- Structure and Reactivity of the Distonic and Aromatic Radical Cations of Tryptophan
- Journal of the American Society for Mass Spectrometry
- Volume | Issue number
- 24 | 4
- Pages (from-to)
- Document type
- Faculty of Science (FNWI)
- Van 't Hoff Institute for Molecular Sciences (HIMS)
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(center dot)) and a canonical aromatic pi (Trp(center dot+)) 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 p-radical cation was produced via CID of the ternary [Cu-II(terpy)(Trp)](center dot 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 pi 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 pi 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 similar to 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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- With erratum: Piatkivskyi, A., Osburn, S., Jaderberg, K., Grzetic, J., Steill, J.D., Oomens, J., Zhao, J., Lau, J.K.C., Verkerk, U.H., Hopkinson, A.C., Siu, K.W.M. & Ryzhov, V. (2013). Erratum to: Structure and reactivity of the distonic and aromatic radical cations of tryptophan. --- Journal of the American Society for Mass Spectrometry, 24 --- (10), 1620.
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