Negative-charge driven fragmentations for evidencing zwitterionic forms from doubly charged coppered peptides.
In aqueous solution, amino acids (AA) and peptides are known to exist as zwitterions over a large pH range. However, in the gas phase, i.e. in electrospray (ESI), the zwitterionic form becomes unfavorable owing to the absence of stabilizing effects from intermolecular solvation. Nevertheless, during mass spectrometry experiments, the presence of a metallic cation can reinforce the zwitterionic character of the molecule and thus influence its fragmentation under low energy collision-induced dissociation (CID) conditions. The [M + Cu(II)](2+) complexes of six pentapeptides (YGGFL, YGGFL(NH(2)), YGGFK, YGGFQ, KYGGF and QYGGF) were analyzed by collision to highlight the presence of zwitterions. The experiments were performed on a 3D-ion trap equipped with an orthogonal ESI source. For each peptides studied, negative-charge driven fragmentations on globally positively charged ions were observed. These fragmentation mechanisms, generally observed in the negative mode, suggest the competitive deprotonation of the C-terminal carboxylic acid or of the tyrosine side-chain residue for each peptide studied and thus a zwitterionic form to preserve the charge balance. Moreover, the specific loss of (CH(3)--C(6)H(4)--O)(*) characterizes YGGFK compared to YGGFQ and the specific loss of styrene characterizes KYGGF compared to QYGGF. These results allow the differentiation of the two couples of isobaric pentapeptides. An unusual loss of NH(4) (+), which occurred from the N-terminus, was also observed for YGGFL, YGGFL(NH(2)), YGGFK and YGGFQ. Finally, the reduction of Cu(II) to Cu(I), concomitant with the (CH(3)--C(6)H(4)--O)(*) release, was pointed out for YGGFK.