Excitonic States and Excitation Energy Transfer in Plant Light-Harvesting Complexes in Different Molecular Environments

Abstract

Light-harvesting complex II (LHCII), the main light-harvesting antenna complex of plants, is a functionally flexible and mobile component of the photosynthetic membrane. Excitonic interactions between chlorophylls in the complex are the basis for fast and efficient excitation energy transfer (EET). In addition to its light-harvesting function, LHCII plays regulatory roles, e.g. balancing the energy flow between Photosystem II and Photosystem I (PSI) and activating photoprotective non-photochemical quenching (NPQ). These functions are controlled by the innate structural flexibility of the complex and its intermolecular interactions in the thylakoid membranes. The general aim of this thesis work is to clarify the changes in the molecular and excitonic structure of LHCII that are incurred by interactions with its environment, and how these changes affect EET in the complex and in the thylakoid membrane. Using CD and anisotropic CD (ACD) spectroscopy, we have identified specific changes in the excitonic states in LHCII related to protein-protein interactions in aggregates or induced by detergents and. Time-resolve fluorescence showed that the molecular environment strongly affects the excitation lifetime, hence the light-harvesting function, of LHCII. Due to self-segregation, LHCII formed protein-dense domains in reconstituted membranes wherein fluorescence quenching occurred with a mechanism similar to NPQ in vivo. The dynamics of EET in LHCII was followed by ultrafast two-dimensional electronic spectroscopy (2DES), resolving simultaneous uphill and downhill energy transfer pathways. Exciton equilibration in the Chl a domain was found to occur on timescales up to 5 ps at physiological temperature. EET in plant PSI–LHCI and isolated PSI core complexes were observed for the first time by 2DES. A refined kinetic model was proposed, according to which primary charge separation in PSI occurs after full equilibration of the excitations in the core antenna, with an effective time constant of 3–4 ps. Long-distance EET between LHCIIs and between LHCII and PSI was detected in reconstituted membranes. LHCII acted as efficient antenna of PSI increasing its functional antenna size by up to 50% with minor loss of photochemical efficiency.