Collision induced dissociation of deprotonated guanine: Fragmentation of pyrimidine ring and water adduct formation

This investigation of collision induced dissociation of deprotonated guanine was conducted by using sequential ion trap tandem mass spectrometry and isotopically labelled guanine analogs to clarify the complex dissociation reactions of pyrimidine ring of deprotonated guanine. The fragmentation patterns confirmed that the gas-phase dissociation processes are initiated from the pyrimidine ring through charge site controlled reactions involving charge redistribution, proton transfer and nucleophilic attack to generate the three primary fragment products by the elimination of ammonia, cyanamide and isocyanic acid. Our findings shed light on the process of pyrimidine ring opening and closure prior to the decomposition of deprotonated guanine and therefore the identity of N1 and exocyclic N2 is lost in deprotonated guanine as a result of scrambling. Intriguing association products of water addition to fragment ions have been structurally characterized to establish the role of ketene group in the selective gas-phase reaction of water addition and the formation of water adducts. The elimination of CO2 from negatively charged water adducts provides evidence for the covalent attachment of water to the ketene moiety. The established mechanisms of the dissociation reactions of pyrimidine ring should provide a basis for the structural elucidation of guanine relevant species modified on its active sites. To lend support for our proposals, collision induced dissociation study of 8-phenyl-2′-deoxyguanosine adduct was performed.