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Phosphorylation is one of the most important protein modifications, andelectron-transfer dissociation tandem mass spectrometry (ETD-MS/MS) is apotentially useful method for the sequencing of phosphopeptides, includingdetermination of the phosphorylation site. Notably, ETD-MS/MS typicallyprovides useful information when the precursor contains more than threepositive charges. It is not yet used as an analysis method for large-scalephosphopeptide production due to difficulties occurring in the production ofacidic phosphopeptides having more than three positive charges. To increase thecharge state of phosphopeptides, we used dinuclear metal complexes, whichselectively bind to the phosphate group in phosphopeptides with the addition ofpositive charge(s). Dinuclear copper, zinc, and gallium complexes were testedand it was found that the type of metal present in the complex stronglyaffected the affinity of the phosphorylated compounds and their ETDfragmentation. The dinuclear copper complex interacted weakly with thephosphate groups and ETD-induced peptide fragmentation was largely suppressedby the presence of Cu2+, which worked as an electron trap. The dinucleargallium complex was strongly bound to a phosphate group. However, the ligandbinding to gallium acted as an electron trap and the presence of dinucleargallium complex in the precursor for ETD-MS/MS hampered the sequencing of thephosphopeptides, as in the case of dinuclear copper complexes. In contrast,dinuclear zinc complexes efficiently bind to phosphopeptides with an increasein the charge state, facilitating phosphopeptide sequencing by ETD-MS/MS. Thefragmentation of the ligand and peptide backbone in the dinuclearzinc--phosphopeptide complex were competitively induced by ETD. These processesare influenced by the ligand structure and so the detailed ETD fragmentationpathways were investigated using density functional theory calculations.

Influence of the Metals and Ligands in Dinuclear Complexes on Phosphopeptide Sequencing by Electron Transfer Dissociation Tandem Mass Spectrometry