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 photosystemII 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 770nm. 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 nmis 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.
