Light Harvesting in the Far-Red: LHCII monomers reconstituted in vitro with Chlorophyll d and f altered by removal of specific chlorophylls

The Light-Harvesting Complex of Photosystem II (LHCII) is a membrane protein responsible for the majority of photon capture in higher plants, as well as in many more oxygen-evolving photosynthetic species. LHCII is designed to absorb light in the visible region, being able to access only about 50% of the incident solar light. Increasing the absorption cross-section of plants has been widely proposed as a method to improve photosynthetic yield. This can be achieved by introducing far-red-absorbing chlorophylls, namely chlorophyll d and chlorophyll f, in LHCII. Previous studies have found the incorporation to be successful and functional in light-harvesting, with little to no interference with the excited state decay or energy equilibration processes within the complex. In this thesis, by removing the lowest energy chlorophylls in LHCII monomeric subunit, we obtained information on the occupancy of these sites and we investigated the photoprotective mechanism involving the carotenoids. In fact, during high-light exposure, the amount of energy harvested exceeds the capacity of turnover of the reaction centers. In this case, excited chlorophylls can go through intersystem crossing and form triplet states that can react with molecular oxygen forming radical oxygen species (ROS), which can damage the entire system. To avoid this dangerous effect, photosynthetic systems have developed mechanisms of photoprotection, such as triplet-triplet energy transfer (TTET) with carotenoids in close proximity.