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 and synthetic amphibian peptides containing Asp and/or Glu exhibit characteristic γ backbone cleavage ions that identify the positions of these residues in the peptide. A theoretical study suggests that the Glu cleavage involves an S N i reaction of the carboxylate anion from the Glu α side chain to form a deprotonated cyclic lactone. The presence of either Asp or Glu or other residues that effect pronounced side‐chain cleavages (e.g. Ser or Thr) results in the normal α and β backbone cleavages being reduced in comparison to those cleavages which originate from side chains. Copyright © 2001 John Wiley & Sons, Ltd.