Excited State Proton Transfer Is Not Involved in the Ultrafast Deactivation of Guanine-Cytosine Pair in Solution

Different derivatives of Guanine (G) and Cytosine (C), which sterically enforce the Watson-Crick (WC) conformer, have been studied in CHCl3 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 CHCl3 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.