Functional domain size in aggregates of light-harvesting complex II and thylakoid membranes

The functional domain size for efficient excited singlet state quenching was studied in artificial aggregates of the main light-harvesting complex II (LHCIIb) from spinach and in native thylakoid membranes by picosecond time-resolved fluorescence spectroscopy and quantum yield measurements. The domain size was estimated from the efficiency of added exogenous singlet excitation quenchers—phenyl-p-benzoquinone (PPQ) and dinitrobenzene (DNB). The mean fluorescence lifetimes τav were quantified for a range of quencher concentrations. Applying the Stern–Volmer formalism, apparent quenching rate constants kq were determined from the dependencies on quencher concentration of the ratio τ0av/τav, where τ0av is the average fluorescence lifetime of the sample without addition of an exogenous quencher. The functional domain size was gathered from the ratio kq′/kq, i.e., the apparent quenching rate constants determined in aggregates (or membranes), kq′, and in detergent-solubilised LHCII trimers, kq, respectively. In LHCII macroaggregates, the resulting values for the domain size were 15–30 LHCII trimers. In native thylakoid membranes the domain size was equivalent to 12–24 LHCII trimers, corresponding to 500–1000 chlorophylls. Virtually the same results were obtained when membranes were suspended in buffers promoting either membrane stacking or destacking. These domain sizes are orders of magnitude smaller than the number of physically connected pigment–protein complexes. Therefore our results imply that the physical size of an antenna system beyond the numbers of a functional domain size has little or no effect on improving the light-harvesting efficiency.

► The functional domain size of photosynthetic pigment–protein assemblies was measured. ► The domain size in LHCII aggregates comprised 15–30 complexes. ► The domain size in native thylakoid membranes comprised 12–24 complexes. ► The functional domain sizes were much smaller than the physical sizes. ► Light-harvesting efficiency cannot be enhanced by simply enlarging the antenna.