Determination of calcium binding sites in gas-phase small peptides by tandem mass spectrometry

Low-energy (LE) and high-energy (HE) collisionally activated decompositions (CAD) of calcium/peptide complexes of the form [M - H + Ca]⁺ and [M + Ca]²⁺ reflect the site of calcium binding in various gas-phase peptides that are models of the calcium binding site III of rabbit skeletal troponin C. The Ca²⁺ binding sites involve an aspartic acid, glutamic acid, and asparagine, which are in the metal-binding loops of calcium-binding proteins. Both fast atom bombardment (FAB) and electrospray ionization (ESI) were used to generate the metal/peptide complexes. When submitted to LE CAD, ESI-produced Ca²⁺/peptide complexes undergo fragmentations that are controlled by Ca²⁺ binding and provide information on the Ca²⁺ binding site. The LE CAD spectra are simple, indicating that Ca²⁺ binding involves specific oxygen ligands including acidic side chains and that only a few low-energy fragmentation channels exist. The HE CAD spectra of FAB-produced Ca²⁺/peptide complexes are more complex, owing to the introduction of high internal energy into the precursor ion. Interactions of the other alkaline-earth metal ions Mg²⁺ and Ba²⁺ with these peptides reveal that the ligand preferences of these metal ions are slightly different than those of Ca²⁺.