Uncommon Modifications and Dehydration Reaction in Analysis of Lysozyme by Matrix-Assisted Laser Desorption Ionization Tandem Time of-Flight Mass Spectrometry

Tryptic digest of Lysozyme were analyzed by matrix-assisted laser desorption ionization tandem time of flight-mass spectrometry (MALDI-TOF/TOF-MS). Six unique peptides were identified with high confidence by database searching and the Mascot score is 420 with the protein coverage of 54%. Furthermore, several uncommon modifications including deamidation of Asn residue, deamidation of Asn plus oxidation of Met, deamidation of Asn plus oxidation of Met and oxidation of Trp were observed for peptide IVSDGNGMNAWVAWR (98→112). Furthermore, a dehydration reaction was observed in the process of MALDI for this peptide. All above results showed that the Asparagine at 103rd position has a highly chemical reactivity. In addition, the modification of propionamide (Cystine) was observed for another two peptides. This study indicated that, for those of unmatched data by database searching, the data utilization and the certainty of analysis result would be increased by using manual explanation and selecting very low abundant ions in primary spectrum. This method is also helpful for finding potential post-translational modifications.

Graphical abstractDespite of the weak signal of m/z 1658.81 ion in MALDI-TOF spectrum of tryptic digests of lysozyme, the MS/MS data provide a sufficient evidence for intramolecular dehydration reaction occurred at 103Asp residue of peptide 98IVSDGDGMNAWVAWR112 by manual interpretation and comparation.Figure optionsDownload full-size imageDownload as PowerPoint slide