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

Summary Photosystem I (PSI) converts photons into electrons with a nearly 100% quantum efficiency. Its minimal energy requirement for photochemistry corresponds to 700-nm photon, representing the well-known "red limit" of oxygenic photosynthesis. Recently, some cyanobacteria containing red-shifted pigment chlorophyll f have been shown harvest up 800 nm. To investigate mechanism responsible converting such low-energy photons, we applied steady-state and time-resolved spectroscopies chlorophyll-f-containing PSI chlorophyll-a-only various cyanobacterial strains. Chlorophyll-f-containing displays less optimal energetic connectivity between its pigments. Nonetheless, it consistently traps long-wavelength excitations surprisingly high efficiency, which can only be achieved by lowering required photochemistry, i.e., "breaking red limit". We propose that charge separation occurs via charge-transfer state reconcile this finding available structural data excluding involvement in photochemistry.