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

Abstract

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.

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