Two-electron transfer reactions in proteins: bridge-mediated and proton-assisted processes

Area of Science:

• Chemical Kinetics
• Quantum Chemistry
• Biophysical Chemistry

Background:

• Two-electron transfer (TET) reactions are crucial in biological and chemical systems.
• Understanding the kinetics of TET in donor-bridge-acceptor (DBA) systems is complex.
• Previous models often simplified the intricate pathways involved.

Purpose of the Study:

• To investigate nonadiabatic TET reactions in DBA systems under fast vibrational relaxation.
• To simplify complex multiexponential kinetics into more manageable models.
• To elucidate the contributions of stepwise and concerted TET mechanisms.

Main Methods:

• Theoretical investigation of nonadiabatic two-electron transfer (TET).
• Analysis within the approximation of fast vibrational relaxation.
• Kinetic modeling of electron transfer pathways.

Main Results:

• Multiexponential TET kinetics reduce to three-state kinetics when bridging states are sparsely populated (<10^-2).
• Under further simplification (small intermediate state population), kinetics approximate single-exponential behavior.
• The overall transfer rate includes both stepwise and concerted TET contributions.
• Concerted TET is governed by a two-electron superexchange coupling involving bridging and intermediate states.

Conclusions:

• The study provides a simplified kinetic framework for analyzing complex TET reactions.
• The findings highlight the importance of concerted TET mechanisms, particularly in biological systems.
• The model successfully explains experimental data for enzyme reduction, suggesting a concerted TET mechanism for proton-assisted reactions.