title: Differentiation of three pairs of positional isomers of hybrid peptides with repeats of phenylalanine-beta3-h-valine/beta3-valine-phenylalanine by electrospray ionization tandem mass spectrometry.
abstract: Electrospray ionization ion trap mass spectrometry has been used to distinguish three pairs of positional isomers of a new series of N-blocked hybrid peptides derived from repeats of phenylalanine(D)-beta3-h-valine/beta3-h-valine-phenylalanine(D) (FbetaV/betaVF) non-natural amino acids. MSn of protonated isomeric peptides produces characteristic fragmentation involving the peptide backbone, the Boc group and the side chain. FbetaV-peptides can be distinguished from betaVF-peptides by the loss of R-OH from [M+H-Boc+H]+, which is either of relatively low abundance or totally absent for the latter peptides. In contrast, betaVF-peptides show abundant Mannich base characteristic ions by the elimination of ammonia, and imine due to a retro-Mannich cleavage. This fragmentation is absent for FbetaV-peptides. When beta-valine is at the C-terminus, abundant b+(n-1) ions are produced. This is ascribed to the probable formation of a stable diketopiperazine structure, and this has been supported by the loss of H2O and CO in the CID spectra of b+(n-1) ions. The hybrid dipeptide acids have also been distinguished in negative ion mass spectrometry.
authors: National Centre for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad 500 007, India.
authors: V
authors: V
authors: Ramesh
authors: P
authors: P
authors: Nagi Reddy
authors: R
authors: R
authors: Srinivas
authors: G
authors: G
authors: Srinivasulu
authors: A C
authors: AC
authors: Kunwar
mesh_terms: D007536:Isomerism; D010455:Peptides / Q000032:analysis*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D010455:Peptides