Differentiation between selected pairs of tripeptide diastereomers by tandem mass spectrometry on a hybrid tandem mass spectrometer

Abstract

Low-energy collisionally activated decomposition (CAD) and unimolecular decomposition of the [M+H]+ ions for X-L-Pro-L-Phe, where X is L-Ala, D-Ala, L-Asp, or D-Asp, allow easy differentiation between the LLL and DLL diastereomers. Tandem mass spectrometric (MS/MS) studies of the [M+H]+ ions formed by fast-atom bombardment (FAB) at various ion kinetic energies (Elab values) on a hybrid tandem instrument produced ions of different intensities for the diastereomers. The ratio of NH3 to H2O loss is 0.3 for the L-Ala peptide but 1.7 for the D-Ala isomer at 5 eV. In some L-Ala spectra, the [M+H-NH3]+ ion does not appear at all. The y2 ion is up to twice as abundant in the L-Ala spectra as in the D-Ala, while the b2 ion is somewhat more abundant for CAD of the D-Ala peptide for most collision energies investigated. The D-Asp peptide produces a b2 ion that is more than half-again as abundant as in the case of the L-Asp isomer, and an [M+H-H2O]+ ion that is up to twice as abundant in the D-Asp CAD spectra as in those of the L-Asp. The y1, a2, and phenylalanine immonium ions are each up to twice as abundant in the L-Asp spectra as in those of the D-Asp isomer. The major differences are correlated with force-field calculations on hydrogen-bonded tautomers.

Separation of peptide isomers and conformers by ultra performance liquid chromatography

“…Site-specific racemization in peptides is known as a rare post-translational modification [4,5]. Complete chromatographic separations of diastereomeric peptides by RP-LC have been reported [4], as well as minor quantitative differences between their tandem mass spectra [6]. These might be useful for their recognition, in case MS/ MS spectra of pure reference compounds are available.…”

Structures of b and a Product Ions from the Fragmentation of Argentinated Peptides

“…Much effort has been spent on understanding fragmentation mechanisms and the resulting product ion structures of protonated peptides, which leads to a relative wealth of information. − ,− Recent efforts from Wysocki's, − Gaskell's, − Boyd's, − and other's 31-36 groups have established the “mobile proton theory”, namely that the “external” proton on a protonated peptidepresumably first attached to the N-terminus or to the basic site on a side chainis easily transferred to amide nitrogen atoms on the peptide backbone upon collision activation, thus producing a heterogeneous population of precursor ion structures. (This is to be contrasted with an alternative view in which the ionization process produces a heterogeneous population of ionized structures whose protonation sites are fixed.)…”

“…Soc. 1991 Gaskell's, [19][20][21][22][23][24][25][26][27] Boyd's, [28][29][30] and other's [31][32][33][34][35][36] groups have established the "mobile proton theory", namely that the "external" proton on a protonated peptidespresumably first attached to the N-terminus or to the basic site on a side chainsis easily transferred to amide nitrogen atoms on the peptide backbone upon collision activation, thus producing a heterogeneous population of precursor ion structures. (This is to be contrasted with an alternative view in which the ionization process produces a heterogeneous population of ionized structures whose protonation sites are fixed.)…”

Mass Spectrometry in Pharmaceutical Analysis

“…42 Although the interpretation of such chemical information remains largely a difficult process limited to skilled analysts, comparative information can be used to analyze related peptides for slight chemical differences such as presented by changes at a chiral center. 43 An alternative to the LC/MS chiral method presented above was devised using comparative MS/MS of synthetic standards. 39 Just as in section 6 above, it was of interest to know when a D-amino acid isomer of the peptide Arg-Lys-Asp-Val-Tyr (RKDVY) appeared during stability studies or as a result of production procedures.…”