Fragmentation of tunichrome Sp-1 is dominated by an unusual gas-phase intramolecular rearrangement.
Tunichrome Sp-1 is a modified pentapeptide from the ascidian Styela plicata, having the structure H-DOPA-DOPA-Gly-Pro-dcDeltaDOPA (where DOPA = 3,4-dihydroxyphenylalanine and dcDeltaDOPA = decarboxy-(E)-alpha,beta-dehydro-DOPA). The tandem mass spectrum of the peptide is dominated by a number of abundant fragment ions that involve a gas-phase rearrangement where the dcDeltaDOPA group becomes covalently attached to the N-terminus. The high degree of rearrangement in Sp-1 compared with a related octapeptide, plicatamide, allowed for detailed multiple mass spectrometric (MS(n)) (up to n = 6) experiments, and hence permitted a detailed assessment of the origin and routes to the formation of the various rearrangement ions. Analyses on both a triple-quadrupole and a quadrupole time-of-flight mass spectrometer were made to ascertain whether the gas-phase rearrangements observed for tunichrome Sp-1 were unique to an ion trap mass spectrometer (i.e. the hypothesis being that perhaps the extended trapping times were required to facilitate this unusual gas-phase rearrangement). Interestingly, analyses on both the triple-quadrupole and quadruple time-of-flight mass spectrometers revealed an identical phenomenon, with the rearrangement fragment ions present at approximately the same abundance as the non-rearranged a-, b- and y-type sequence ions. We suggest that the smaller size of tunichrome Sp-1 compared with plicatamide facilitates the transfer of the dcDeltaDOPA group in this gas-phase rearrangement. This rearrangement was not observed for peptide analogs of tunichrome Sp-1 that did not contain the dcDeltaDOPA at the C-terminus, confirming that the presence of dcDeltaDOPA is critical for the rearrangement.
Taylor
Steven W
SW
Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California-San Diego, La Jolla, CA 92093-0204, USA. swtaylor@ucsd.edu
Kassel
Daniel B
DB
Tincu
J Andy
JA
Craig
A Grey
AG
D000818:Animals; D005740:Gases / Q000737:chemistry*; D015394:Molecular Structure; D009842:Oligopeptides / Q000737:chemistry*; D010446:Peptide Fragments / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D014561:Urochordata / Q000737:chemistry
D005740:Gases; D009842:Oligopeptides; D010446:Peptide Fragments; C455663:tunichrome sp-1