Fast Atom Bombardment and Tandem Mass Spectrometry of Covalently Modified Nucleosides and Nucleotides: Adducts of Pyrrolizidine Alkaloid Metabolites

Twelve different modified nucleoside and nucleotide adducts containing guanosine, adenosine, uridine, or thymidine bonded to various pyrrolizidine alkaloid metabolites were investigated by using fast atom bombardment (FAB) and tandem mass spectrometry (MS/MS). The goal is to compare and understand the collisionally activated decompositions of gas-phase (M – H)~, (M + H)+, and (M + K)⁺ions, where M is the modified nucleoside or nucleotide. The (M – H)^_ions of nucleoside adducts fragment simply to lose the sugar moiety whereas the (M – H)^-of the nucleotides decompose by losing either the sugar or the alkaloid and by forming the nucleic acid base as B^-. The abundances of the base-containing ions correlate with the known stabilities of the bases. Decomposition of the (M + H)⁺leads to liberation of the alkaloid as an electrophile. The protonated nucleic acid base (BH₂ ⁺) and the chemically modified base are also formed, and their abundances correlate with the proton affinities of the bases. (M + K)⁺ions decompose differently than the (M + H)⁺to give principally ions containing the base moiety and potassium. K⁺appears to be principally affiliated with the base or nucleoside moieties, indicating that these structural units have high potassium ion affinities. The proton, on the other hand, is more closely associated with the base and/or the modified base. The fragmentation reactions provide information relating to the molecular weight of the covalently bonded alkaloid and to the nature and sites of modification of the base, sugar, and alkaloid.