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DNA Excitonics

Final Report Summary - DEX (DNA Excitonics)

The nucleobases that make up DNA strongly absorb ultraviolet light. This leaves electrons in an excited state, which could damage the molecule. However, it is well known that the electrons quickly relax, thereby transferring energy to harmless nuclear motions. The mechanism for this relaxation process is still under debate, even for relatively simple molecules like the nucleobases. The reason that it is difficult to study this process lies both in the ultrafast time scale and in the inherent complexity of coupled electronic and nuclear dynamics. The fact that the process happens so fast means that experiments need to employ very short laser pulses in the ultraviolet part of the spectrum. The challenge for theory is to describe the motion of electrons and nuclei, which is a problem with many coupled degrees of freedom, in a liquid environment.

During the course of this project, progress in both experiment and theory was achieved in the host institution. Experimentalists developed a new optical technique (UV 2DES spectroscopy), which enabled unprecedented new measurements of the photophysics of nucleobases. This work led to the surprising insight that all four nucleobases show similar behaviour, and that an intermediate dark electronic state is involved in the ultrafast relaxation process. Stimulated and guided by these experimental insights, the contribution from this Marie Curie project was to develop a new theoretical method to enable comparison with experiment. In detail, the new calculations propagate damped dynamics in a model system with two conical intersections. In close collaboration with experimental colleagues, model parameters were optimized by calculating observables and directly comparing those with experimental results. The outcome of this effort is an increased understanding of the principles that underlie the photostability of DNA nucleobases.