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Ligand dynamics in a protein internal cavity.
Jan M Kriegl1, Karin Nienhaus, Pengchi Deng
1Department of Biophysics, University of Ulm, D-89069 Ulm, Germany.
Summary
We studied carbon monoxide (CO) rotation within a myoglobin mutant. Increasing temperature alters CO orientation, revealing insights into protein dynamics and internal electric fields.
Area of Science:
- Biophysics
- Protein Dynamics
- Spectroscopy
Background:
- Myoglobin is a protein that binds oxygen.
- Mutant L29W-S108L of myoglobin was engineered for specific cavity properties.
- Carbon monoxide (CO) serves as a probe for studying protein internal environments.
Purpose of the Study:
- To investigate the temperature dependence of Infrared (IR) stretch bands of CO.
- To analyze CO rotation dynamics within the Xe 4 cavity of myoglobin mutant L29W-S108L.
- To quantify the protein's internal electric field and CO orientation potential.
Main Methods:
- Infrared (IR) spectroscopy at cryogenic temperatures.
- Quantitative analysis using a dynamic model for constrained CO rotation.
- Combined classical and quantum-mechanical analysis of spectral data.
Main Results:
- Observed pronounced changes in CO IR band areas and positions with temperature.
- Identified a doublet splitting of the CO stretch band due to a strong local electric field, indicating two preferred CO orientations.
- Quantified the energy barrier (V0 ≈ 2 kJ/mol) and oscillation frequency (ω ≈ 25 cm⁻¹) for CO rotation.
- Estimated the electric field within the protein cavity to be approximately 10 MV/cm.
Conclusions:
- CO rotation in the myoglobin mutant cavity is constrained by a static potential.
- Temperature influences CO orientation, causing spectral peaks to converge.
- The study provides precise measurements of protein internal electric fields and rotational dynamics.