Negative ion mass spectrometry of peptides : an aid to structure determination

Abstract

Amphibians contain a rich chemical arsenal in their skin glands, vital for their defence against predators, both large and small. The peptides secreted by the frogs have a range of biological activities. These include both antibacterial and anticancer activity, others are neuropeptides, while some exhibit antifungal and antimalarial activity. Peptides are usually sequenced using positive ion mass spectrometry ( MS ). However, negative ion MS can also provide valuable sequencing information. Under negative ion MS conditions, the presence of a Cys residue is immediately identified by the facile side chain loss of H [subscript 2] S. The position of the Cys residue is determined by the formation of a side chain induced backbone cleavage ion. When a Cys residue is in the C - terminal position of a peptide, the spectrum is dominated by the loss of both H [subscript 2] S and CO [subscript 2]. Negative ion MS can also be used to identify and sequence intramolecular disulfide bridged peptides. The disulfide bridge is immediately identified by the facile loss of H [subscript 2] S [subscript 2] from the parent anion. Once the disulfide bridge is cleaved, further amide cleavages provide much of the sequence of the peptide, including the residues originally within the disulfide link. When one of the disulfide bridged Cys residues is in the C - terminal position, the major fragmentation is the loss of H [subscript 2] S [subscript 2] and CO [subscipt 2] from the parent ion. The negative ion mass spectra of citropin 1.1 and synthetically modified analogues show an unusual loss of an internal Val residue from the ( M - H ) - parent ion. This rearrangement requires the decomposing anion to have an α - helical structure. The skin secretions of Litoria peronii or Peron ' s Tree Frog contain five novel peptides which have been named peroneins. Four pro - peroneins are present in the summer secretions only. The biologically active peptides caerulein 1.1, caerin 1.1 and caerin 2.1 are also present in the glandular secretions.