Distinction and quantitation of leucine-isoleucine isomers and lysine-glutamine isobars by electrospray ionization tandem mass spectrometry (MSn, n=2, 3) of copper(II)-diimine complexes

Electrospray ionization of mixtures of isomeric and isobaric amino acids wa s investigated with the goal of distinguishing and quantifying the componen ts. Isomeric amino acids leucine and isoleucine were readily distinguished and quantified in 90:10 to 10:90 binary mixtures using hue-stage (MS2) and three-stage (MS3) tandem mass spectrometric dissociations of ternary Cu2+-2 ,2'-bipyridyl (bpy) complexes, [Cu(AA - Pf)bpy](+). The complexes self-asse mbled in solution upon mixing the components and provided a convenient mean s of efficient derivatization that increased the efficiency of amino acid i onization by electrospray and shifted the mass of the analytes to a region which was free of solvent interferences, Low-energy dissociations of [Cu(AA - H)bpy](+) complexes in a quadrupole ion trap were achieved at >90% conve rsions and >80% trapping efficiencies for the MS2 and MS3 precursor and fra gment ions. Isobaric amino acids glutamine and lysine were also distinguish ed through MS2 and MS3 of their ternary complexes with Cu2+ and bpy, ESI of [Cu(Gln - H)bpy](+) was enhanced in the presence of [Cu(Lys - H)bpy](+), w hich resulted in non-linear response at low Lys concentrations, Copyright ( C) 2000 John Whey & Sons, Ltd.