Charge-state selective fragmentation analysis for protonated peptides in infrared multiphoton dissociation

We investigate the charge-state selective cleavage in gas-phase protonated peptides by using electrospray ionization (ESI) Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer (MS). The singly and multiply protonated peptides (Angiotensin II, Angiotensin I, Urotensin II, Bradykinin, Substance P) at the selected charge state were cleaved with the techniques of infrared multiphoton dissociation (IRMPD) in FTICR MS. Systematically changing IR laser power, the fragment ions were assessed to determine the cleaved amino bonds in the peptides at the selected charge state, then to quantitatively obtain the dissociation efficiency for each fragment. The results show that the fragment ions are observed at the selective cleavage of Asp-Xaa in singly charged Angiotensin I, Angiotensin II and Urotensin II ions that contain an acidic residue (Asp) and a basic residue (Arg or Lys), while the fragment ions arising from cleavage at Xaa-Pro dominate in those doubly charged peptides. It is observed that the dissociation channel of Asp-Xaa requires higher energy than that of Xaa-Pro. The charge selective fragmentation is not seen for Bradykinin and Substance P containing strong basic amino residues without an acidic residue. (C) 2004 Elsevier B.V. All rights reserved.