DNA excited-state dynamics: Ultrafast internal conversion and vibrational cooling in a series of nucleosides

To better understand DNA photodamage, several nucleosides were studied by f emtosecond transient absorption spectroscopy. A 263-nm, 150-fs ultraviolet pump pulse excited each nucleoside in aqueous solution, and the subsequent dynamics were followed by transient absorption of a femtosecond continuum p ulse at wavelengths between 270 and 700 nm. A transient absorption band wit h maximum amplitude near 600 mn was detected in protonated guanosine at pH 2. This band decayed in 191 +/- 4 ps in excellent agreement with the known fluorescence lifetime, indicating that it arises from absorption by the low est excited singlet state. Excited state absorption for guanosine and the o ther nucleosides at pH 7 was observed in the same spectral region, but deca yed on a subpicosecond time scale by internal conversion to the electronic around state. The cross section for excited state absorption is very weak f or all nucleosides studied, making some amount of two-photon ionization of the solvent unavoidable. The excited state lifetimes of Ado, Guo, Cyd, and Thd were determined to be 290, 460, 720, and 540 fs, respectively (uncertai nties are +/-40 fs). The decay times are shorter for the purines than for t he pyrimidine bases, consistent with their lower propensity for photochemic al damage. Following internal conversion, vibrationally highly excited grou nd state molecules were detected in experiments on Ado and Cyd by hot groun d state absorption at ultraviolet wavelengths. The decays are assigned to i ntermolecular vibrational energy transfer to the solvent. The longest time constant observed for Ado is approximately 2 ps, and we propose that solute -solvent H-bonds are responsible for this fast rate of vibrational cooling. The results show for the first time that excited singlet state dynamics of the DNA bases can be directly studied at room temperature. Like sunscreens that function by fight absorption, the bases rapidly convert dangerous ele ctronic energy into heat, and this property is likely to have played a crit ical role in life's early evolution on earth.