title: Structure and reactivity of the distonic and aromatic radical cations of tryptophan.
abstract: In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric forms of tryptophan radical cations-a distonic indolyl N-radical (H3N(+) - TrpN(•)) and a canonical aromatic π (Trp(•+)) radical cation. The distonic radical cation was generated by nitrosylating the indole nitrogen of tryptophan in solution followed by collision-induced dissociation (CID) of the resulting protonated N-nitroso tryptophan. The π-radical cation was produced via CID of the ternary [Cu(II)(terpy)(Trp)](•2+) complex. CID spectra of the two isomeric species were found to be very different, suggesting no interconversion between the isomers. In gas-phase ion-molecule reactions, the distonic radical cation was unreactive towards n-propylsulfide, whereas the π radical cation reacted by hydrogen atom abstraction. DFT calculations revealed that the distonic indolyl radical cation is about 82 kJ/mol higher in energy than the π radical cation of tryptophan. The low reactivity of the distonic nitrogen radical cation was explained by spin delocalization of the radical over the aromatic ring and the remote, localized charge (at the amino nitrogen). The lack of interconversion between the isomers under both trapping and CID conditions was explained by the high rearrangement barrier of ca.137 kJ/mol. Finally, the two isomers were characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy in the ~1000-1800 cm(-1) region. It was found that some of the main experimental IR features overlap between the two species, making their distinction by IRMPD spectroscopy in this region problematic. In addition, DFT theoretical calculations showed that the IR spectra are strongly conformation-dependent.
authors.affiliation: Department of Chemistry and Biochemistry, and Center for Biochemical and Biophysical Studies, Northern Illinois University, DeKalb, IL 60115, USA.
authors.forename: Andrii
authors.initials: A
authors.lastname: Piatkivskyi
authors.forename: Sandra
authors.initials: S
authors.lastname: Osburn
authors.forename: Kendall
authors.initials: K
authors.lastname: Jaderberg
authors.forename: Josipa
authors.initials: J
authors.lastname: Grzetic
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Steill
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: Junfang
authors.initials: J
authors.lastname: Zhao
authors.forename: Justin Kai-Chi
authors.initials: JK
authors.lastname: Lau
authors.forename: Udo H
authors.initials: UH
authors.lastname: Verkerk
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
authors.forename: Victor
authors.initials: V
authors.lastname: Ryzhov
mesh_terms: D002412:Cations / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D008958:Models, Molecular; D021241:Spectrometry, Mass, Electrospray Ionization; D013329:Structure-Activity Relationship; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D002412:Cations; D005609:Free Radicals; D014364:Tryptophan