Multiple alkali metal ion complexes of tripeptides: An investigation by means of electrospray tandem mass spectrometry

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Abstract
The [M – H + 2X]+ and [M – 2H + 3X]+ ions, where X = Li, Na or K, are the most abundant peptide-containing ions observed in the positive electrospray mass spectra of nine tripeptide solutions adjusted to pH 12 with lithium, sodium or potassium hydroxide. The collision-induced dissociations of these ions reveal structures in which the alkali metal ions are both attached to carbonyl oxygen atoms, and centrally coordinated to the deprotonated carboxylate terminus and the peptide nitrogen atoms. It is speculated that the precursor ions of [M – H + 2X]+ and [M – 2H + 3X]+ exist in solution and desorb to produce these ions directly via desolvation or indirectly via desolvation and collision-induced dissociation in the lens region.

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