A study of b₁+H₂O and b₁-ions in the product ion spectra of dipeptides containing N-terminal basic amino acid residues

Abstract

The product ion spectra of approximately 200 dipeptides were acquired under low-energy conditions using a triple quadrupole mass spectrometer. The spectra of dipeptides containing an N-terminal arginine (R), histidine (H), or lysine (K) were observed to yield a b 1 ϩ H 2 O ion corresponding to the protonated basic amino acid. This was equivalent to the y 1 -ion in the corresponding C-terminal isomer. The formation of a b 1 ϩ H 2 O ion was not a significant fragmentation channel in any dipeptides analyzed including those containing a C-terminal basic amino acid unless they also contained an N-terminal basic amino acid. Occurring simultaneously and under equal energy conditions an apparent b 1 -ion was formed, which has its corresponding C-terminal equivalent in the y 1 -H 2 O ion. Energy resolved mass spectrometry (ERMS), deuterium labeling, and accurate mass experiments as well as data reported were used to show the relationships between the b 1 ϩH 2 O and b 1 -ions in the dipeptides containing an N-terminal basic amino acid and the y 1 and y 1 -H 2 O ions in the corresponding C-terminal isomers. (J Am Soc Mass Spectrom 2007, 18, 1414 -1422) © 2007 American Society for Mass Spectrometry C ollision induced dissociation (CID) in a triple quadrupole mass spectrometer utilizing electrospray ionization (ESI) is a widely used technique for obtaining product ion spectra for the characterization of peptides. The spectrometric information in the product ion spectrum acquired at a single collision energy is limited and can be significantly enhanced utilizing energy resolved mass spectrometry (ERMS), where product ion spectra are acquired at increasingly greater collision cell energies. This results in an increase in the internal energy of the precursor ion, which leads to further dissociation. A plot of relative intensity versus collision energy of selected fragment ions leads to the generation of breakdown curves [1,2]. Evaluation of these breakdown curves can provide information on fragmentation mechanisms such as distinguishing between competitive and consecutive fragmentation pathways, the stability of product ions, and identification of first and subsequent generation product ions [3][4][5][6][7][8]; identification of isomers and tautomers [9 -11]; and development of library searchable product ion spectra [12,13]. Since the internal energy of the precursor ion is not rigorously known, breakdown curves provide a qualitative picture of a fragmentation mechanism.Using these analytical tools, our laboratory analyzed ϳ200 dipeptides and have made some noteworthy observations in the product ion spectra of dipeptides containing N-terminal arginine (R), histidine (H), and lysine (K) and their C-terminal isomers. We used the single letter abbreviations for amino acids in dipeptides throughout this paper.The first was the observation of a fragment ion that corresponded to the mass of the protonated molecule (M ϩ H) ϩ of the basic amino acid. This was observed at m/z ϭ 175 for arginine, m/z ϭ 156 for histidine, and m/z ϭ 147 fo...

Structural elucidation of isocyanate-peptide adducts using tandem mass spectrometry

“…Further evidence of binding to the Nterminus is obtained through the observation of the [Y ϩ NCO] ϩ immonium ion and an unusual b 1 ion. Formation of acylium b 1 ions is generally considered to result in the loss of CO to form the stable a 1 immonium ion, however, a number of reports have described the formation of stable cyclic b 1 ions [26,27].…”

Mass Spectrometry Analysis of 2-Nitrophenylhydrazine Carboxy Derivatized Peptides

“…Hydrophilic peptides with two or more basic groups have a tendency to undergo C-terminal elimination (loss of the C-terminal residue) upon CAD, a phenomenon already documented in the literature [24][25][26][27][28][29][30][31][32][33][34][35][36] and recently found to be a charge-dependent, side-chain assisted C-terminal rearrangement [29]. By blocking the C-terminal carboxy group, NPHylation completely eliminates such C-terminal rearrangement for both charge states (SF4), once again demonstrating that a free C-terminal carboxy group is required for C-terminal elimination side reactions [26,29,36].…”

Characterization of impurities in the bulk drug lisinopril by liquid chromatography/ion trap spectrometry

“…The product ion at m/z 263 corresponded to the dipeptide N-terminal a 1 ion with the combined loss of proline and CO, while ion at m/z 246 was formed from subsequent loss of NH 3 . The product ion at m/z 309 was formed through a rearrangement (Florêncio et al, 1998;Hiserodt et al, 2007), with the elimination of dihydropyrrole and CO, and could be used to characterize the 6-amino-2-(1-carboxy-3-phenylpropyl-amino)-hexanoic acid structure. The product ion at m/z 227 was formed by the neutral loss of 2-amino-4-phenylbutanoic acid (179 Da).…”