Electron-Capture Dissociation and Collision-Induced Dissociation of Lanthanide Metal–Ligand Complexes and Lanthanide Metal–Ligand Complexes Bound to Phosphopeptides

Collision-induced dissociation (CID) and electron-capture dissociation (ECD) of lanthanide metal(III)–ligand complexes and lanthanide metal(III)–ligand–phosphopeptide complexes have been investigated using a Fourier transform-ion cyclotron resonance mass spectrometer (FT-ICR MS). Ternary adducts were formed for [LnL³⁺ + solvent anion(s)^n–]^(3–n)+ [Ln = europium, terbium and ytterbium, L = heptadentate ligand, solvent anion = acetate or trifluoromethane-sulphonate (triflate)]. Results show that ECD provides much more diagnostically useful information than CID. ECD was found to systematically cleave N–C bonds in the “arms” of the ligand, similar to the N–C_α cleavage of peptides, generating two fragmentation sites per arm of the ligand. The most intense and informative fragment ions were obtained from the terbium complex and it is believed that this is a consequence of terbium's greater reduction potential: the greater the reduction potential, the greater the ligand fragmentation; the lower the reduction potential, the more likely the molecule is to relinquish the solvent anion. The choice of solvent is also shown to be important. In general, the complexes studied fragment easily to lose CH₃CO₂H; however, particularly for ECD, the complexes retain the triflate anion causing the ligand to fragment instead, thus providing much more useful information. The triflate anion can be displaced by a phosphopeptide to create a lanthanide metal–ligand–phosphopeptide adduct. ECD is able to sequence the phosphopeptide, locating the site of phosphorylation bound to [LnL]³⁺ and also confirm the identity of the ligand. Small differences in the fragmentation of the lanthanide metal–ligand–phosphopeptide adducts follow the trend Eu < Tb < Yb suggesting that charge density may now be a more significant factor than metal ion reduction potential. ECD analysis of the triflate salts of the terbium complexes is most informative in developing a method to optimise structural information that can be gained from this group of molecules by mass spectrometry.