Abstract
SummaryThe major light-harvesting complex of photosystem II (LHCII) is the main contributor to sunlight energy harvesting in plants. The flexible design of LHCII underlies a photoprotective mechanism whereby this complex switches to a dissipative state in response to high light stress, allowing the rapid dissipation of excess excitation energy (non-photochemical quenching, NPQ). In this work, we locked single LHCII trimers in a quenched conformation after immobilization of the complexes in polyacrylamide gels to impede protein interactions. A comparison of their pigment excited-state dynamics with quenched LHCII aggregates in buffer revealed the presence of a new spectral band at 515 nm arising after chlorophyll excitation. This is suggested to be the signature of a carotenoid excited state, linked to the quenching of chlorophyll singlet excited states. Our data highlight the marked sensitivity of pigment excited-state dynamics in LHCII to structural changes induced by the environment.
Highlights
Photosynthesis relentlessly fuels energy to our biosphere, capturing photons of sunlight and storing their energy into stable chemical bonds
In the spectral region of Qy band of chlorophylls a (Chl-a), the immobilization of light-harvesting complex of photosystem II (LHCII) in gel causes a 2-nm blue shift of the Qy maximum, which peaks at 674 nm in buffer and at 672 nm in gel
It is obvious that maximal loss of absorption is at 680 nm, whereas maximal gain is at 660 nm, some changes in pigment-protein interaction affecting the Chl-a molecules absorbing at these wavelengths are expected
Summary
Photosynthesis relentlessly fuels energy to our biosphere, capturing photons of sunlight and storing their energy into stable chemical bonds. Due to the different dynamical and spectral properties of the obtained signals, various models of quenching mechanisms that include Car participation have been proposed: incoherent slow energy transfer from Chl to Car (Ruban et al, 2007), dissipative excitonic interactions (Bode et al, 2009; Ma et al, 2003; Park et al, 2018), and reductive mechanisms involving electron transfer (Ahn et al, 2008; Avenson et al, 2008; Dall’Osto et al, 2017; Park et al, 2017) The proposal of the former mechanism, first documented in aggregates of major LHCII trimers, has been strengthened by the observation of the same quenching pathway in various systems. Most of these studies provide evidence of the Car S1 state to be the quencher, yet the spectrum of the quencher reported recently for CP29 suggests the involvement of another Car excited state, S* (Mascoli et al, 2019)
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