The mechanisms of photoprotection in natural and artificial photsynthetic systems

Future energy systems are likely to utilise solar energy. Plants, algae and cyanobacteria convert solar energy to a chemical form and store it as energy rich compounds, primarily carbohydrates, by a process known as photosynthesis. In photosynthetic syste ms, sunlight is captured by light-harvesting (or antenna) complexes, whose main function is to hand over, with as little waste as possible, their energy to a reaction centre (the part where this energy is converted, through a long chain of events, into ch emical energy). When it is carrying out these chemical reactions, a reaction centre is said to be closed (since it cannot accept energy from a light-harvesting complex). This means that an antenna which cannot deliver its energy to a closed reaction centr e must somehow dissipate its energy through a channel that is not harmful to the organism. Carotenoids are involved both in efficient utilisation of sunlight and in playing a protective role; however, the carotenoids in the reaction centre of the photosys tem II of higher plants are unable to play this role, and it has been proposed that alpha-tocopherol (vitamin E) provides photoprotection in this case. One of the principal objects of the project is to measure the efficiencies of the processes through whi ch a carotenoid accepts and donates energy to other pigments, and to explore how a light-harvesting complex dissipates its excess excitation energy. To this end, various natural and artificial antenna systems will be investigated; attention will be focuss ed on the possibility that a carotenoid can act not only as a donor of electronic energy to chlorophyll pigments, but also as a quencher of chlorophyll, and the proposal, based on this result, that carotenoids take part in dissipating excess excitation in an antenna. Another principal objective is to investigate secondary electron transport in the D proteins of the reaction centre of photosystem II.