title: Diastereomeric differentiation of norbornene amino acid peptides by electrospray ionization tandem mass spectrometry.
abstract: A new class of diastereomeric pairs of non-natural amino acid peptides derived from butyloxycarbonyl (Boc-)protected cis-(2S,3R)- and trans-(2S,3S)-beta-norbornene amino acids including a monomeric pair have been investigated by electrospray ionization (ESI) tandem mass spectrometry using quadrupole time-of-flight (Q-TOF) and ion-trap mass spectrometers. The protonated cis-BocN-beta-nbaa (2S,3R) (1) (betanbaa = beta-norbornene amino acid) eliminates the Boc group to form [M+H-Boc+H](+), whereas an additional ion [M+H-C(4)H(8)](+) is formed from trans-BocN-beta-nbaa (2S,3S) (2). Similarly, it is observed that the peptide diastereomers (di-, tri- and tetra-), with cis-BocN-beta-nbaa (2S,3R)- at the N-terminus, initially eliminate the Boc group to form [M+H-Boc+H](+) which undergo further fragmentation to give a set of product ions that are different for the peptides with trans-BocN-beta-nbaa (2S,3S)- at the N-terminus. Thus the Boc group fragments differently depending on the configuration of the amino acid present at the N-terminus. It is also observed that the peptide bond cleavage in these peptides is less favoured and most of the product ions are formed due to retro-Diels-Alder fragmentation. Interestingly, sodium-cationized peptide diastereomers mainly yield a series of retro-Diels-Alder fragment ions which are different for each diastereomer as they are formed starting from [M+Na-Boc+H](+) in peptides with cis-BocN-beta-nbaa (2S,3R)- at the N-terminus, and [M+Na-C(4)H(8)](+) in peptides with trans-BocN-beta-nbaa (2S,3S)- at the N-terminus. All these results clearly indicate that these diastereomeric pairs of peptides yield characteristic product ions which help distinguish the isomers.
authors.affiliation: National Centre for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad 500 007, India.
authors.forename: B
authors.initials: B
authors.lastname: Raju
authors.forename: V
authors.initials: V
authors.lastname: Ramesh
authors.forename: A
authors.initials: A
authors.lastname: Sudhakar
authors.forename: M
authors.initials: M
authors.lastname: Ramesh
authors.forename: V U M
authors.initials: VU
authors.lastname: Sarma
authors.forename: S
authors.initials: S
authors.lastname: Chandrasekhar
authors.forename: R
authors.initials: R
authors.lastname: Srinivas
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D013237:Stereoisomerism
chemical_list: D000596:Amino Acids; D010455:Peptides