No 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...

No abstract

Two fragmentations of (CH3)3SiO-CR2-X-CR2-OSi(CH3)3, where X is a rigid group such as a triple bond or an aromatic ring, are losses of a methyl or R group (where R is H or alkyl). The metastable-ion dissociations of [M-R]+ and [M-CH3]+ ions include the rearrangement of a trimethylsilyl (TMS) cation and a (CH3)2Si = O neutral species through an ion-neutral complex. On the basis of tandem mass spectrometry (MS/MS), exact mass measurement and isotopic labeling experiments, it has been established that the two trimethylsilyloxy groups in the TMS ethers interact across a wide range of distances via an ion-neutral complex. The migration of a TMS cation occurs when the group that is bound to carbon is expelled as a radical by an oxygen-directed cleavage to give a trimethylsilylated oxonium ion. If, on the other hand, a methyl radical is lost from the silicon atom, then (CH3)2Si = O migrates. The mobilities of the TMS cation and the neutral (CH3)2Si = O are governed by the capability of the rigid group to delocalize charge.