In plants, antenna supercomplexes (SCs) play two opposing roles; efficiently transfering absorbed energy to reaction center (photosynthesis) and harmlessly dissipating excessively absorbed light energy as heat (photoprotection).1 The former is to generate sugars and chemical energies for plant survival, instead the latter aims to avoid inevitably generated deleterious oxygen-related byproducts (i.e. reactive oxygen species). The radiation-less relaxation is called non-photochemical quenching (NPQ) and is critical for plant survival and fitness.2 NPQ is predominantly mediated by a rapid response to photon flux density, or energy-dependent quenching (qE).3, 4 qE can regulate reversibly photosynthesis depending on low lumen pH,5 de-epoxidized xanthophylls, e.g. zeaxanthin (Z),6 and the antenna-associated membrane protein PsbS7. All these components of qE are linearly correlated to charge transfer (CT) quenching involving an electron transfer from Z to chlorophyll(s) (Chls) in thylakoid membranes of C-3 plant Arabidopsis thaliana.8-10

Antenna SCs in photosystem II (PSII) are composed of LHCII trimers, major peripheral antenna light-harvesting complexes (LHCs), and minor chlorophyll protein complexes (mCPs, i.e. CP24, CP26, and CP29). All the LHCs contain chlorophylls (Chls) a and b, and carotenoids (Cars), i.e. lutein (L), violaxanthin (V), and neoxanthin (N). In light-adapted plants, specifically V can be converted into Z by an enzyme i.e. violaxanthin de-epoxidase which is activated under low pH. To pinpoint where CT quenching occurs in antenna SCs, we explored antenna LHCs systematically from LHCII trimer to individual LHC monomers; each comparing with Z-bound and V-bound LHCs which are analogs to light-adapted and dark-adapted conditions, respectively. Recently we found CT quenching in all isolated mCPs (CP24, CP26, and CP29),9, 11, 12 while we could not observe any trace of Z•+ in LHCII trimer.9 Furthermore, especially in CP29 we revealed molecular architecture of CT quenching including a Chl dimer (Chls A5 and B5) and Z.11 mCPs are proximately located in the middle between LHCII antenna and D1/D2 core complexes where the reaction centers (RCs) are located, a perfect geometry to regulate the downstream energy flow from antenna LHCs to RCs.

In addition, we suggested ultrafast excitation energy transfer (EET) from mostly light-absorbed LHCII to mCPs (specifically CP29) where CT quenching occurs.13 We postulated a kinetic model of CT quenching and compared the differential transient absorption (TA) traces of thylakoid membranes and of CP29 to get the EET rate.13

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