Negative ion fragmentations of deprotonated peptides: backbone cleavages directed through both Asp and Glu

The collision-induced spectra of [M - H](-) ions of a variety of natural an d synthetic amphibian peptides containing Asp and/or Glu exhibit characteri stic gamma backbone cleavage ions that identify the positions of these resi dues in the peptide. A theoretical study suggests that the Glu cleavage inv olves an S(N)i reaction of the carboxylate anion from the Glu alpha side ch ain to form a deprotonated cyclic lactone. The presence of either Asp or Gl u or other residues that effect pronounced side-chain cleavages (e.g. Ser o r Thr) results in the normal alpha and beta backbone cleavages being reduce d in comparison to those cleavages which originate from side chains. Copyri ght (C) 2001 John Wiley & Sons, Ltd.