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 Gin to differentiate between Lys and Gin 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 gamma and epsilon 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 gamma 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.