Optimizing the MALDI-TOF-MS observation of peptides containing disulfide bonds

The observation of peaks corresponding to both disulfide-bonded and reduced peptides in matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra of disulfides could suggest that the samples are either mixtures prior to analysis or that the measurement process has converted single compounds into mixtures. This is an important distinction when characterizing potentially disulfide-bonded peptides obtained from proteolyzed proteins or from oxidized synthetic peptides. It is well documented that disulfides can undergo in-source decay (ISD) when using a 337-nm laser. However, the mixed matrix 2-(4-hydroxyphenylazo)benzoic acid:alpha-cyano-4-hydroxycinnamic acid (1:10) not only suppresses the ISD reduction of disulfides to thiols but allows the same low threshold laser power generally used with alpha-cyano-4-hydroxycinnamic acid to be applied.