Exciton Transfer Simulations in a Light-Harvesting 2 Complex Reveal the Transient Delocalization Mechanism.

Area of Science:

• Photosynthesis research
• Quantum biology
• Computational chemistry

Background:

• The efficiency of exciton transfer in light-harvesting (LH) complexes is crucial for photosynthesis.
• Debate exists regarding the role of electronic coherences versus other mechanisms in this efficiency.
• The LH2 complex from Rhodospirillum molischianum serves as a model system.

Purpose of the Study:

• To investigate the mechanism behind the high efficiency of exciton transfer in the LH2 complex.
• To simulate exciton dynamics using a novel computational approach.
• To elucidate the role of electronic properties and delocalization in energy transfer.

Main Methods:

• Development of a neural network model for pigments within the LH2 complex.
• Nonadiabatic molecular dynamics (NAMD) simulations employing coupled quantum mechanical/molecular mechanics (QM/MM) embedding.
• Analysis of hundreds of picosecond-long simulation trajectories to calculate exciton occupations.

Main Results:

• Simulated exciton transitions within the B800 and B850 rings align with experimental data.
• Identified incoherent hopping in the B800 ring and more delocalized transfer in the B850 ring.
• Reorganization energies and excitonic couplings were found to be comparable, supporting the 'transient delocalization' model.

Conclusions:

• The 'transient delocalization' model, involving occasional large delocalization events of localized excitons, explains efficient B850 exciton transport.
• The unique electronic properties of bacteriochlorophyll, specifically minimal reorganization energies, are key to this efficiency.
• This study provides a deeper understanding of exciton dynamics in light-harvesting complexes.