Quantum Coherence in Photosystem II: Environment-Assisted Transport and the Role of Vibronic Coupling in Biological Light-Harvesting

In the paper's words

Review of quantum coherence in PSII light-harvesting, ENAQT, vibronic coupling.

The abstract, verbatim, and its opening sentence is the claim. Every line below is quoted from the text.

Numbers it reports

1. Novoderezhkin et al. [8] employed modified Redfield theory to model exciton dynamics in the full PSII core, predicting coherence lifetimes of 100-200 fs at room temperature, with coherent contributions to the overall transfer efficiency estimated at $10-20\%$.

   from 2.3 Photosystem II Core Complex

2. The spectral density of chlorophyll-$a$ in the $Q_y$ band (650-700 nm) creates significant overlap, making it difficult to isolate individual pigment contributions in 2DES experiments.

   from 2.3 Photosystem II Core Complex

3. Proposed experiment: Measure PSII oxygen evolution as a function of illumination flicker frequency (1 Hz to 1 kHz), comparing wild-type with mutants that alter chromophore site energies.

   from Q1: Is quantum coherence functional or epiphenomenal?

4. The ENAQT paradigm, formalized by Mohseni, Rebentrost, Lloyd, and Aspuru-Guzik (2008) [9], proposes that environmental noise can enhance, rather than degrade, quantum transport efficiency in systems with static disorder.

   from 3.1 Environment-Assisted Quantum Transport (ENAQT)

5. Experimental evidence for ENAQT in photosynthetic systems remains indirect but suggestive: the FMO complex's energy transfer efficiency is robust against mutations that alter pigment site energies [10], consistent with a mechanism that does not require fine-tuning of electronic Hamiltonian parameters.

   from 3.1 Environment-Assisted Quantum Transport (ENAQT)

6. Evidence from mutation studies in FMO [10] and LHCII [13] suggests that coherence lifetimes can be altered without significantly degrading overall efficiency, consistent with an epiphenomenal interpretation.

   from Q1: Is quantum coherence functional or epiphenomenal?

How it is done

1. However, in densely packed antenna complexes where chromophore separations are $\sim 1$ nm or less, electronic couplings can approach $100-500 \text{ cm}^{-1}$, and the Förster picture breaks down.

   from 1. Introduction

2. The Hierarchical Equations of Motion (HEOM) approach [11,12] provides a numerically exact framework for non-Markovian dynamics in systems with arbitrary spectral densities.

   from 3.2 Vibronic Coupling and Non-Markovian Dynamics

3. The modified Redfield approach has been extensively applied to PSII core complex modeling [8], yielding predictions that agree qualitatively with HEOM.

   from 3.3 Modified Redfield and Förster Theory

4. Proposed experiment: Measure PSII oxygen evolution as a function of illumination flicker frequency (1 Hz to 1 kHz), comparing wild-type with mutants that alter chromophore site energies.

   from Q1: Is quantum coherence functional or epiphenomenal?

What would falsify it

1. These results initiated an intense research program investigating whether quantum coherence plays a functional role in photosynthetic efficiency or is merely a spectroscopic artifact observable only under artificial conditions.

   from 1. Introduction

2. (2020) [5], is that both electronic and vibrational coherences contribute, with vibronic (mixed electronic-vibrational) coherence being the dominant observable at room temperature.

   from 2.1 The FMO Complex

3. Falsifiability: This would be disconfirmed if oxygen evolution efficiency shows no differential sensitivity to flicker frequency between wild-type and site-energy mutants.

   from Q1: Is quantum coherence functional or epiphenomenal?

Where the claim stops

1. The discovery of long-lived quantum coherence in photosynthetic light-harvesting complexes—first reported in the Fenna-Matthews-Olson (FMO) complex in 2007—fundamentally challenged the assumption that biological systems operate exclusively in the classical regime at physiological temperatures.

   from Abstract

2. Photosynthesis converts approximately 130 TW of solar energy into chemical free energy annually, making it the dominant energy conversion process on Earth [established].

   from 1. Introduction

3. However, in densely packed antenna complexes where chromophore separations are $\sim 1$ nm or less, electronic couplings can approach $100-500 \text{ cm}^{-1}$, and the Förster picture breaks down.

   from 1. Introduction

Where to read

1. Abstract153 words · 1 min
2. 1. Introduction210 words · 1 min
3. 2. Spectroscopic Evidence309 words · 1 min
4. 3. Theoretical Frameworks274 words · 1 min
5. 4. Photosystem II as a Model System158 words · 1 min
6. 5. Open Questions173 words · 1 min
7. 6. Design Principles for Artificial Light-Harvesting69 words · 1 min