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, ND3 is reacted with sodiated glycine oligomers, Glyn, where n = 1-5, and the results are interpreted using density functional calculations. Experimentally, Gly1Na +, Gly4Na+, and Gly5Na+ all undergo three fast exchanges with ND3, while Gly 2Na+ and Gly3Na+ undergo one fast and two slow exchanges with ND3. The methyl esters Gly 3OMeNa+ 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.
| Original language | English |
|---|---|
| Pages (from-to) | 6485-6490 |
| Number of pages | 6 |
| Journal | Journal of the American Chemical Society |
| Volume | 126 |
| Issue number | 20 |
| DOIs | |
| Publication status | Published - 2004 May 26 |
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Gas-phase H/D exchange of solidated glycine oligomers with ND3 : Exchange kinetics do not reflect parent ion structures. / Cox, Heather A.; Julian, Ryan R.; Lee, Sang-Won; Beauchamp, J. L.
In: Journal of the American Chemical Society, Vol. 126, No. 20, 26.05.2004, p. 6485-6490.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Gas-phase H/D exchange of solidated glycine oligomers with ND3
T2 - Exchange kinetics do not reflect parent ion structures
AU - Cox, Heather A.
AU - Julian, Ryan R.
AU - Lee, Sang-Won
AU - Beauchamp, J. L.
PY - 2004/5/26
Y1 - 2004/5/26
N2 - 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, ND3 is reacted with sodiated glycine oligomers, Glyn, where n = 1-5, and the results are interpreted using density functional calculations. Experimentally, Gly1Na +, Gly4Na+, and Gly5Na+ all undergo three fast exchanges with ND3, while Gly 2Na+ and Gly3Na+ undergo one fast and two slow exchanges with ND3. The methyl esters Gly 3OMeNa+ 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.
AB - 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, ND3 is reacted with sodiated glycine oligomers, Glyn, where n = 1-5, and the results are interpreted using density functional calculations. Experimentally, Gly1Na +, Gly4Na+, and Gly5Na+ all undergo three fast exchanges with ND3, while Gly 2Na+ and Gly3Na+ undergo one fast and two slow exchanges with ND3. The methyl esters Gly 3OMeNa+ 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.
UR - http://www.scopus.com/inward/record.url?scp=2442637587&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=2442637587&partnerID=8YFLogxK
U2 - 10.1021/ja049834y
DO - 10.1021/ja049834y
M3 - Article
C2 - 15149245
AN - SCOPUS:2442637587
VL - 126
SP - 6485
EP - 6490
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 20
ER -