The Electronic Origin of Far-Red-Light-Driven Oxygenic Photosynthesis.

Photosystem-II uses sunlight to trigger charge separation and catalyze water oxidation. Intrinsic properties of chlorophyll a pigments define a natural "red limit" of photosynthesis at ≈680 nm. Nevertheless, charge separation can be triggered with far-red photons up to 800 nm, without altering the nature of light-harvesting pigments. Here we identify the electronic origin of this remarkable phenomenon using quantum chemical and multiscale simulations on a native Photosystem-II model. We find that the reaction center is preorganized for charge separation in the far-red region by specific chlorophyll-pheophytin pairs, potentially bypassing the light-harvesting apparatus. Charge transfer can occur along two distinct pathways with one and the same pheophytin acceptor (Pheo D1 ). The identity of the donor chlorophyll (Chl D1 or P D1 ) is wavelength-dependent and conformational dynamics broaden the sampling of the far-red region by the two charge-transfer states. The two pathways rationalize spectroscopic observations and underpin designed extensions of the photosynthetically active radiation limit.