Collision-induced dissociation of protonated guanine

Guanine and its naturally or xenobiotically modified derivatives occur widely in nucleic acids and other natural products, yet the potential applications of tandem mass spectrometry in structural studies is limited by the complexity of dissociation pathways of the guanine nucleus. The low energy-collision induced dissociation (CID) of protonated guanine has been studied using analogs independently labeled with N-15 at each of the five nitrogen atoms and by O-18 at position 6. Following activation under multi-collision conditions, protonated guanine dissociates along two major pathways starting with expulsion of either NH3 or cyanamide. The reactions are rationalized in terms of initial opening of the pyrimidine ring, which results in loss of identity of ring atom N-1 and exocyclic N-2. The equivalence of N-1 and N-6 is retained under low energy (single collision) conditions, and upon methyl substitution at either nitrogen atom. Other reactions show the pyrimidine ring rather than the imidazole moiety to be the principal site of reactivity, analogous to the dissociation chemistry of protonated adenine. (C) 1997 Elsevier Science B.V.