Journal-Discussion- BioPhysics Activity

This study explores how quantum coherence among pigment molecules in purple bacteria enhances light-harvesting efficiency.

Photosynthesis converts light energy into chemical energy, sustaining life on Earth.
It involves capturing solar energy and transforming it into a proton gradient across cellular membranes, driving biological processes.
Evolution has led to diverse photosynthetic systems from a common ancestor, employing universal physical principles.

Light harvesting begins with photon absorption by chlorophyll or carotenoid pigments embedded in proteins, forming light-harvesting complexes (LHCs).
The important step is converting electronic excitation energy into a charge-separated state in the reaction center (RC).
Emerson and Arnold (1932) showed that hundreds of pigments cooperate in light harvesting, a concept that intrigues biophysicists.
Pigment cooperation displays a hierarchical pattern, exhibiting both strong and weak electronic interactions crucial for efficient harvesting.

Coherent Spread: Within strongly interacting pigment groups, electronic excitation spreads coherently.
Incoherent Spread: Between weakly coupled pigments, excitation share occurs incoherently through random transfer, termed Förster resonant energy transfer (FRET).
Exciton dynamics combine both coherent and incoherent transfer mechanisms, influencing efficiency.
An intermediate coupling regime can yield some coherent spread, still a subject of investigation.

Quantum coherence significantly contributes to efficient light harvesting, based on experimental results that show oscillations of exciton states.
Speculations about how much initial quantum state coherence affects harvesting efficiency persist.
The coherence arises from strong coupling among chlorophyll molecules in close proximity in the photosynthetic apparatus.

The RC converts short-lived excitation into a sustainable charge gradient.
It consists of four bacteriochlorophyll pigments and two bacteriopheophytins that collaboratively convert absorbed light into charge separation.
Following photon absorption, excitation energy transfers through various pigments towards the reaction center, sustaining charge-separated states.

BChls serve as conduits, funneling energy to the RC through high FRET rates, crucial for sustaining energy supply.
Electron transfer to BChls poses a risk of intercepting trips to charge state Q2 - + SP + . Fortunately, its range is less compared to the FRET mechanism.
This implies a corridor for energy transfer where BChls prevent loss of excitation energy and maintain efficient FRET.
FRET employs the overlap of emission and absorption spectra of donor and acceptor molecules.

The LH1 complex contains 32 bacteriochlorophylls close enough for optimal energy transfer but distanced from SP to avoid electron tunneling loss.
Strong electronic interactions exist among neighboring BChls, allowing for coherent excitation sharing despite high temperatures.
Energy transfer rates from LH1 BChls to SP BChls show significant improvements due to quantum coherence, enabling an effective light-harvesting design.

In low-light habitats, purple bacteria developed LH2 complexes to improve photon absorption by extending exciton dynamics.
LH2 consists of smaller, homologous proteins around LH1−RC, forming two distinct BChl rings, enhancing excitation feed to the RC.
Coupled exciton mechanisms in LH2 facilitate quick excitation transfer, enhancing the overall light-harvesting efficiency.

Purple bacteria have about 100 light-harvesting proteins assembled into chromatophores, consisting of numerous LH2 and LH1−RC complexes.
The chromatophore’s architecture promotes electrical potential generation through multiple processes: absorption, excitation migration, electron transfer, fluorescence, and internal conversion.
Quantum coherence increases FRET rates, improving the chances of excitation reaching the RC and corresponding overall efficiency gains.

Quantum coherence plays a pivotal role in enhancing light-harvesting efficiency in purple bacteria.
It achieves this by modifying energy levels and dropping resonance between pigment clusters, contributing significantly to the overall biochemical energy conversion process.

Corresponding Author: K. Schulten (kschulte@ks.uiuc.edu)
J. Strümpfer, M. Şener, and K. Schulten are affiliated with the Center for Biophysics and Computational Biology and the Department of Physics at the University of Illinois at Urbana-Champaign.