Photophysical properties of 5-methylcytidine.

Cytosine methylation, which determines the hot spots for DNA photo-damage, is shown to induce a red-shift of the nucleoside absorption spectrum, making the chromophore more vulnerable to solar radiation, and a tenfold increase of the fluorescence lifetime, making excited statereactions more probable. A femtosecond investigation of the excited state deactivation reveals a quite complex mechanism.