The Role of Xanthophylls in the Mechanism of Nonradiative Energy Dissipation in Photosynthesis
H. A. Frank, P.I.
Supported by the National Science Foundation*
Photosynthetic 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 work, studies of several different xanthophylls in native, reconstituted, and genetically-modified pigment-protein complexes are being carried out. The experiments include steady state and time-resolved absorption and fluorescence spectroscopy and electrochemical determinations using novel methodologies recently developed in our own laboratory. 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 carotenoids in solution and bound in the pigment-protein complexes. The molecules to be studied are lutein, violaxanthin, antheraxanthin, zeaxanthin, diadinoxanthin, diatoxanthin, b-carotene, lycopene, spheroidene and a series of spheroidene analog molecules. These molecules differ systematically in their structures, extents of p-electron conjugation, excited state energies and oxidation potentials. The major objective of this proposal is 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.*This material is based upon work supported by the National Science Foundation under Grant No. 0314380. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.