Photophysics of isolated nucleobases and their hydrates: femtosecond dynamics study by photoelectrons, photoions and fragments detection

The photophysical properties of the nucleobases are believed to play an essential role in the response of DNA molecules to UV radiation exposition. The aim of the present study is a characterization of the various intrinsic relaxation pathways of these molecules. Femtosecond dynamics of the excited states of nucleobases (adenine, guanine, cytosine,thymine, uracil) and model derivatives were studied in supersonic jet by pump-probe photoionisation followed by velocity imaging of the photoelectrons or time-of-flight mass spectrometry of the ions. The chosen derivatives limit the number of tautomers possibly observed and model the biologically relevant structures of the bases. Our excellent time-resolution (80 fs) and accurate internal calibration demonstrated the existence of an ultra-fast relaxation pathway in the purine bases consisting of two successive steps of ∼100 fs and ∼1 ps, respectively. In the case of the pyrimidine bases, a third long-lived component was evidenced, and the typical lifetimes measured are in the range of 50-100 fs/2-6 ps/ns. The fragmentation phenomenon, widely considered so far as a perturbation of the parent ions signal, is demonstrated to provide a complementary diagnostic tool for the characterization of the excited states probed especially in terms of vibrational energy content. Comparison of these results to the most recent ab initio theoretical calculations allows us to propose different relaxation mechanisms for purine and pyrimidine bases and to explain the different effects of hydration on the excited states of these molecules. The study of small nucleobase-water clusters already shows trends that can be compared to those obtained in solution. (author)