Energy transfer and fluorescence quenching in the lightharvesting complexes of photosystem II from higher plants

Non-photochemical quenching (NPQ) is the process by which plants exposed to high light conditions dissipate the potentially harmful excess energy as heat. It is thought to involve conformational changes in the light-harvesting complexes of photosystem II (LHCII). The work reported here involves an investigation of LHCII from various perspectives, describing energy transfer between the pigments bound as well as the role of the protein in NPQ. The 510 nm band in the 77K absorption spectra of LHCII trimers belongs to one of the luteins (lutein 2) in each monomer. The red-shift of this band may be caused by specific interaction(s) between the monomers during their association into trimers. The presence of the red-shifted lutein 2 in the unusual Lhcb3-Lhcb5 trimers from antisense Lhcb2 Arabidopsis plants is consistent with this interpretation. This lutein was found to be efficient in transferring energy to chlorophyll a. Analysis of the spectroscopic features of spinach thylakoids before and after de-epoxidation suggests the occurrence of a conformational change in the light-harvesting antenna, resulting in the remaining violaxanthin becoming more strongly involved in energy transfer to the PSII core. The quenching mechanism in LHCII was investigated. LHCII immobilised in a gel matrix showed quenching without protein aggregation, the transition to the quenched state involving a conformational change in which the neoxanthin and lutein 1 domains were affected. By monitoring the twisting of the neoxanthin molecule detected by resonance Raman spectroscopy, the same conformational change that accompanies the formation of the quenched state in vitro was observed in vivo upon NPQ induction. Transient absorption spectroscopy applied to purified LHCII in the quenched state showed the pathway for energy dissipation, involving energy transfer from chlorophyll a to the S1 excited state of lutein 1, which then decays to its ground state, dissipating the energy as heat