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Hypervalent radical formation probed by electron transfer dissociation of zwitterionic tryptophan and tryptophan‐containing dipeptides complexed with Ca 2+ and 18‐crown‐6 in the gas phase
Author(s) -
Fujihara Akimasa,
Matsuo Sou,
Tajiri Michiko,
Wada Yoshinao,
Hayakawa Shigeo
Publication year - 2015
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/jms.3628
Subject(s) - chemistry , electron transfer dissociation , hypervalent molecule , photochemistry , dissociation (chemistry) , electron capture dissociation , electron transfer , collision induced dissociation , hydrogen atom , mass spectrometry , crystallography , medicinal chemistry , tandem mass spectrometry , organic chemistry , reagent , alkyl , chromatography
The relationship between peptide structure and electron transfer dissociation (ETD) is important for structural analysis by mass spectrometry. In the present study, the formation, structure and reactivity of the reaction intermediate in the ETD process were examined using a quadrupole ion trap mass spectrometer equipped with an electrospray ionization source. ETD product ions of zwitterionic tryptophan (Trp) and Trp‐containing dipeptides (Trp‐Gly and Gly‐Trp) were detected without reionization using non‐covalent analyte complexes with Ca 2+ and 18‐crown‐6 (18C6). In the collision‐induced dissociation, NH 3 loss was the main dissociation pathway, and loss related to the dissociation of the carboxyl group was not observed. This indicated that Trp and its dipeptides on Ca 2+ (18C6) adopted a zwitterionic structure with an NH 3 + group and bonded to Ca 2+ (18C6) through the COO − group. Hydrogen atom loss observed in the ETD spectra indicated that intermolecular electron transfer from a molecular anion to the NH 3 + group formed a hypervalent ammonium radical, R‐NH 3 , as a reaction intermediate, which was unstable and dissociated rapidly through N–H bond cleavage. In addition, N–C α bond cleavage forming the z 1 ion was observed in the ETD spectra of Trp‐GlyCa 2+ (18C6) and Gly‐TrpCa 2+ (18C6). This dissociation was induced by transfer of a hydrogen atom in the cluster formed via an N–H bond cleavage of the hypervalent ammonium radical and was in competition with the hydrogen atom loss. The results showed that a hypervalent radical intermediate, forming a delocalized hydrogen atom, contributes to the backbone cleavages of peptides in ETD. Copyright © 2015 John Wiley & Sons, Ltd.