Formation of deprotonated 2-imidazoline-4(5)-one product ions in the collision-induced dissociation of some serine-containing dipeptides

•Deprotonated 2-imidazoline-4(5)-one product ions from dipeptides were observed.•C-terminal Ser allows formation of deprotonated 2-imidazoline-4(5)-one product ions.•A deprotonated diketopiperazine is proposed as a fragmentation intermediate.•Cis dipeptide conformation and enolate formation facilitate cyclized MS product ions.•Density functional theory calculations support the proposed fragmentation mechanism.

A deprotonated 2-imidazoline-4(5)-one product ion was observed as a major fragment in the collision-induced dissociation (CID) of several dipeptides containing serine at the C-terminal and an amino acid with an alkyl substituent at the N-terminal. This fragment becomes predominant at high collision energies. The same type of product ion was seen in the CID of cyclo(GlyGly). Labeling GlySer with O-18 suggests that the fragmentation may proceed through a symmetrical intermediate such as a deprotonated diketopiperazine. Density functional theory calculations of GlySer provided a possible mechanism for the fragmentation.

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