[Paper Review] Probing the Role of the Eighth Bacteriochlorophyll in holo-FMO Complex by Simulated Two-Dimensional Electronic Spectroscopy

Shu-Hao Yeh, Sabre Kais|arXiv (Cornell University)|Aug 15, 2014

Spectroscopy and Quantum Chemical Studies1 references4 citations

TL;DR

This study uses simulated two-dimensional electronic spectroscopy based on the hierarchical equation of motion (HEOM) to investigate the functional role of the eighth bacteriochlorophyll (BChl 8) in the holo-FMO complex of *Prosthecochloris aestuarii*. The results show that BChl 8 enhances excitonic energy transfer by strengthening coupling in the 8→6→3→1 pathway and increasing wave function overlap between excitons 4 and 5 in the 7→4,5→2→1 pathway, while also slightly reducing dephasing rates, thereby improving energy transfer efficiency and coherence lifetime.

ABSTRACT

The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular wire between the outer antenna system and the reaction center (RC); it is an important model system to study the excitonic energy transfer. Recent crystallographic studies report the existence of an additional (eighth) bacteriochlorophyll a (BChl a). To understand the functionality of this eighth BChl, we simulated the two-dimensional electronic spectra of both the 7-site (apo form) and the 8-site (holo form) variant of the FMO complex from green sulfur bacteria, Prosthecochloris aestuarii. By comparing the difference between the spectrum, it was found that the eighth BChl can affect two different excitonic energy transfer pathways, these being: (1) directly involve in the first pathway 6 $ ightarrow$ 3 $ ightarrow$ 1 of the apo form model by passing the excitonic energy to exciton 6; and (2) increase the excitonic wave function overlap between excitons 4 and 5 in the second pathway (7 $ ightarrow$ 4,5 $ ightarrow$ 2 $ ightarrow$ 1) and thus increase the possible downward sampling routes across the BChls.

Motivation & Objective

• To understand the functional role of the newly identified eighth bacteriochlorophyll (BChl 8) in the FMO complex of *Prosthecochloris aestuarii*.
• To investigate how BChl 8 influences excitonic energy transfer (EET) pathways in the holo-FMO variant compared to the apo-FMO (7-site) form.
• To examine the effects of temperature and static disorder on EET dynamics and 2D electronic spectra using a high-temperature approximation of the HEOM method.
• To determine whether BChl 8 contributes to enhanced quantum coherence and reduced dephasing in the FMO complex.

Proposed method

• The study employs a model Hamiltonian derived from crystallographic data (PDB ID: 3EOJ) for both apo-FMO (7 sites) and holo-FMO (8 sites), incorporating site energies and excitonic couplings from prior theoretical work.
• Two-dimensional electronic spectra are simulated using the high-temperature approximation (HTA) of the hierarchical equation of motion (HEOM), which accounts for non-Markovian dynamics and system-bath interactions.
• The system Hamiltonian includes site energies (εj) and excitonic couplings (Jjk) between bacteriochlorophylls, with BChl 8 assigned the highest site energy and positioned near the baseplate.
• Simulations are performed at 77 K, 125 K, and 150 K to assess temperature dependence of dephasing rates.
• Static disorder is introduced via Gaussian noise (σ = 25 cm⁻¹) on site energies to assess robustness of spectral features.
• Dephasing rates are extracted by fitting oscillatory cross-peak signals (e.g., 2,3 → 1) with multi-exponential functions, excluding data with pulse overlap (T < 96 fs).

Experimental results

Research questions

• RQ1 How does the presence of the eighth bacteriochlorophyll (BChl 8) alter the excitonic energy transfer pathways in the FMO complex?
• RQ2 Does BChl 8 enhance excitonic wave function overlap or coupling strength in specific transfer pathways?
• RQ3 How does temperature affect the dephasing rates of key excitonic transitions (e.g., 3→1) in apo- and holo-FMO?
• RQ4 To what extent does static disorder influence the visibility of cross-peaks involving BChl 8 in 2D electronic spectra?
• RQ5 Does BChl 8 reduce excitonic dephasing, thereby improving energy transfer efficiency and coherence lifetime?

Key findings

• The 8th BChl enhances the 8→6→3→1 energy transfer pathway by increasing the excitonic coupling, despite the low wave function overlap between excitons 3 and 1.
• BChl 8 increases the excitonic wave function overlap between excitons 4 and 5 in the 7→4,5→2→1 pathway, facilitating more efficient downward energy transfer.
• The dephasing rate for the 3→1 transition is reduced in holo-FMO (γ₃₁(T)/T = 0.24 ± 0.03) compared to apo-FMO (γ₃₁(T)/T = 0.31 ± 0.08), indicating reduced energy loss to the environment.
• Dephasing rates increase linearly with temperature in both forms, consistent with experimental observations, though absolute values are smaller than in experiments.
• The cross-peaks (8,6) and (8,3) persist even under 25 cm⁻¹ static disorder, suggesting their presence is robust and not merely an artifact of disorder.
• Oscillatory features in the 2D spectra persist for at least ~600 fs, indicating long-lived quantum coherence in both apo- and holo-FMO complexes.