Disentangling the low-energy states of the major light-harvesting complex of plants and their role in photoprotection

https://doi.org/10.1016/j.bbabio.2014.02.014Get rights and content
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Highlights

  • Single-molecule study on LHCII trimers

  • Contribution of low-energy states (red states) to photoprotection

  • Emission at 700 nm is enhanced under qE conditions but unrelated to quenching.

  • Single-molecule results are explained in the context of numerous ensemble results.

Abstract

The ability to dissipate large fractions of their absorbed light energy as heat is a vital photoprotective function of the peripheral light-harvesting pigment–protein complexes in photosystem II of plants. The major component of this process, known as qE, is characterised by the appearance of low-energy (red-shifted) absorption and fluorescence bands. Although the appearance of these red states has been established, the molecular mechanism, their site and particularly their involvement in qE are strongly debated. Here, room-temperature single-molecule fluorescence spectroscopy was used to study the red emission states of the major plant light-harvesting complex (LHCII) in different environments, in particular conditions mimicking qE. It was found that most states correspond to peak emission at around 700 nm and are unrelated to energy dissipative states, though their frequency of occurrence increased under conditions that mimicked qE. Longer-wavelength emission appeared to be directly related to energy dissipative states, in particular emission beyond 770 nm. The ensemble average of the red emission bands shares many properties with those obtained from previous bulk in vitro and in vivo studies. We propose the existence of at least three excitation energy dissipating mechanisms in LHCII, each of which is associated with a different spectral signature and whose contribution to qE is determined by environmental control of protein conformational disorder. Emission at 700 nm is attributed to a conformational change in the Lut 2 domain, which is facilitated by the conformational change associated with the primary quenching mechanism involving Lut 1.

Abbreviations

CT
charge transfer
F680
fluorescence near 680 nm
F700
fluorescence near 700 nm
SM
single molecule

Keywords

NPQ
Photoprotection
Photosystem II
Light-harvesting complex
Single-molecule spectroscopy (SMS)
Protein dynamics

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This article is part of a Special Issue entitled: “18th European Bioenergetic Conference”.