Negative ion production from peptides and proteins by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Negative ion production from peptides and proteins was investigated by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Although most research on peptide and protein identification with ionization by MALDI has involved the detection of positive ions, for some acidic peptides protonated molecules are not easily formed because the side chains of acidic residues are more likely to lose a proton and form a deprotonated species. After investigating more than 30 peptides and proteins in both positive and negative ion modes, M-H ions were detected in the negative ion mode for all peptides and proteins although the matrix used was 2,5-dihydroxybenzoic acid (DHB), which is a good proton donor and favors the positive ion mode production of M+H ions. Even for highly basic peptides without an acidic site, such as myosin kinase inhibiting peptide and substance P, good negative ion signals were observed. Conversely, gastrin I (1-14), a peptide without a highly basic site, will form positive ions. In addition, spectra obtained in the negative ion mode are usually cleaner due to absence of alkali metal adducts. This can be useful during precursor ion isolation for MS/MS studies.