Ultrafast molecular photophysics investigated by UV spectroscopy.

Many molecules of biological interest present strong absorption bands in the ultraviolet (UV) spectral range. Their photo-stability is determined by the primary steps after photo-excitation. Tracking these processes is technically challenging, since they evolve in ultrafast time scales (typically tens of femtoseconds). Thanks to the extreme temporal resolution of the ultrafast spectroscopy beamline that we have developed, we have been able to track for the first time the relaxation of the wave packet in RNA/DNA nucleosides after the absorption of UV light. In particular, we have observed that the excited state of uridine relaxes in about $100 {fs}$ by means of a conical intersection, while in thymidine this mechanism takes an order of magnitude longer. Our results, supported by simulations from first principles based on mixed quantum mechanics /molecular mechanics (QM/MM), allow us to identify ring puckering as the dominant deactivation channel and rationalize the difference in decay times with larger inertia of the methyl group in thymidine with respect to hydrogen in uridine.