title: Electrospray ionization tandem mass spectrometric study of protonated and alkali- cationized α/ε-hybrid peptides: differentiation of a pair of dipeptide positional isomers.
abstract: A new class of Boc-N-protected hybrid peptides derived from L- Ala and ε6-Caa (L-Ala = L-Alanine, Caa = C-linked carboamino acid derived from D-xylose) have been studied by positive ion electrospray ionization (ESI) ion-trap tandem mass spectrometry (MS/MS). MSnspectra of protonated and alkali-cationized hybrid peptides produce characteristic fragmentation involving the peptide backbone, the tert-butyloxycarbonyl (Boc) group, and the side chain. The dipeptide positional isomers are differentiated by the collision-induced dissociation (CID) of the protonated and alkali-cationized peptides. The CID of [M + H]+ion of Boc-NH-L-Ala-ε-Caa- OCH3(1) shows a prominent [M + H - C4H8]+ion, which is totally absent for its positional isomer Boc-NH-ε-Caa-L-Ala-OCH3(6), which instead shows significant loss of t-butanol. The formation of the [M + Cat - C4H8]+ion is totally absent and [M + Cat - Boc + H]+is prominent in the CID of the [M + Cat]+ion of Boc-NH-L-Ala-ε-Caa- OCH3(1), whereas the former is highly abundant and the latter is of low abundance for its positional isomer Boc-NH-ε-Caa-L-Ala-OCH3(6). It is observed that 'b' ions are abundant when oxazolone structures are formed through a five-membered cyclic transition state in tetra-, penta-, and hexapeptides and the cyclization process for larger 'b' ions led to an insignificant abundance. However, the significant 'b' ion is formed in ε,α-dipeptide, which may have a seven-membered substituted 2-oxoazepanium ion structure. The MSnspectra of [M + Cat - Boc + H]+ions of these peptides are found to be significantly different to those of [M + H - Boc + H]+ions. The CID spectra of [M + Cat - Boc + H]+ions of peptide acids containing L-Ala at the C-terminus show an abundant N-terminal rearrangement ion, [bn+ 17 + Cat]+, which is absent for the peptide acids containing ε-Caa at the C-terminus. Thus, the results of these hybrid peptides provide sequencing information, the structure of the cyclic intermediate involved in the formation of the rearrangement ion, and distinguish a pair of dipeptide positional isomers.
authors.affiliation: National Centre for Mass Spectrometry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India.
authors.forename: A
authors.initials: A
authors.lastname: Ramesh Babu
authors.affiliation: National Centre for Mass Spectrometry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India.
authors.forename: G
authors.initials: G
authors.lastname: Raju
authors.affiliation: National Centre for Mass Spectrometry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India.
authors.forename: C
authors.initials: C
authors.lastname: Purna Chander
authors.affiliation: Organic and Biomolecular Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India.
authors.forename: B
authors.initials: B
authors.lastname: Shoban Babu
authors.affiliation: National Centre for Mass Spectrometry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India. srini@iict.res.in.
authors.forename: R
authors.initials: R
authors.lastname: Srinivas
authors.affiliation: Organic and Biomolecular Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India. sragampeta@yahoo.co.in.
authors.forename: G V M
authors.initials: GV
authors.lastname: Sharma
mesh_terms: D000468:Alkalies / Q000737:chemistry*; D002412:Cations; D004151:Dipeptides / Q000032:analysis* / Q000737:chemistry*; D008635:Mescaline; D011522:Protons; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D000468:Alkalies; D002412:Cations; D004151:Dipeptides; D011522:Protons; C032893:isomescaline; D008635:Mescaline