Characterization of phosphate-containing metabolites by calcium adduction and electron capture dissociation

Several phosphate-containing metabolites, including nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), adenosine 5'-diphosphate ribose (ADP-r), adenosine 5'-triphosphate (ATP), and guanosine 5'-triphosphate (GTP), have been characterized with electron capture dissociation (ECD) and sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) tandem mass spectrometry (MS/MS) in positive-ion mode. Calcium complexation was used to successfully produce abundant doubly charged cationic precursor ions with or without hydration. This approach enabled application of ECD to acidic metabolites for the first time. Fragmentation pathways observed in ECD and SORI-CAD of calcium-adducted phosphate-containing metabolites were complementary. Unique fragmentation was observed in ECD compared to SORI-CAD MS/MS, including ribose cross-ring cleavage for NAD and NADP, and generation of hydrated product ions, including cross-ring fragments, for hydrated ATP and GTP. A combination of ECD and CAD appears promising for maximizing structural information about metabolites.