Theoretical examination of quantum coherence in a photosynthetic system at physiological temperature

Abstract

The observation of long-lived electronic coherence in a photosynthetic pigment-protein complex, the Fenna-Matthews-Olson (FMO) complex, is suggestive that quantum coherence might play a significant role in achieving the remarkable efficiency of photosynthetic electronic energy transfer (EET), although the data were acquired at cryogenic temperature [Engel GS, et al. (2007) Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems. Nature 446:782-786]. In this paper, the spatial and temporal dynamics of EET through the FMO complex at physiological temperature are investigated theoretically. The numerical results reveal that quantum wave-like motion persists for several hundred femtoseconds even at physiological temperature, and suggest that the FMO complex may work as a rectifier for unidirectional energy flow from the peripheral light-harvesting antenna to the reaction center complex by taking advantage of quantum coherence and the energy landscape of pigments tuned by the protein scaffold. A potential role of quantum coherence is to overcome local energetic traps and aid efficient trapping of electronic energy by the pigments facing the reaction center complex.

Fig. 1.

Seven BChl molecules belonging to the monomeric subunit of the FMO complex. The complex is oriented with BChl 1 and 6 toward the baseplate protein whereas BChl 3 and 4 define the target region in contact with the reaction center complex. The spiral strands are α-helices that are part of protein environment.

Fig. 2.

Time evolution of the population of each BChl in the FMO complex. Calculations were done for cryogenic temperature, T = 77 K. The reorganization energy and the phonon relaxation time are set to be λ_j = 35 cm⁻¹ and τ_c = γ_j⁻¹ = 50 fs, respectively.

Fig. 3.

Time evolution of the population of each BChl in the FMO complex. Calculations were done for physiological temperature, T = 300 K. The other parameters are the same as in Fig. 2.

Fig. 4.

Time evolution of the population of each BChl in the FMO complex. Calculations were done for the same parameters as in Fig. 3, except for the phonon relaxation times τ_c = γ_j⁻¹.

Fig. 5.

The energy landscapes along the two primary transfer pathways in the FMO complex: baseplate → BChls 1 → 2 → 3 → 4 (A) and baseplate → BChls 6 → 5, 7, 4 → 3 (B). The relatively strong couplings between BChls are depicted by solid lines.