Correlation of Car S1 → Chl with Chl → Car S1 Energy Transfer Supports the Excitonic Model in Quenched Light Harvesting Complex II

Recently, excitonic carotenoid-chlorophyll interactions have been proposed as a simple but effective model for the down-regulation of photosynthesis in plants. The model was proposed on the basis of quenching-correlated electronic carotenoid-chlorophyll interactions (Car S-1 -> Chl) determined by Car S-1 two-photon excitation and red-shifted absorption bands. However, if excitonic interactions are indeed responsible for this effect, a simultaneous correlation of quenching with increased energy transfer in the opposite direction, Chl Q(y) -> Car S-1, should be observed. Here we present a systematic study on the correlation of Car S-1 -> Chl and Chl -> Car S-1 energy transfer with the occurrence of red-shifted bands and quenching in isolated LHCII. We found a direct correlation between all four phenomena, supporting our conclusion that excitonic Car S-1-Chl interactions provide low-lying states serving as energy traps and dissipative valves for excess excitation energy.