Electronic coupling in the excited electronic state of stacked DNA base homodimers

Hans-Hermann Ritze,² Pavel Hobza¹ and Dana Nachtigallová¹

¹Center for Biomolecules and Complex Molecular Systems, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo na´m. 2, 166 10 Praha 6, Czech Republic

²Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Strasse 2A, D-12489 Berlin, Germany.

The nature of the excited states of DNA bases, in particular the electronic coupling between adjacent bases in their excited state is addressed in this contribution. One of the possibilities how DNA can protect itself from UV damage is a fast transfer of excitation energy which prevents the localization (trapping) of electronic energy and subsequent reaction. The energy transfer becomes faster with increasing electronic coupling between excited states.

The electronic coupling results from both the orbital overlap in the short-range limit and interaction of transition dipole moments (dipole-dipole interaction) in the intermediate-range limit. It is a generally accepted view that the extent of the delocalization depends on the base sequence as well as on the structure of DNA helix which effects their mutual orientation.

In this contribution we report the results of ab initio study of the electronic coupling between two adjacent stacked DNA bases where the orbital overlap is the dominant part. These studies were performed for adjacent cytosine, thymine, and adenine in both A- and B-DNA conformations.

It is shown that the electronic coupling depends considerably on the geometry of two adjacent bases. In addition, the character of the lowest excited states of thymine complex is different compared to that of cytosine complex. We believe that these studies can bring some contribution to the understanding of an observed different photochemical behavior, with respect to cyclobutane formation, in particular.