TANDEM MASS-SPECTROMETRY OF PEPTIDES .3. DIFFERENTIATION BETWEEN LEUCINE AND ISOLEUCINE BASED ON NEUTRAL LOSSES

The neutral species generated during the collisionally activated dissociation (CAD) of the protonated dipeptides Ala-Leu, Ala-Ile, Leu-Ala and Ile-Ala are post-ionized by collision and detected in neutral fragment-reionization ((+)N(f)R(+)) mass spectra. These spectra are dominated by fragment ions diagnostic for the C-terminal amino acid, as confirmed by comparison with reference collision-induced dissociative ionization (CIDI) mass spectra of authentic Ala, Leu and Ile. CIDI of Leu and Ile yields distinct and structurally indicative products. These are also observed upon (+)N(f)R(+) of [Ala-Leu]H+ and [Ala-Ile]H+, respectively, thus allowing one to deduce the correct sequence. In contrast, [Leu-Ala]H+ and [Ile-Ala]H+, which possess identical C-termini, form no differentiating neutral losses upon CAD.