Dissipation Pathways in a Photosynthetic Complex

Area of Science:

• * Molecular biophysics and quantum dynamics.
• * Photosynthesis and light-harvesting complexes.
• * Computational chemistry and condensed matter physics.

Background:

• * Understanding molecular energy flow is key to chemical reactions, material properties, and photosynthesis.
• * Elucidating specific molecular pathways for energy transfer in complex systems is challenging.
• * The Fenna-Matthews-Olson (FMO) complex mediates energy transfer from light-harvesting chlorosomes to the photosynthetic reaction center in green sulfur bacteria.

Purpose of the Study:

• * To investigate how photon excitation energy is dissipated within the FMO complex.
• * To isolate the contributions of protein and pigment vibrational modes to energy dynamics.
• * To identify specific vibrational modes responsible for energy dissipation pathways.

Main Methods:

• * Developed an efficient computational implementation of a theory for dissipation pathways in open quantum systems.
• * Utilized second-order perturbation theory in electronic couplings.
• * Employed a state-of-the-art FMO model with structured, chromophore-specific spectral densities.

Main Results:

• * Energy dissipation is dominated by low-frequency vibrational modes (<800 cm^-1) near resonance with pigment electronic state energy gaps.
• * In-plane breathing modes (∼200 cm^-1) of bacteriochlorophylls are identified as crucial for dissipation.
• * Higher-frequency intramolecular vibrations (>800 cm^-1) do not contribute to dissipation.
• * The FMO complex transiently borrows energy from the environment to dissipate excess photonic energy.

Conclusions:

• * Low-frequency vibrational modes play a critical role in directing energy dissipation in the FMO complex.
• * Energy transfer dynamics involve a complex exchange with the thermal environment, not strictly unidirectional.
• * Findings can inform the design of artificial light-harvesting devices and chemical/quantum control systems.