Backbone Cleavages of [M – H] − Anions of Peptides. New Backbone Cleavages following Cyclisation Reactions of a C-Terminal [CONH] − Group with Ser Residues and the Use of the γ Backbone Cleavage Initiated by Gln to Differentiate between Lys and Gln Residues

The [M – H]⁻ anions of some citropin, caerin and aurein peptides, which contain both Ser residues and a C-terminal CONH₂ functionality, show a number of fragmentations which occur through Ser residues. The first, the loss of CH₂O from the Ser side chain, always produces pronounced peaks in the spectrum. There are two other fragmentations which are competitive with CH₂O loss: these involve cleavage of the backbone of the peptide. These fragmentations involve loss of RNH₂ and (RNH₂ + H₂O) from the [M – H]⁻ ion of a peptide of general structure RNHCH(CH₂OH)CO——NHCHR'CONH₂. This effectively breaks the peptide backbone between the NH and CH of Ser. We have called these fragmentations γ and ε cleavages, respectively. These processes are initiated by molecular recognition between the C-terminal CONH⁻ and the Ser side chain, with the formation of an H-bonded complex between the CONH⁻ oxygen and the OH group of Ser. The ion complex, so formed, holds the nucleophilic NH⁻ and the electrophilic CH group (of Ser) in positions which allow cyclisation to proceed and produce an intermediate which then initiates those independent processes which involve losses of RNH₂ and (RNH₂ + H₂O), respectively. These processes have been studied theoretically using a model system at the AM1 level of theory. The isobaric residues Gln and Lys should be distinguishable using negative-ion mass spectrometry, because the Gln residue should effect a γ backbone cleavage, whereas Lys cannot. The study of a number of peptides containing Gln and/or Lys, indicates that this is the case, except when Gln is close to the C-terminal position. The Gln backbone cleavage ions are of small abundance when there are other residues present, for example Asp, which may effect facile backbone cleavage.