Exciton Delocalization Promotes Far-Red Absorption in a Tetrameric Chlorophyll <i>a</i> Light-Harvesting Complex from <i>Trachydiscus minutus</i>

Photosynthetic organisms employ light-harvesting complexes (LHCs) to optimize energy capture under variable light conditions. The freshwater eustigmatophyte <i>Trachydiscus minutus</i> accumulates a red-shifted violaxanthin-chlorophyll protein (rVCP) that contributes to far-red light harvesting using only chlorophyll (Chl) <i>a</i> molecules, without chemical modification or substitution of pigments. Based on high-resolution cryo-EM and multiscale quantum chemical calculations, we uncovered a heterodimer-based tetrameric architecture, representing a unique oligomerization mode among LHCs. Within each heterodimer, Chls <i>a</i> are distinctively arranged adjacent to the terminal emitter, forming an unprecedentedly extended chlorophyll cluster. Quantum chemical calculations reveal three strong exciton-coupled pigment domains, two of which reside in the large cluster and solely account for the intense far-red absorption near 700 nm without contributions from charge-transfer states. Our structural and quantum chemical characterizations of far-red light harvesting reveal a molecular mechanism of red spectral tuning that relies on protein-controlled excitonic coupling of identical Chl <i>a</i> pigments, as demonstrated here in this eustigmatophyte, highlighting diverse adaptations for harvesting spectrally shifted, low-energy light.