The Recombination Triplet State in the Far-Red Light Adapted Photosystem II Is Located at the Chl _D1 Site and Resides on the Red-Most Chlorophyll of the Reaction Center

The energetic limits of Photosystem II (PSII) photochemical reactivity required reconsideration after the discovery of far-red light acclimation responses in cyanobacteria. Insights into PSII functionality following the inclusion of the red-shifted Chlorophylls <i>d</i> and <i>f</i> can be obtained by extending the current knowledge on spectroscopic and structural properties of its reaction center (RC). The photoinduced triplet states, which represent selective endogenous probes, were therefore investigated in far-red adapted PSII by magnetic resonance techniques. Zero-field splitting tensor analysis combined with spin-polarization dynamics arising from radical pair recombination unambiguously identifies an intrinsically low-energy-absorbing chlorophyll participating in charge separation reactions. The triplet-<i>minus</i>-singlet (T-S) spectrum associated with the recombination triplet state, obtained by microwave selection, showed a sharp 725 nm bleaching demonstrating the dominant involvement of this red-shifted chlorophyll in the lowest RC exciton. Moreover, spectral simulations provided strong evidence in favor of its localization at the Chl_D1 position, making it the most likely site of primary photochemistry.