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Published on: April 24, 2014
Histidine-containing radicals in the gas phase
Frantisek Turecek1, Chunxiang Yao, Y M Eva Fung
1Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, Washington 98195, USA. turecek@chem.washington.edu
The Journal of Physical Chemistry. B
|April 25, 2009
Summary
Gas-phase histidine radicals were generated and studied. Nalpha-histidylglycine radicals showed unusual stability due to an exothermic isomerization involving proton transfer and a unique salt-bridge intermediate.
Area of Science:
- Physical Chemistry
- Chemical Physics
- Computational Chemistry
Background:
- Understanding the stability and reactivity of radical species is crucial in various chemical and biological processes.
- Histidine radicals, important in biological systems, have been challenging to study in isolation due to their transient nature.
- Previous studies often focused on solution-phase or indirect methods, limiting detailed mechanistic insights.
Purpose of the Study:
- To generate and characterize histidine-containing radicals in the gas phase.
- To investigate the dissociation pathways and stability of these radicals using advanced experimental and computational techniques.
- To elucidate the structural and energetic factors governing radical stability, particularly for Nalpha-histidylglycine.
Main Methods:
- Gas-phase generation of histidine radicals via femtosecond electron transfer and collisional electron transfer.
- Mass spectrometry techniques (reionization to cations and conversion to anions) to probe radical dissociation and stability.
- High-level computational chemistry (DFT/B3-PMP2) and Rice-Ramsperger-Kassel-Marcus (RRKM) theory for structural and energetic analysis.
Main Results:
- Radicals derived from histidine-N-methylamide and Nalpha-acetylhistidine-N-methylamide dissociated rapidly, primarily via side-chain fragmentation or C(alpha)CO bond cleavage.
- Nalpha-glycylhistidine and Nalpha-histidylglycine radicals exhibited backbone NCalpha bond dissociation, with Nalpha-histidylglycine radicals showing significant stability.
- Computational analysis revealed an exothermic isomerization in the Nalpha-histidylglycine radical, catalyzed by proton transfer, forming a stable cation radical-COO salt-bridge intermediate.
Conclusions:
- The stability of Nalpha-histidylglycine radicals is attributed to an internal, exothermic isomerization facilitated by the carboxyl group and steric accessibility.
- This isomerization involves a novel cation radical-COO salt-bridge, offering new insights into radical stabilization mechanisms.
- The study provides a detailed understanding of gas-phase histidine radical chemistry, bridging experimental observation with theoretical prediction.
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