Chlorophyll triplet excited states are byproducts of photosynthetic processes that can indirectly harm biological membranes by forming highly reactive oxygen species. A crucial photoprotective mechanism evolved by plants to counter this threat involves the triplet energy transfer from chlorophylls to carotenoid molecules, in which triplet states are not reactive. In the major light-harvesting complex of photosystem II (LHCII), the two central luteins play an important role in the mechanism, but it has been shown that carotenoid triplets are formed even when other carotenoids replace them in their binding sites. In this work, we have investigated carotenoid triplet formation in LHCII isolated from Arabidopsis thaliana npq1lut2 plants, in which violaxanthin replaces lutein. Although transient absorption spectroscopy showed altered singlet excited-state dynamics in the mutant LHCII without lutein, these antennae formed carotenoid triplets that were spectrally and dynamically identical to the wild-type protein. We conclude that lutein-binding sites in LHCII have conserved characteristics to ensure efficient triplet energy transfer to the carotenoid molecules that they accommodate, making the identity of the carotenoid trivial per se.

Institute of Microbiology Academy of Sciences of the Czech Republic Opatovický mlýn 379 81 Třeboň Czech Republic

Institute of Physics Faculty of Science University of South Bohemia Branišovská 1760 370 05 České Budějovice Czech Republic

School of Biological and Chemical Sciences Queen Mary University of London Mile End Road E1 4NS London U K

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Violaxanthin and Zeaxanthin May Replace Lutein at the L1 Site of LHCII, Conserving the Interactions with Surrounding Chlorophylls and the Capability of Triplet-Triplet Energy Transfer