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Structure and Reactivity of the Distonic and Aromatic Radical Cations of Tryptophan |
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| Authors: | Andrii Piatkivskyi Sandra Osburn Kendall Jaderberg Josipa Grzetic Jeffrey D. Steill Jos Oomens Junfang Zhao Justin Kai-Chi Lau Udo H. Verkerk Alan C. Hopkinson K. W. Michael Siu Victor Ryzhov | |
| Affiliation: | 1. Department of Chemistry and Biochemistry, and Center for Biochemical and Biophysical Studies, Northern Illinois University, DeKalb, IL, 60115, USA 2. FOM Institute for Plasma Physics, Rijnhuizen 14, 3439 MN, Nieuwegein, The Netherlands 3. University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands 4. Institute for Molecules and Materials (IMM), FELIX facility, Radboud University Nijmegen, Heyendaalseweg 135, 6525AJ, Nijmegen, The Netherlands 6. Sandia National Laboratories, Livermore, CA, 94550-0969, USA 5. Department of Chemistry and Centre for Research in Mass Spectrometry, York University, Toronto, ON, Canada |
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| 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 [CuII(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.  | |
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