Fragmentation of a novel marine peptide, plicatamide, involves an unusual gas-phase intramolecular rearrangement

During our characterization of plicatamide 1, a modified octapeptide: Phe-Phe-His-Leu-His-Phe-His-dc Delta DOPA (where dc Delta DOPA = decarboxy-(E)-alpha,beta -dehydro-3,4-dihydroxyphenylalanine) from the blood cells of the ascidian Styela plicata, we noted a series of fragment ions from the [M + H](+) ion which could not be assigned. There was no evidence in the H NMR spectrum to support an alternative molecular structure and the series of fragment ions were not present in the tandem mass spectrometry analysis of the [M + Na](+) ion. Ln addition, there was no evidence that the sample was a mixture of isobaric compounds. We propose that an unusual C-terminal to N-terminal rearrangement is responsible for the series of fragment ions from the [M + H](+) ion. This rearrangement was not observed in peptide analogs of plitatamide which did not contain the dc Delta DOPA at the C-terminus suggesting that this moiety is critical for the rearrangement. The proposed reaction is analogous to that recently reported by Vachet et al. involving a fragment ion formed from leucine enkephalin. (C) 2001 American Society for Mass Spectrometry.