Congratulations to Alessandro Agostini on his new paper! Check it out.
Photosynthetic eukaryotic algae can harvest far-red light by tuning chlorophyll a (Chl a) absorption, though the underlying mechanism has remained unclear. In this study, optically detected magnetic resonance (ODMR) combined with pulsed EPR spectroscopy was used to probe red-adapted light-harvesting complexes (rVCP) from the freshwater alga Trachydiscus minutus. This combined spectroscopic approach pinpointed the pigments responsible for far-red light absorption to an excitonic cluster of Chl a molecules located near one of the two central carotenoids in the core of the antenna. Our combined spectroscopic and phylogenetic approach revealed that the red shift originates from a histidine-to-asparagine substitution at the magnesium-ligation site and a small residue at the i-4 position of a chlorophyll in this cluster, forming an [A/G]xxxN chlorophyll a binding motif. This work demonstrates how carotenoid and chlorophyll triplet states provide direct insight into pigment arrangement and electronic interactions and can be used as internal probes to elucidate the molecular basis of spectral tuning.
