Through photosynthesis, green plants are able to harvest energy from sunlight and convert it to chemical energy at an energy transfer efficiency rate of approximately 97 percent. If scientists can create artificial versions of photosynthesis, the dream of solar power as the ultimate green and renewable source of electrical energy could be realized. However, a potential pitfall for any sunlight-harvesting system is that if the system becomes overloaded with absorbed solar energy, it most likely will suffer some form of damage. Plants solve this problem on a daily basis with a photo-protective mechanism called energy-quenching. Excess energy, detected by changes in pH levels, is safely dissipated from one molecular system to another, where it can then be routed down relatively harmless chemical reaction pathways.
As Fleming once explained, This defense mechanism is so sensitive to changing light conditions, it will even respond to the passing of clouds overhead.
In 2005, Fleming and his research group identified zeaxanthin, a member of the carotenoid family of pigment molecules, as the safety outlet in the photo-protection of green plants. A plants light harvesting system consists of two protein complexes, Photosystem I and Photosystem II. Each complex features antennae made up of chlorophyll and carotenoid molecules that gain extra excitation energy when they capture photons. Fleming and his group found that intense exposure to light triggers the formation of zeaxanthin molecules in Photosystem II. These zeaxanthin molecules interact with excited chlorophyll molecules and dissipate the excess energy via a charge-transfer mechanism - zeaxanthin gives up an electron to the chlorophyll, bringing the chlorophylls energy back down to its ground state and turning the zeaxanthin into a radical cation which, unlike an excited chlorophyll molecule, is a non-oxidizing agent. However, until now a critical piece to
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory