Dependence of chlorophyll fluorescence quenching on the lipid-to-protein ratio in reconstituted light-harvesting complex II membranes containing lipid labels

The quenching of chlorophyll-a fluorescence was investigated in plant light-harvesting complex II (LHCII) embedded in reconstituted membranes containing thylakoid lipids and a lipid label. The proteoliposomes were further separated by density and the protein and lipid contents of the fractions were quantified spectrophotometrically, allowing tighter control over the L/P ratios in a wide range of values. Using time-resolved fluorescence, we found a strong correlation between the fluorescence quenching and the L/P ratio, in line with other studies reporting progressive quenching at low L/P ratios, presumably triggered by self-clustering in the membrane. The average fluorescence lifetimes decreased to 0.3 ns at L/P ratios below 50:1; these values are comparable to the quenching observed in plants under excess-light conditions and are accompanied by a similar far-red fluorescence signature. It is hypothesized that plants can exploit the intrinsic quenching propensity of LHCII-only membrane domains to safely store extra antenna units.