Abstract:
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.
