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 on PSII functionality following the inclusion of the red shifted Chlorophylls d and f can be obtained by extending the current knowledge on spectroscopic and structural properties of its reaction center (RC). The photo-induced triplet states, that 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 identify an intrinsically low-energy-absorbing chlorophyll participating to charge separation reactions. The triplet- minus -singlet (T–S) spectrum associated to 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.