| Abstract
| - We present a detailed study of the optical absorption spectra of DNA bases and base pairs, carried out bymeans of time dependent density functional theory. The spectra for the isolated bases are compared to availabletheoretical and experimental data and used to assess the accuracy of the method and the quality of the exchange-correlation functional. Our approach turns out to be a reliable tool to describe the response of the nucleobases.Furthermore, we analyze in detail the impact of hydrogen bonding and π-stacking in the calculated spectrafor both Watson−Crick base pairs and Watson−Crick stacked assemblies. We show that the reduction of theUV absorption intensity (hypochromicity) for light polarized along the base-pair plane depends strongly onthe type of interaction. For light polarized perpendicular to the basal plane, the hypochromicity effect isreduced, but another characteristic is found, namely a blue shift of the optical spectrum of the base-assemblycompared to that of the isolated bases. The use of optical tools as fingerprints for the characterization of thestructure (and type of interaction) is extensively discussed.
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