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

Abstract

<h2>Summary</h2> 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 <i>f</i> 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-<i>f</i>-containing PSI and chlorophyll-<i>a</i>-only PSI of various cyanobacterial strains. Chlorophyll-<i>f</i>-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 <i>f</i> in photochemistry.