Abstract
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 Ca 2+ 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 Ca 2+/peptide complexes undergo fragmentations that are controlled by Ca 2+ binding and provide information on the Ca 2+ binding site. The LE CAD spectra are simple, indicating that Ca 2+ 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 Ca 2+/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 Mg 2+ and Ba 2+ with these peptides reveal that the ligand preferences of these metal ions are slightly different than those of Ca 2+.
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