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 castenholziibinds 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. castenholziiand its carotenoid (Car)-less mutant (m-LH–RC) and found that Car can boost the B800 → B880 EET rate from (2.43 ps)−1to (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.