NOVEL FRAGMENTATION PROCESS OF PEPTIDES BY COLLISION-INDUCED DECOMPOSITION IN A TANDEM MASS-SPECTROMETER - DIFFERENTIATION OF LEUCINE AND ISOLEUCINE

The mass spectra produced upon collision-induced decomposition of the protonated molecules of peptides often exhibit peaks that correspond to ions that are formed by cleavage of the -N-CR- bond along the peptide chain followed by cleavage of the beta , gamma bond if R has the general structure -C// beta -C// gamma -R prime . Most important is the differentiation of the amino acids leucine and isoleucine, which is generally difficult or impossible by mass spectrometry. Aromatic amino acids do not undergo this fragmentation because it would involve cleavage of a C-Ar bond, neither does alanine, which would involve loss of a hydrogen radical, nor does glycine, which lacks a beta , gamma bond.