title: How to discriminate between leucine and isoleucine by low energy ESI-TRAP MSn.
abstract: In peptide sequencing experiments involving a single step tandem mass acquisition, leucine and isoleucine are indistinguishable because both are characterized by a 113 Da mass difference from the other peptide fragments in the MS2 spectrum. In this work, we propose a new method to distinguish between these two amino acids in consecutive MSn experiments, exploiting a gas-phase fragmentation of isoleucine that leads to a diagnostic 69 Da ion. We used this method to assess the Leu/Ile residues of several synthetic peptides. The procedure was then tested on a tryptic digest of myoglobin, assigning the correct amino acid in the majority of the peptides. This work was performed with an old and low-resolution instrument, thus demonstrating that our method is suitable for a wide number of ion trap mass spectrometers, not necessarily expensive or up-to-date.
authors.affiliation: Department of Experimental Medicine-Biochemistry Section and Center of Excellence for Biomedical Research (DIMES), University of Genoa, Genoa, Italy. andrea.armirotti@unige.it
authors.forename: Andrea
authors.initials: A
authors.lastname: Armirotti
authors.forename: Enrico
authors.initials: E
authors.lastname: Millo
authors.forename: Gianluca
authors.initials: G
authors.lastname: Damonte
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D006736:Horses; D007532:Isoleucine / Q000032:analysis*; D007930:Leucine / Q000032:analysis*; D008969:Molecular Sequence Data; D009211:Myoglobin / Q000737:chemistry; D010455:Peptides / Q000032:analysis / Q000737:chemistry; D020539:Sequence Analysis, Protein; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D009211:Myoglobin; D010455:Peptides; D007532:Isoleucine; D007930:Leucine