Thermal determination of the abundances of c and z type fragment ions generated by in-source decay of peptide ions in matrix-assisted laser desorption ionization

Abstract

In our previous study on the spectral variation in matrix-assisted laser desorption ionization (MALDI) of peptides, we found that quantitatively reproducible spectra could be acquired by selecting those associated with the same early plume temperature, Tearly. The study was limited to the cases in which the in-source decay (ISD) of peptide ions occurred via b-y channels. Study was extended in this work to the cases in which ISD via c-z channels also occurred significantly, taking MALDI of substance P and chromogranin A using 2,5-dihydroxybenzoic acid (DHB) as examples. The observation made for these peptides were the same as in the previous cases, i.e., MALDI spectra became reproducible when those associated with the same Tearly were selected. The result suggests that the MALDI spectrum of a peptide is thermally determined regardless of the mechanism contributing to ISD of the peptide ion. This, in turn, suggests that c-z channels in ISD of peptide ions are not initiated by electron capture. Also, if hydrogen radical capture initiates these channels, these radicals would be generated by the homolytic cleavage of an XH bond in the ground electronic state of a matrix molecule, not by excited state photolysis.