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 deltaDOPA (where dc deltaDOPA = 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 1H 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. In 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 plicatamide which did not contain the dc deltaDOPA 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.