The objective of this research is to elucidate how the structural features of carotenoids determine their ability to carry out the photochemical roles of light-harvesting and photoprotection in vivo. These properties of carotenoids are essential for the survival of photosynthetic organisms. The systems being studied in this investigation are carotenoids in vitro and also contained in several different photosynthetic bacterial reaction center and antenna complexes that have been isolated, purified, pigment-reconstituted, pigment-exchanged, crystallized or genetically altered. X-ray crystallography, nuclear magnetic resonance (NMR) and resonance Raman spectroscopy will be used as direct structural probes. Optical and electron paramagnetic resonance (EPR) spectroscopy will allow the correlation of structure with spectroscopic observables and function. These data will provide a basis from which to understand more complex systems; e.g. higher plant pigment-protein complexes whose properties are more complex than the bacterial systems. The specific aims of this work are: (1) To examine the structures of carotenoids in photosynthetic pigment-protein complexes; (2) To explore the relationships between the carotenoid structures and their spectroscopic properties; (3) To determine the relationships between the carotenoid structures and their mechanisms of light-harvesting and photoprotection.