Fragmentation Characteristics and Utility of Immonium Ions for Peptide Identification by MALDI-TOF/TOF-Mass Spectrometry

One of significant characteristics of matrix-assisted laser desorption/ionization tandem time-of-light mass spectrometry (MALDI-TOF/TOF-MS) high-energy collision induced dissociation (CID) is to produce abundant immonium (IM) ions that can offer a wealth of information for peptide composition. However, MALDI-TOF/TOF is generally used for routine protein identification based on database search or de novo sequencing combined with chemical derivation. Consequently, the characteristics of IM ions may not be fully explored and utilized. Here, a total of 239 MS/MS spectra were used to explore the fragmentation features of IM ions with MALDI TOF/TOF spectrometry and their application for peptide identification. IM ion signals for 14 kinds of amino acids including His with a positive rate of more than 50% were observed. The results indicated that the chemical nature of the amino acids and position effect were two main factors that affect the intensity of fragment ions. In addition, false positive IM ions were mainly derived from Arg, Lys, Leu and Ile residues or mixture peptides. Besides the compositional information, partial sequence information was also obtained by comparison of the relative intensity of IM ions. These findings are helpful when performing manual interpretations and can help improving current peptide search algorithms.

Besides a nearly complete series of b- and y- ion, another advantage of high energy CID performed on TOF/TOF is that the method can produce immonium ions, which can be used to distinguish isobaric peptides such as DGEAAENTDAQK and DSVAAENTDAQK. A strong Val IM ion was observed at m/z 72, which indicated that there are more than one Valine residues in sequence or it occupies a favorable position effect.Figure optionsDownload as PowerPoint slide