Identifying The Quencher Of Excited State Energy In Photosynthetic Antennae

Excess energy dissipation in plants in high-light conditions requires the formation of a quenching site. Although several different quenching mechanisms have been proposed, all of them involve pigment-pigment interactions between chromophores coordinated to the antenna complexes of Photosystem II. The best quencher-candidates are Chlorophyll-Chlorophyll and Chlorophyll-carotenoid pairs, likely belonging to the same Lhcb complex, which switches between a light-harvesting and a dissipative state, in this way changing the strength of the interaction. In principle all the antenna complexes can contain a quencher, as suggested by the analysis of Arabidopsis Lhcb-depleted lines, which have shown that none of the Lhcb is per se necessary for NPQ, although the absence of all of them leaves the system unprotected. This suggests that more than one antenna complex can act as a quencher and thus should contain a quenching site. Previous proposals have suggested a role for Chl 612 interacting with site L1 and Chl 603 interacting with site L2, but also other Chls located in the proximity of the carotenoids can be a putative quenching site. The spectroscopic properties of most of the Chls coordinated to several Lhcb complexes, their interactions with neighbouring carotenoids and their effect on the excited state lifetimes of the complexes have been investigated by combining mutation analysis with time-resolved spectroscopy. The experiments have been performed both in solution, where the light-harvesting conformation dominates, and in aggregates, which are widely used to mimic the quenching state in vivo. The experiments indentify several quenching sites in the aggregates.