Ultrafast electronic deactivation of DNA bases in aqueous solution

One of the primary mechanisms of DNA damage occurs following irradiation with high energy ultraviolet (UV) light. Consequently, the study of excited state dynamics of nucleic acid bases upon UV excitation is essential towards understanding and mediating DNA photodamage. Due to the extremely short sub-picosecond (10-12 s) lifetimes, most time-resolved studies on DNA bases are hampered by the ability to generate and manipulate short UV laser pulses. The development of an ultrashort UV-pulse (∼30 fs) source reported here now makes it possible to characterize very rapid dynamics that is simply not observable in previous lower time resolution experiments. Results of experiments combining broadband UV/Visible dispersed probing with simultaneous polarization resolution are presented for isolated free adenine and uracil derivatives in aqueous solution at room temperature. Both spectral and dynamical data is acquired providing the most detailed view of the excited-state dynamics to date.; For thymidine, the spectra of all transient species have been identified and compare surprisingly well with corresponding time-resolved photoelectron spectra from the same intermediate states in the gas-phase. The proposed model for electronic deactivation is, thus, analogous to gas-phase dynamics where there is an intermediate state between the optically bright pipi* and the final ground state during electronic relaxation. Additional experimental studies for uridine show ultrafast branching in the initial pipi* state: a fraction of the excited-state population decays via internal conversion to the ground state, the rest of the population decays to the npi* state.; For adenosine, ultrafast electronic relaxation from the excited pipi* states to the ground state is likely to take place via a pipi*/S 0 conical intersection. However, gas-phase time-resolved photoelectron spectroscopy predict the same npi* state intermediate relaxation model. Therefore, the deactivation mechanism is different for the gas and solution phase dynamics. This provides direct evidence of the solvent effect on the excited-state potential energy surfaces, especially in the region of conical intersections.