The application of negative ion electrospray mass spectrometry for the sequencing of underivatized disulfide-containing proteins: insulin and lysozyme

Abstract

Negative ion electrospray mass spectra of the peptides produced by tryptic and chymotrypsin digests of bovine insulin, and from the tryptic digest of lysozyme identify at least 80% of the sequences of these proteins. In particular, negative ion mass spectrometry identifies and positions disulfide moieties, and is the method of choice for identifying this post-translational modification in these two proteins. Intramolecular disulfide functionality is identified by the fragmentation [(M − H)− − H2S2]− in a digest peptide, and CID of that fragment anion provides amino acid sequencing information. Digest peptides containing an intermolecular disulfide structure undergo facile and diagnostic cleavages. Each cleavage produces a peptide fragment from which CID MS/MS data provide sequencing information.