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The gas-phase H/D exchange mechanism of protonated amino acids


A mass spectrometry and Density Functional Theory study of gas-phase H/D exchange in protonated Ala, Cys, Ile, Leu, Met, and Val is reported. Site-specific rate constants were determined and results identify the α-amino group as the protonation site. Lack of exchange on the Cys thiol group is explained by the absence of strong intramolecular hydrogen bonding within the reaction complex. In aliphatic amino acids the presence of a methyl group at the β-C atom was found to lower the site-specific H/D exchange rate for amino hydrogens. Study of the exchange mechanism showed that isotopic exchange occurs in two independent reactions: in one, only the carboxylic hydrogen is exchanged and in the other, both carboxylic and amino group hydrogens exchange. The proposed reaction mechanisms, calculated structures of various species, and a number of structural findings are consistent with experimental data.


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Correspondence to Marko Rožman.

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Published online September 29, 2005

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Rožman, M. The gas-phase H/D exchange mechanism of protonated amino acids. J Am Soc Mass Spectrom 16, 1846–1852 (2005).

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  • Isotopic Exchange
  • Aliphatic Side Chain
  • Protonated Peptide
  • Aliphatic Amino Acid
  • Strong Intramolecular Hydrogen Bonding