DIFFERENTIATION OF DERIVATIZED LEUCINE AND ISOLEUCINE BY TANDEM MASS-SPECTROMETRY UNDER LIQUID SECONDARY-ION MASS-SPECTRAL CONDITIONS

In the liquid secondary ion mass spectra of N-acetyl, N-benzoyl and N- pivaloyl leucine and isoleucine the relative abundance of [MH - (H2O,C O)](+) ions is greater in the case of N-substituted isoleucine. The di fference is more distinctive in the collision-induced dissociation spe ctra of [M + H](+) ions, The [MH - COOH](+.) ions from N-substituted l eucine and isoleucine selectively decompose to give ions corresponding to the loss of isopropyl and ethyl radicals respectively. This select ive 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 fo r the corresponding N-benzoyl derivatives. (C) 1998 John Wiley & Sons, Ltd.