Single-Molecule Identification of Quenched and Unquenched States of LHCII.

In photosynthetic light harvesting, absorbed sunlight is converted to electron flow with near-unity quantum efficiency under low light conditions. Under high light conditions, plants avoid damage to their molecular machinery by activating a set of photoprotective mechanisms to harmlessly dissipate excess energy as heat. To investigate these mechanisms, we study the primary antenna complex in green plants, light-harvesting complex II (LHCII), at the single-complex level. We use a single-molecule technique, the Anti-Brownian Electrokinetic trap, which enables simultaneous measurements of fluorescence intensity, lifetime, and spectra in solution. With this approach, including the first measurements of fluorescence lifetime on single LHCII complexes, we access the intrinsic conformational dynamics. In addition to an unquenched state, we identify two partially quenched states of LHCII. Our results suggest that there are at least two distinct quenching sites with different molecular compositions, meaning multiple dissipative pathways in LHCII. Furthermore, one of the quenched conformations significantly increases in relative population under environmental conditions mimicking high light.