Crystal structure of plant light‐harvesting complex shows the active,energy‐transmitting state

Plants dissipate excess excitation energy as heat by non‐photochemical quenching (NPQ). NPQ has been thought to resemble in vitro aggregation quenching of the major antenna complex, light harvesting complex of photosystem II (LHC‐II). Both processes are widely believed to involve a conformational change that creates a quenching centre of two neighbouring pigments within the complex. Using recombinant LHC‐II lacking the pigments implicated in quenching, we show that they have no particular role. Single crystals of LHC‐II emit strong, orientation‐dependent fluorescence with an emission maximum at 680 nm. The average lifetime of the main 680 nm crystal emission at 100 K is 1.31 ns, but only 0.39 ns for LHC‐II aggregates under identical conditions. The strong emission and comparatively long fluorescence lifetimes of single LHC‐II crystals indicate that the complex is unquenched, and that therefore the crystal structure shows the active, energy‐transmitting state of LHC‐II. We conclude that quenching of excitation energy in the light‐harvesting antenna is due to the molecular interaction with external pigments in vitro or other pigment–protein complexes such as PsbS in vivo, and does not require a conformational change within the complex.