Enhanced detection of thiol peptides by matrix-assisted laser desorption/ionization mass spectrometry after selective derivatization with a tailor-made quaternary ammonium tag containing maleimidyl group.

The goal of this study was to develop a new quaternary ammonium tag for the selective labeling of thiol peptides to improve their detection by using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The tag, 1-[3-(4-maleimidylphenoxy)propyl]trimethylammonium bromide (MPPTAB), was designed by using the maleimidyl moiety as a selective labeling group for thiols, and a positively charged quaternary ammonium moiety for improving ionization efficiency; the two moieties were combined by a bifunctional linker p-aminophenol. The properties of the tag were investigated with glutathione as a model peptide, and the derivatization conditions were also optimized with reversed-phase high-performance liquid chromatography. The results show that the effective derivatization of thiol peptides can be achieved at the reactant ratio of 3:1 (MPPTAB to the peptide) with a reaction time of 2 h at 37 degrees C, and the excess MPPTAB can be removed with 2-mercaptoethanol. The comparative study by mass spectrometry indicates that, after derivatization, the signal intensity of thiol peptides with more polar residues such as GPVEDAITAAIGRVAC is increased by 3-5 times, while that with less polar residues like CLLLGGTMV and LCMFVPGYT can be enhanced by more than 100 times. This may be explained in terms of the introduction of the positive charge and the improvement of hydrophobicity: the former raises the ionization efficiency of the tagged peptides, and the latter facilitates their solubility in a hydrophobic matrix, which in turn increases the signal intensity. The present strategy may have a potential use in detecting low-abundant thiol peptides.