Abstract
Cysteine sulfonic acid‐containing peptides, being typical acidic peptides, exhibit low response in matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry. In this study, matrix conditions and the effect of diammonium hydrogencitrate (DAHC) as additive were investigated for ionization of cysteine sulfonic acid‐containing peptides in MALDI. A matrix‐free ionization method, desorption/ionization on porous silicon (DIOS), was also utilized to evaluate the effect of DAHC. When equimolar three‐component mixtures of peptides carrying free cysteine, cysteine sulfonic acid, and carbamidomethyl cysteine were measured by MALDI using a common matrix, α‐cyano‐4‐hydroxycinnamic acid (CHCA), no signal corresponding to cysteine sulfonic acid‐containing peptide could be observed in the mass spectrum. However, by addition of DAHC to CHCA, the peaks of cysteine sulfonic acid‐containing peptides were successfully observed, as well as when using 2,4,6‐trihydroxyacetophenone (THAP) and 2,6‐dihydroxyacetophenone with DAHC. In the DIOS mass spectra of these analytes, the use of DAHC also enhanced the peak intensity of the cysteine sulfonic acid‐containing peptides. On the basis of studies with these model peptides, tryptic digests of oxidized peroxiredoxin 6 were examined as a complex peptide mixture by MALDI and DIOS. In MALDI, the peaks of cysteine sulfonic acid‐containing peptides were observed when using THAP/DAHC as the matrix, but this was not so with CHCA. In DIOS, the signal from cysteine sulfonic acid‐containing peptides was suppressed; however, the use of DAHC significantly enhanced the signal intensity with an increase in the number of observed peptides and increased signal‐to‐noise ratio in the DIOS spectra. The results show that DAHC in the matrix or on the DIOS chip decreases discrimination and suppression effects in addition to suppressing alkali‐adduct ions, which leads to a beneficial effect on protonation of peptides containing cysteine sulfonic acid. Copyright © 2005 John Wiley & Sons, Ltd.