Formation of (b n−1 + H 2O) Ions by Collisional Activation of MALDI-Formed Peptide [M + H] + Ions in a QqTOF Mass Spectrometer

• Published in: Journal of the American Society for Mass Spectrometry 2007-06, Vol.18 (6), p.1024-1037
• Main Authors: She, Yi-Min, Krokhin, Oleg, Spicer, Victor, Loboda, Alexandre, Garland, Gideon, Ens, Werner, Standing, Kenneth G., Westmore, John B.
• Format: Article
• Language: English
• Subjects: Activation; Amino acids; Amino Acids - chemistry; Analytical, structural and metabolic biochemistry; Biological and medical sciences; Capsid Proteins - chemistry; Carbonyls; Carboxyl group; Chains; Fundamental and applied biological sciences. Psychology; General aspects, investigation methods; Histidine; Hydrogen Bonding; Ions; Lysine; Mass spectrometry; Peptides; Proteins; Solutions; Spectrometry, Mass, Electrospray Ionization - methods; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization - methods; Water - chemistry
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1016/j.jasms.2007.02.008

Collisional activation of [M + H] + parent ions from peptides of n amino acid residues may yield a rearrangement that involves loss of the C-terminal amino acid residue to produce (b n−1 + H 2O) daughters. We have studied this reaction by a retrospective examination of the m/z spectra of two collections of data. The first set comprised 398 peptides from coat protein digests of a number of plant viruses by various enzymes, where conditions in the tryptic digests were chosen so as to produce many missed cleavages. In this case, a large effect was observed—323 (b n−1 + H 2O) daughter ions (∼81%), including 185 (∼46%) “strong” decays with ratios (b n−1 + H 2O)/(b n−1 ) > 1. The second set comprised 1200 peptides, all from tryptic digests, which were carried out under more stringent conditions, resulting in relatively few missed cleavages. Even here, 190 (b n−1 + H 2O) ions (∼16%) were observed, including 87 (> 7%) “strong” decays, so the effect is still appreciable. The results suggest that the tendency for (b n−1 + H 2O) ion formation is promoted by the protonated side chain of a non-C-terminal basic amino acid residue, in the order arginine ⪢ lysine ≥ histidine, and that its (non-C-terminal) position is not critical. The results can be interpreted by a mechanism in which hydrogen bonding between the protonated side chain and the ( n − 1) carbonyl oxygen facilitates loss of the C-terminal amino acid residue to give a product ion having a carboxyl group at the new C-terminus.