Abstract
Twenty proteinic amino acids were ionized in the presence of sodium cation by electrospray ionization (ESI) method and the product ions were analyzed. Among them, asparagine, glutamine, serine, and threonine generated the [ nM− nH +( n + 1)Na] + cluster ions. The cluster ion intensity distributions showed a magic number at n = 3 of six-coordinated sodium complex structures, (M−H +Na) 3Na +. The neutral species of (M−H +Na) formed by proton-sodium cation exchange of asparagine, glutamine, serine, and threonine could have stable cyclic structures of 7-, 8-, 6-, and 6-membered rings, respectively. They also had two ligand sites of ketone and amine. We concluded that formation of the proton-sodium cation exchanged species plays an important role in the cluster ion formation.
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