Different derivatives of Guanine (G) and Cytosine (C), which sterically enforce the Watson-Crick (WC) conformer, have been studied in CHCl 3 by means of broad-band transient absorption spectroscopy. Our experiments rule out the involvement of an Excited State Proton Transfer (ESPT), which dominates the excited state decay of GC in the gas phase. Instead, the ultrafast dynamics via internal conversion occurs in a polar environment mainly by relaxation in the monomer moieties. Time-dependent density functional theory (TD-DFT) calculations in solution indeed indicate that population transfer from the bright excited states toward the charge transfer state is not effective in CHCl 3 and a noticeable energy barrier is associated with the ESPT reaction. ESPT is therefore not expected to be a main deactivation route for GC pairs within DNA. © 2011 American Chemical Society.

*** IBB - CNR ***

Department of Physical Chemistry, Heinrich-Heine University, University-str. 1, D-40225 Düsseldorf, Germany

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany

Istituto Biostrutture e Bioimmagini-CNR, Via Mezzocannone 16, I-80134 Napoli, Italy

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