Ultrafast electronic relaxation in guanosine is promoted by hydrogen bonding with cytidine

The excited electronic state lifetime of the guanosine-cytidine (G center dot center dot center dot C) Watson-Crick (WC) base pair has been directly measured in comparison to free G and C. Measurements have been carried out in solution in chloroform, where the formation of H-bonded base pairs is strongly favored, using the technique of femtosecond fluorescence up-conversion spectroscopy. The results show that the formation of the H-bonded WC pair leads to steep acceleration of the ultrafast nonradiative electronic deactivation compared to the free nucleosides, especially G, which can be explained by an intermolecular G-to-C electron-induced proton-transfer mechanism in the excited state. The results are of vital interest for bridging the huge gap between the well-known electronic properties of the isolated nucleobases and the strikingly different dynamics of DNA molecules.