title: Differentiation of positional isomers of hybrid peptides containing repeats of β-nucleoside derived amino acid (β-Nda-) and L-amino acids by positive and negative ion electrospray ionization tandem mass spectrometry (ESI-MSn).
abstract: A new class of positional isomeric pairs of -Boc protected oligopeptides comprised of alternating nucleoside derived β-amino acid (β-Nda-) and L-amino acid residues (alanine, valine, and phenylalanine) have been differentiated by both positive and negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-MS(n)). The protonated dipeptide positional isomers with β-Nda- at the N-terminus lose CH(3)OH, NH(3), and C(2)H(4)O(2), whereas these processes are absent for the peptides with L-amino acids at the N-terminus. Instead, the presence of L-amino acids at the N-terminus results in characteristic retro-Mannich reaction involving elimination of imine. A good correlation has been observed between the conformational structure of the peptides and the abundance of y(n)(+) and b(n)(+) ions in MS(n) spectra. In the case of tetrapeptide isomers that are reported to form helical structures in solution phase, no y(n)(+) and b(n)(+) ions are observed when the corresponding amide -NH- participates in the helical structures. In contrast, significant y(n)(+) and b(n)(+) ions are formed when the amide -NH- is not involved in the H-bonding. In the case of tetra- and hexapeptides, it is observed that abundant b(n)(+) ions are formed, presumably with stable oxazolone structures when the C-terminus of the b(n)(+) ions possessed L-amino acid and the β-Nda- at the C-terminus appears to prevent the cyclization process leading to the absence of corresponding b(n)(+) ions.
authors.affiliation: National Center for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad, 500607 India.
authors.forename: B
authors.initials: B
authors.lastname: Raju
authors.forename: M
authors.initials: M
authors.lastname: Ramesh
authors.forename: R
authors.initials: R
authors.lastname: Srinivas
authors.forename: S
authors.initials: S
authors.lastname: Chandrasekhar
authors.forename: N
authors.initials: N
authors.lastname: Kiranmai
authors.forename: V U M
authors.initials: VU
authors.lastname: Sarma
mesh_terms: D000596:Amino Acids / Q000737:chemistry; D007536:Isomerism; D009705:Nucleosides / Q000737:chemistry*; D009842:Oligopeptides / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D000596:Amino Acids; D009705:Nucleosides; D009842:Oligopeptides