Fragmentation of protonated oligopeptides XLDVLQ (X=L, H, K or R) by surface induced dissociation: additional evidence for the 'mobile proton' model

The fragmentation of a series of singly protonated peptides, X-Leu-Asp-Val-Leu-Gln (XLDVLQ, X=Leu(L), His(H), Lys(K), or Arg(R)), was investigated by surface induced dissociation (SID) in a tandem quadrupole mass spectrometer. The SID collision energies required for the fragmentation were found to increase with increasing gas-phase basicity of the 'X' amino acid residue. The results are consistent with previous observations reported for other series of peptides and can be explained based on the 'mobile proton' model. Enhanced cleavage at the C(O)-N bond located C-terminal to the Asp residue (Asp-Xxx) was observed only in the presence of Arg, the most basic common amino acid residue. The results suggest that the acidic hydrogen of the Asp side chain becomes significant as an alternative source of proton to promote the 'charge-directed' cleavage of the amide linkage of Asp-Xxx (e.g., via a cyclic intramolecular hydrogen bond), when the 'ionizing' proton is 'sequestered' by the Arg residue. Lower abundances of side chain cleavage d ions by SID were observed relative to those previously detected by high energy CID in sector and sector-hybrid instruments. (C) 1999 Elsevier Science B.V. All rights reserved.