Differentiation of derivatized leucine and isoleucine by tandem mass spectrometry under liquid secondary ion mass spectral conditions

Abstract

In the liquid secondary ion mass spectra of N-acetyl, N-benzoyl and N-pivaloyl leucine and isoleucine the relative abundance of [MH - (H₂O,CO)]⁺ ions is greater in the case of N-substituted isoleucine. The difference is more distinctive in the collision-induced dissociation spectra of [M + H]⁺ ions. The [MH - COOH]⁺. ions from N-substituted leucine and isoleucine selectively decompose to give ions corresponding to the loss of isopropyl and ethyl radicals respectively. This selective decomposition can be used to estimate the percentage of leucine or isoleucine in a mixture. Peptides containing leucine or isoleucine as the N-terminus residue also give this characteristic fragmentation for the corresponding N-benzoyl derivatives.