Electrospray ionization mass spectroscopic analysis of peptides modified with N-ethylmaleimide or iodoacetanilide

The cysteine-specific modifiers we reported previously, N-ethylmaleimide (NEM) and iodoacetanilide (IAA), have been applied to label cysteine residues of peptides in combination with electrospray ionization mass spectrometry (ESI-MS/MS), and their scope in proteomic studies was examined. Peptides modified with N-ethylmaleimide (NEM) or iodoacetanilide (IAA) showed significant enhancement in ionization efficiencies. These modifiers were also found to remain intact in tandem mass spectrometry. Both combinations of N-ethylmaleimide (NEM) and d(5)-N-ethylmaleimide (d(5)-NEM), and iodoacetanilide (IAA) and (13)C(6)-iodoacetanilide ((13)C(6)-IAA) were also shown to be applicable to quantitative analysis of a peptide.