Inter- and Intramolecular Proton Transfer in an Isolated (Cytosine-Guanine)H <superscript>+</superscript> Pair: Direct Evidence from IRMPD Spectroscopy.

The collision-induced dissociation of the protonated cytosine-guanine pair was studied using tandem mass spectrometry (MS 3 ) coupled to infrared multiple photon dissociation spectroscopy with the free electron laser at Orsay (CLIO) to determine the structure of the CH + and GH + ionic fragments. The results were rationalized with the help of electronic structure calculations at the density functional theory level with the B3LYP/6-311++G(3df,2p) method. Several tautomers of each fragment were identified for the first time, some of which were previously predicted by other authors. In addition, two unexpected and minor tautomers were also found: cytosine keto-imino [CKI(1,2,3,4)H + ] and guanine keto-amino [GKA(1,3,7)H + ]. These results highlight the importance of the DNA base tautomerization assisted by inter- and intramolecular proton or hydrogen transfer within the protonated pairs.