Tunneling in ligand binding to heme proteins

Area of Science:

• Biophysics
• Physical Chemistry
• Molecular Biology

Background:

• Hemoglobin's function involves reversible binding of ligands like carbon monoxide.
• Understanding ligand rebinding kinetics is crucial for hemoglobin function studies.

Purpose of the Study:

• To investigate the mechanism of carbon monoxide rebinding to the beta chain of hemoglobin after photodissociation.
• To elucidate the transition from classical to quantum mechanical behavior in this process.

Main Methods:

• Photodissociation of carbon monoxide from hemoglobin using a laser flash.
• Spectroscopic analysis to monitor rebinding kinetics at various temperatures.
• Analysis of temperature dependence to differentiate between classical and quantum mechanisms.

Main Results:

• Intramolecular rebinding of carbon monoxide to hemoglobin's beta chain observed below 200 K.
• Classical over-the-barrier motion dominates rebinding above 25 K.
• Quantum mechanical tunneling becomes the dominant mechanism below 25 K.
• A consistent energy spectrum with a peak at 4.0 kJ/mol describes both mechanisms.
• Barrier width is dependent on barrier height, following d(E) ≈ 0.05 nm X (E/Epeak)^1.5.

Conclusions:

• The study reveals a transition in the carbon monoxide rebinding mechanism in hemoglobin's beta chain from classical to quantum tunneling.
• The energy landscape governing this process is characterized by a specific energy spectrum and a barrier width dependent on barrier height.