In-source decay characteristics of peptides in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

AbstractIn-source decay (ISD) of peptides, coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, has been examined to determine the influence of the matrix, the susceptibility of amino-acid residues to ISD, and the effect of extraction delay times. Out of nine di- and tri-hydroxybenzoic acids and three cinnamic derivatives tested, the most suitable matrix for ISD was 2,5-dihydroxybenzoic acid. The amine bond at Xxx–Gly and Xxx–Val residues was less susceptible than other amino-acid residues to ISD; however, the more sensitive residue(s) were not as clear. Using a peptide that gave the yn- and (zn + 2)-series product ions, it was confirmed that amide-bond cleavage (formation of the yn-series ions) accompanied metastable peaks, whereas metastable peaks were never observed with amine-bond cleavage [formation of the (zn + 2)-series ions]. Furthermore, abundant cn-series ions, which originate from amine-bond cleavage on the peptide backbone, were observed whenever a minimum delay time of 38 ns or continuous extraction was used to obtain spectra. These data indicate that amine-bond cleavage in ISD takes place on the ionization time scale before the energy randomization is completed