Equilibrium/nonequilibrium initial configurations in forward/reverse electron transfer within mixed-metal hemoglobin...

Area of Science:

• Biochemistry
• Physical Chemistry
• Protein Dynamics

Background:

• Protein-protein electron transfer (ET) involves complexes in various configurations.
• Charge-separated intermediates exist in nonequilibrium states with maximal ET matrix elements.

Purpose of the Study:

• To investigate the differential viscosity dependence of forward and backward ET rate constants in a protein photocycle.
• To test the hypothesis that conformational changes influence ET reaction dynamics.

Main Methods:

• Studied the photocycle of a [alpha2(Zn),beta2(Fe3+N3-)] mixed-metal hemoglobin hybrid at pH 7.
• Analyzed the viscosity dependence of forward (kf) and backward (kb) electron transfer rate constants.

Main Results:

• Forward ET rate constant (kf) decreased with increasing viscosity.
• Backward ET rate constant (kb) strongly increased with increasing viscosity, contrary to expectations for simple diffusion control.

Conclusions:

• Protein conformation significantly impacts electron transfer efficiency.
• Viscosity differentially affects forward and backward ET, suggesting conformational gating controls the photocycle.
• The findings support a model where conformational dynamics, not just diffusion, govern electron transfer rates.