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

Abstract

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.