Carotenoid-Mediated Long-Range Energy Transfer in the Light Harvesting–Reaction Center Complex from Photosynthetic Bacterium Roseiflexus castenholzii

The light harvesting–reaction center complex (LH–RC) of Roseiflexus castenholzii binds bacteriochlorophylls a (BChls a), B800 and B880, absorbing around 800 and 880 nm, respectively. We comparatively investigated the interband excitation energy transfer (EET) dynamics of the wild-type LH–RC (wt-LH–RC) of Rfl. castenholzii and its carotenoid (Car)-less mutant (m-LH–RC) and found that Car can boost the B800 → B880 EET rate from (2.43 ps)−1 to (1.75 ps)−1, accounting for 38% acceleration of the EET process. Interestingly, photoexcitation of wt-LH–RC at 800 nm induced pronounced excitation dynamics of Car despite the insufficient photon energy for direct Car excitation, a phenomenon which is attributed to the BChl–Car exciplex 1[B800(↑↑)···Car(↓↓)]*. Such an exciplex is suggested to play an essential role in promoting the B800 → B880 EET process, as corroborated by the recently reported cryo-EM structures of wt-LH–RC and m-LH–RC. The mechanism of Car-mediated EET will be helpful to deepen the understanding of the role of Car in bacterial photosynthesis.