Electron transfer in a crystalline cytochrome with four hemes

Area of Science:

• Biophysics
• Biochemistry
• Computational Chemistry

Background:

• Long-range electron transfer (100 μm) observed in small tetraheme cytochrome (STC) crystals.
• Electron transfer rates in STC crystals deviate from semiclassical theories, showing unexpected temperature dependence.

Purpose of the Study:

• Investigate the mechanisms behind observed electron transfer in STC crystals.
• Explain discrepancies between experimental data and semiclassical electron transfer theory.

Main Methods:

• Molecular dynamics simulations of crystalline and monomeric STC.
• Development of new procedures for time-dependent quantum mechanical energy differences and electronic coupling.
• Calculation of electron transfer rate constants using a parameter-free expression.

Main Results:

• Water interactions are the dominant factor in reactant and product state energy gap fluctuations.
• Calculated rate constant for inter-subunit electron transfer at 300 K matches experimental values.
• Calculated activation energy for crystal electron transfer is lower than observed; intra-subunit transfer is slower than in solution.

Conclusions:

• Molecular dynamics simulations provide insights into STC electron transfer mechanisms.
• Water's role in energy gap fluctuations is crucial for understanding electron transfer rates.
• Discrepancies in activation energy suggest further investigation into crystal-specific effects is needed.