Photosynthesis: from the antenna to the reaction center

From the antenna to the reaction center: downhill energy transfer in the photosynthetic apparatus of Chlorobium tepidum.

The photosynthetic apparatus of the green sulfur bacteria (chlorobium tepdidum) spurred a long lasting discussion about quantum coherence in biology, mostly focused on its subunit, the Fenna Matthews Olson complex. An account of the discovery of the FMO Protein is given by Olson in Photosynth. Res 80, 2004.

Recent experiments by Dostál, Pšenčík, and Zigmantas (Nat. Chem 8, 2016) show measured time and frequency resolved 2d-spectra of the whole photosynthetic apparatus. These results allow one to trace the energy flow from the antenna down to the reaction center and relate it to theoretical models.

Computed two-dimensional spectrum of the antenna and FMO complex of C. tepidum. “A” denotes the location of the antenna peak, 1-7 the FMO complex states. Within tens of picoseconds, the energy is shuffled from the antenna towards the FMO complex.

In our article [Kramer & Rodriguez  Sci. Reports 7, 2017] , open access] we provide a model of the experimental results using the open quantum system dynamics code described previously. In addition we show how the different pathways in 2D spectroscopy (ground state bleaching, stimulated emission, and excited state absorption) affect the spectra and lead to shifts of “blobs” down from the diagonal places. This allows us to infer the effective coupling of the antenna part to the FMO complex and to assess the relative orientations of the different units. The comparison of theory and experimental result is an good test of our current understanding of the physical processes at work.

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