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Hydrogen-atom abstraction from a model amino acid: dependence on the attacking radical

Citation

Amos, RIJ and Chan, B and Easton, CJ and Radom, L, Hydrogen-atom abstraction from a model amino acid: dependence on the attacking radical, The Journal of Physical Chemistry B, 119, (3) pp. 783-788. ISSN 1520-6106 (2015) [Refereed Article]

Copyright Statement

Copyright 2014 American Chemical Society

DOI: doi:10.1021/jp505217q

Abstract

We have used computational chemistry to examine the reactivity of a model amino acid toward hydrogen abstraction by HO•, HOO•, and Br•. The trends in the calculated condensed-phase (acetic acid) free energy barriers are in accord with experimental relative reactivities. Our calculations suggest that HO• is likely to be the abstracting species for reactions with hydrogen peroxide. For HO• abstractions, the barriers decrease as the site of reaction becomes more remote from the electron-withdrawing α-substituents, in accord with a diminishing polar deactivating effect. We find that the transition structures for α- and β-abstractions have additional hydrogen-bonding interactions, which lead to lower gas-phase vibrationless electronic barriers at these positions. Such favorable interactions become less important in a polar solvent such as acetic acid, and this leads to larger calculated barriers when the effect of solvation is taken into account. For Br• abstractions, the α-barrier is the smallest while the β-barrier is the largest, with the barrier gradually becoming smaller further along the side chain. We attribute the low barrier for the α-abstraction in this case to the partial reflection of the thermodynamic effect of the captodatively stabilized α-radical product in the more product-like transition structure, while the trend of decreasing barriers in the order β > γ > δ ∼ ε is explained by the diminishing polar deactivating effect. More generally, the favorable influence of thermodynamic effects on the α-abstraction barrier is found to be smaller when the transition structure for hydrogen abstraction is earlier.

Item Details

Item Type:Refereed Article
Keywords:radical abstraction, amino acids
Research Division:Chemical Sciences
Research Group:Theoretical and Computational Chemistry
Research Field:Quantum Chemistry
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Chemical Sciences
Author:Amos, RIJ (Dr Ruth Amos)
ID Code:103941
Year Published:2015
Web of Science® Times Cited:14
Deposited By:Chemistry
Deposited On:2015-10-29
Last Modified:2017-10-30
Downloads:0

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