Tautomeric selectivity of the excited-state lifetime of guanine-cytosine base pairs: the role of electron and proton transfer processes

Andrzej L. Sobolewski, Wolfgang Domcke‡,⊥ and Christof Hättig
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, PL-02668, Warsaw, Poland; Department of Chemistry, Technical University of Munich, Lichtenbergstrasse 4, D-85747 Garching, Germany; and Institute of Nanotechnology, Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany

Proceed. Natl. Acad. Sci. USA 102, 17903-17906 (2005).
Edited by F. Fleming Crim, University of Wisconsin, Madison, WI, and approved October 27, 2005 (received for review May 17, 2005)

The UV spectra of three different conformers of the guanine/cytosine base pair were recorded recently with UV-IR double-resonance techniques in a supersonic jet [Abo-Riziq, A., Grace, L., Nir, E., Kabelac, M., Hobza, P. & de Vries, M. S. (2005) Proc. Nadl. Acad. Sci. USA 102, 20-23]. The spectra provide evidence for a very efficient excited-state deactivation mechanism that is specific for the Watson-Crick structure and may be essential for the photostability of DNA. Here we report results of ab initio electronic-structure calculations for the excited electronic states of the three lowest-energy conformers of the guanine/cytosine base pair. The calculations reveal that electron-driven interbase proton-transfer processes play an important role in the photochemistry of these systems. The exceptionally short lifetime of the UV-absorbing states of the Watson-Crick conformer is tentatively explained by the existence of a barrierless reaction path that connects the spectroscopic 1π π* excited state with the electronic ground state via two electronic curve crossings. For the non-Watson-Crick structures, the photochemically reactive state is located at higher energies, resulting in a barrier for proton transfer and, thus, a longer lifetime of the UV-absorbing 1π π* state. The computational results support the conjecture that the photochemistry of hydrogen bonds plays a decisive role for the photostability of the molecular encoding of the genetic information in isolated DNA base pairs.

§To whom correspondence should be addressed. E-mail: domcke@ch.tum.de.

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