Breaking the Red Limit: Efficient Trapping of Long-Wavelength Excitations in Chlorophyll-f-Containing Photosystem I

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.

Automated summary

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.