Long-range electron transfer in biomolecules. Tunneling or hopping?

Area of Science:

• Biophysics
• Biochemistry
• Physical Chemistry

Background:

• Electron transfer (ET) in biomolecules involves direct tunneling (D → A) or multistep hopping (D → X → A) via intermediate sites.
• The dominant mechanism is dictated by the spatial arrangement and electronic properties of donor (D) and acceptor (A) redox centers.

Purpose of the Study:

• To elucidate the conditions determining whether direct tunneling or hopping dominates long-range ET in biomolecules.
• To provide a framework for predicting ET mechanisms based on energy gaps, driving force, and distances.

Main Methods:

• Theoretical analysis of thermal and photoinduced electron transfer mechanisms.
• Formulation of crossover barrier equations (E(C)) to distinguish between tunneling and hopping.

Main Results:

• For thermal ET, hopping dominates when the energy gap (E) is less than the crossover barrier (E(C) = (ΔG/2) + (3/4)k(B)TβR(DA)).
• Single-step tunneling prevails when E > E(C). In proteins, tunneling is favored for typical cofactor distances (<20 Å) and residue energy gaps (>0.5 eV).
• In the activationless regime of photoinduced ET, the crossover barrier is E(C) = (2λkTβR(DA))(1/2) - λ.

Conclusions:

• The interplay between energy gap (E), driving force (ΔG), and distance (R(DA)) determines the dominant ET mechanism.
• The derived expressions for the threshold barrier can predict ET pathways in biological and artificial redox systems.