Photosynthetic organisms use networks of pigment-protein complexes to harvest solar excitations and funnel them to the reaction center where trapping and charge separation occurs. Nature employs a spatial and energetic funnel to move energy from the most distal, high energy pigments of the light-harvesting antenna to the proximal, low energy pigments of the reaction centers. However, in cyanobacterial Photosystem I (PSI), there are sets of strongly coupled pigments known as the red chlorophyll (Chl) that are lower in energy than even the PSI reaction center. We use power-dependent transient absorption spectroscopy to map the connectivity of PSI complexes in intact thylakoid membranes and interrogate the role of the red Chl within the network of PSI complexes. Using PSI-only thylakoid membranes, we observe power-dependent dynamics at the red Chl pool that we attribute to singlet-singlet annihilation arising from inter-monomer hopping between red Chl pools before trapping by the reaction center. Using a diffusion-limited model, we calculate the exciton hopping time observed between monomers of trimeric PSI. We employ simulations to demonstrate the physical basis of the observed power-dependent dynamics and map the relative location of the red Chl within a PSI monomer.

Fluence-dependent transient absorption spectroscopy reveals long-range hopping between PSI red chlorophyll pools in intact cyanobacterial thylakoids

ACS Spring 2022

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