tRNAs, using configuration interaction singles (CIS) and time dependent density functional (TDDFT) methods. Both gas phase excited state properties and solvent effects, modelled through Onsager reaction field method, were considered. In addition to vertical excitation energies, the fluorescence transitions were calculated, based on S₁ equilibrium geometry optimized at CIS level. Our computations show encouraging agreement with known experimental data either directly (TDDFT) or after applying empirical scaling (CIS). The fluorescence Stokes’ shift for the S₀ Ã S₁ transition is computed taking into account the contributions from both intramolecular and solvent reorganization processes. The results suggest that intramolecular relaxation of the S₁ state accounts for the major part of the magnitude of the Stokes’ shift, while the role of solvent reorganization seems to be of less importance.