The Role of Xanthophylls in the Mechanism of Nonradiative Energy Dissipation in Photosynthesis

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Harry A. Frank9816759Photosynthetic organisms contain protective mechanisms by which excess light energy is dissipated before it leads to the photodestruction of the photosynthetic apparatus. Also, energy flow from the light-harvesting pigment-protein complexes to the photosynthetic reaction center is highly regulated. The molecular features that control these processes are not well understood. Xanthophyll pigments have been implicated in the mechanisms, but the precise nature of their involvement is unclear. In this project, several different xanthophylls will be studied. The experiments to be performed include steady state and time-resolved absorption and fluorescence spectroscopy and electrochemical determinations. The spectroscopic experiments will measure the efficiencies and dynamics of energy transfer between the xanthophylls and chlorophyll. The electrochemical experiments will reveal the oxidation potentials of the molecules in solution and bound in the pigment-protein complexes. The major objective of this research to examine each of the molecular factors thought to be important in nonradiative energy dissipation in order to reveal the detailed molecular mechanism by which it takes place in vivo. The factors to be explored include xanthophyll composition, state of aggregation, pH, phosphorylation, position of the energy levels, and the oxidation potentials of the pigments. The question of the involvement of the xanthophylls as direct or indirect quenchers of chlorophyll fluorescence will be examined. The hypotheses that xanthophylls quench chlorophyll fluorescence by energy transfer or electron transfer mechanisms will be tested. The experiments are important in enlarging our view of how photosynthetic organisms protect themselves from excessive light energy absorption and respond to varying environmental conditions while maintaining efficient energy flow essential for survival.Xanthophylls are familiar to anyone who has observed the bright orange and yellow colors of Autumn leaves in New England. However, unbeknownst to most observers, these eye-catching photosynthetic pigments do more than provide aesthetic beauty to our lives. They are critical components in a biological protection system that prevents chlorophyll, the most essential plant pigment, from breaking down under high light stress. In a way that is not yet fully understood, xanthophylls are able to be converted from one form into another and back again in the so-called "Xanthophyll cycle." The molecular components of this cycle appear to be capable of harmlessly deactivating excess excited state energy before it catalyzes the destruction of chlorophyll. Interestingly, the same components may also regulate the flow of absorbed solar energy between photosynthetic proteins. This research will investigate precisely how the protection and flow regulation are accomplished. Protein preparations from several higher plant and algal systems will be systematically investigated. A combination of biochemical, molecular biological, and spectroscopic tools will be used. The overall goal of the research is to elucidate the molecular details of how the Xanthophyll cycle works. A better understanding of how plants convert solar energy into chemical energy and withstand stress under bright solar conditions is expected to be obtained from this work.