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

The collision-induced dissociation of the protonated cytosine-guanine pair was studied using tandem mass spectrometry (MS3) 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 [GI A(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.

Automated summary

The collision-induced dissociation of the protonated cytosine-guanine pair was studied using MS3 coupled to infrared multiple photon dissociation spectroscopy. The structure of the CH+ and GH(+) ionic fragments was determined, and electronic structure calculations were used for rationalization. Several tautomers of each fragment were identified, including two unexpected ones. These results emphasize the importance of tautomerization in DNA base pairs assisted by proton or hydrogen transfer.