Journal
CHEM
Volume 7, Issue 1, Pages 155-173Publisher
CELL PRESS
DOI: 10.1016/j.chempr.2020.10.024
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Funding
- Netherlands Organization for Scientific Research (NWO) via a TOP grant [714.018.001]
- U.S. National Science Foundation [MCB-1613022]
- Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center - DOE, Office of Science, Office of Basic Energy Sciences [DE-SC 0001035]
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Chlorophyll f helps cyanobacteria absorb low-energy photons, but its energetic connectivity with other pigments is not optimal. Despite this, chlorophyll f still consistently traps long-wavelength excitations with high efficiency, which may be achieved by lowering the energy required for photochemistry.
Photosystem I (PSI) converts photons into electrons with a nearly 100% quantum efficiency. Its minimal energy requirement for photochemistry corresponds to a 700-nm photon, representing the well-known red limit of oxygenic photosynthesis. Recently, some cyanobacteria containing the red-shifted pigment chlorophyll f have been shown to harvest photons up to 800 nm. To investigate the mechanism responsible for converting such low-energy photons, we applied steady-state and time-resolved spectroscopies to the chlorophyll-f-containing PSI and chlorophyll-a-only PSI of various cyanobacterial strains. Chlorophyll-f-containing PSI displays a less optimal energetic connectivity between its pigments. Nonetheless, it consistently traps long-wavelength excitations with a surprisingly high efficiency, which can only be achieved by lowering the energy required for photochemistry, i.e., by breaking the red limit We propose that charge separation occurs via a low-energy charge-transfer state to reconcile this finding with the available structural data excluding the involvement of chlorophyll f in photochemistry.
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