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

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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.

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