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Gas-Phase H/D Exchange of Sodiated Glycine Oligomers with ND_3: Exchange Kinetics Do Not Reflect Parent Ion Structures

Cox, Heather A. and Julian, Ryan R. and Lee, Sang-Won and Beauchamp, J. L. (2004) Gas-Phase H/D Exchange of Sodiated Glycine Oligomers with ND_3: Exchange Kinetics Do Not Reflect Parent Ion Structures. Journal of the American Chemical Society, 126 (20). pp. 6485-6490. ISSN 0002-7863. doi:10.1021/ja049834y.

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H/D exchange is a method commonly used to probe molecular structure. The majority of studies in the gas phase have involved protonated molecular ions. The present study gives attention to molecular ions formed by coordination with a sodium ion. In particular, ND_3 is reacted with sodiated glycine oligomers, Gly_n, where n = 1−5, and the results are interpreted using density functional calculations. Experimentally, Gly1Na^+, Gly_4Na^+, and Gly_5N^a^+ all undergo three fast exchanges with ND_3, while Gly2Na+ and Gly3Na+ undergo one fast and two slow exchanges with ND3. The methyl esters Gly3OMeNa+ and Gly5OMeNa+ do not exchange with ND3. In agreement with earlier experimental studies, theoretical calculations show that the lowest-energy conformers of the sodiated glycine oligomers are charge-solvated structures. Calculations further indicate that, in the process of H/D exchange with ND3, sodiated monoglycine and tetraglycine adopt zwitterionic structures, sodiated diglycine adopts a salt-bridge form, and sodiated triglycine takes on an ion-stabilized ion pair form. Sodiated monoglycine and diglycine exchange via an onium-ion mechanism. The proposed exchange mechanisms require a carboxylic acid hydrogen to complete the exchange, which is in agreement with the experimental results showing that no exchange occurs with methyl ester glycine oligomers. These studies clearly demonstrate that, in the process of H/D exchange, noncovalent complexation of the exchange reagent provides the energy required to access intermediates structurally distinct from the parent ions. H/D exchange is facile for these intermediates. Contrary to the assumption often expressed in earlier studies, H/D exchange kinetics may not directly reflect ion structures.

Item Type:Article
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Julian, Ryan R.0000-0003-1580-8355
Beauchamp, J. L.0000-0001-8839-4822
Additional Information:© 2004 American Chemical Society. Received 10 January 2004. Published online 29 April 2004. Published in print 1 May 2004. We gratefully acknowledge support from the Beckman Institute and computational resources provided by the Materials Simulation Center. This work was supported in part by NSF Grant CHE-9727566.
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Caltech Beckman InstituteUNSPECIFIED
Issue or Number:20
Record Number:CaltechAUTHORS:20170420-115451899
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Official Citation:Gas-Phase H/D Exchange of Sodiated Glycine Oligomers with ND3:  Exchange Kinetics Do Not Reflect Parent Ion Structures Heather A. Cox, Ryan R. Julian, Sang-Won Lee, and J. L. Beauchamp Journal of the American Chemical Society 2004 126 (20), 6485-6490 DOI: 10.1021/ja049834y
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76771
Deposited By: Ruth Sustaita
Deposited On:20 Apr 2017 20:22
Last Modified:15 Nov 2021 17:02

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