Unexpected substituent effects in offset pi-pi stacked interactions in water
Mark J Rashkin1, Marcey L Waters
1Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA.
Journal of the American Chemical Society
|February 28, 2002
Summary
This study used dynamic NMR to measure rotational barriers in substituted pyridinium bromides. Electronegative meta-substituents significantly increased rotational barriers, indicating stronger offset-stacking interactions in proteins.
Area of Science:
- Organic Chemistry
- Biophysical Chemistry
- Chemical Physics
Background:
- Offset-stacking interactions are crucial for protein structure and function.
- Understanding these non-covalent interactions requires robust model systems.
- Pyridinium salts offer a tunable platform for studying aromatic interactions.
Purpose of the Study:
- To quantify the impact of meta- and para-substituents on rotational barriers in N-benzyl-2-(2-fluorophenyl)pyridinium bromides.
- To model and understand offset-stacking interactions between aromatic rings in a solution-phase system.
- To correlate substituent effects with the strength of these stacking interactions.
Main Methods:
- Dynamic Nuclear Magnetic Resonance (NMR) spectroscopy was employed to measure rotational barriers.
- Synthesis of meta- and para-substituted N-benzyl-2-(2-fluorophenyl)pyridinium bromides.
- Analysis of spectral changes to determine energy differences between ground and transition states.
Main Results:
- A small change (0.1 kcal/mol) in rotational barrier was observed for para-substituted compounds.
- Electronegative meta-substituents, such as trifluoromethyl (CF3), led to a significant increase in rotational barrier (up to 0.66 kcal/mol).
- The observed effects suggest a combination of stacking and electrostatic interactions.
Conclusions:
- The rotational barrier in this model system effectively reflects the strength of offset-stacking interactions.
- Electronegative meta-substituents substantially enhance these interactions, likely through electrostatic contributions.
- This research provides valuable insights into the forces governing aromatic interactions in biological systems, particularly in proteins.
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