A transformation analogous to the well‐known conversion of an N‐terminal glutamine residue to pyroglutamic acid is the cyclization of an N‐terminal carboxamidomethylated cysteine residue (the normal product of alkylation with iodoacetamide). This yields 5‐oxothiomorpholine‐3‐carboxylic acid, with the same 17 Da mass loss observed in the Gln reaction. Nineteen tryptic peptides with Cys at the N‐terminal were identified for this study, and compared with eight with N‐terminal Gln. When examined by MALDI‐QqTOF and (off‐line HPLC)/MALDI‐QqTOF measurements, these were all found to undergo the cyclization reactions. The average degree of degradation during overnight digestion was found to be ∼51 and ∼34% for Cys and Gln, respectively; more detailed information on the time course of the reactions was obtained for the peptides CCTESLVNR and QYYTVFDR. Taking this modification into account while sequencing is likely to increase the probability of protein identification by peptide mass fingerprinting, especially for cysteine‐rich proteins. Copyright © 2003 John Wiley & Sons, Ltd.
Characterizing degradation products of peptides containing N‐terminal Cys residues by (off‐line high‐performance liquid chromatography)/matrix‐assisted laser desorption/ionization quadrupole time‐of‐flight measurements