Ultrafast carotenoid to pheophorbide energy transfer in a biomimetic model for antenna function in photosynthesis

Carotenoids serve as light-harvesting pigments and as photoprotective agents in photosynthetic organisms^1-9. Their role as antenna pigments involves absorption of photons in the blue-green spectral region followed by highly efficient singlet-singlet energy transfer to a neighbouring chlorophyll. The dependence of both the rate and mechanism of energy transfer on carotenoid-chlorophyll distance and orientation is Unknown. Here, we have directly measured both the rate and efficiency of singlet energy transfer from a carotenoid covalently linked to pyropheophorbide a (PPheo a) in two model compounds, using picosecond transient absorption spectroscopy. In one model the π systems of the carotenoid and PPsheo a possess a maximum edge-to-edge distance of 5 Å, while in the other model this distance is only 2Å. Energy transfer occurs from the carotenoid to PPheo a at the 2-Å distance with a rate constant of 7 ± 2 × 10¹⁰ s^-1 and 53±5% efficiency, while energy transfer at the 5-Å distance occurs at a rate constant of <3 ×10⁹ s^-1 and with <5% efficiency. These results provide evidence that short distances and strong electronic interactions between carotenoids and chlorophylls are necessary to achieve the high energy transfer efficiencies observed in vivo.