title: Evidence for sequence scrambling and divergent H/D exchange reactions of doubly-charged isobaric b-type fragment ions.
abstract: To date, only a limited number of reports are available on structural variants of multiply-charged b-fragment ions. We report on observed bimodal gas-phase hydrogen/deuterium exchange (HDX) reaction kinetics and patterns for substance P b10(2+) that point to presence of isomeric structures. We also compare HDX reactions, post-ion mobility/collision-induced dissociation (post-IM/CID), and sustained off-resonance irradiation-collision induced dissociation (SORI-CID) of substance P b10(2+) and a cyclic peptide with an identical amino acid (AA) sequence order to substance P b10. The observed HDX patterns and reaction kinetics and SORI-CID pattern for the doubly charged head-to-tail cyclized peptide were different from either of the presumed isomers of substance P b10(2+), suggesting that b10(2+) may not exist exclusively as a head-to-tail cyclized structure. Ultra-high mass measurement accuracy was used to assign identities of the observed SORI-CID fragment ions of substance P b10(2+); over 30% of the observed SORI-CID fragment ions from substance P b10(2+) had rearranged (scrambled) AA sequences. Moreover, post-IM/CID experiments revealed the presence of two conformer types for substance P b10(2+), whereas only one conformer type was observed for the head-to-tail cyclized peptide. We also show that AA sequence scrambling from CID of doubly-charged b-fragment ions is not unique to substance P b10(2+).
authors.affiliation: Department of Chemistry and Biochemistry, Baylor University, Waco, TX, 76798, USA.
authors.forename: Behrooz
authors.initials: B
authors.lastname: Zekavat
authors.forename: Mahsan
authors.initials: M
authors.lastname: Miladi
authors.forename: Abdullah H
authors.initials: AH
authors.lastname: Al-Fdeilat
authors.forename: Arpad
authors.initials: A
authors.lastname: Somogyi
authors.forename: Touradj
authors.initials: T
authors.lastname: Solouki
mesh_terms: D000595:Amino Acid Sequence; D003903:Deuterium / Q000737:chemistry*; D041961:Deuterium Exchange Measurement / Q000379:methods*; D007700:Kinetics; D013058:Mass Spectrometry; D008956:Models, Chemical; D010446:Peptide Fragments / Q000737:chemistry*; D010456:Peptides, Cyclic / Q000737:chemistry; D013373:Substance P / Q000737:chemistry
chemical_list: D010446:Peptide Fragments; D010456:Peptides, Cyclic; D013373:Substance P; D003903:Deuterium