Photophysics of photosynthesis. Structure and spectroscopy of reaction centers of purple bacteria

The fundamental photochemical reaction of photosynthetic energy conversion, which transforms the photon energy of light in chemical free energy, takes places in a membrane-bound, pigmented reaction center protein (RC). We examine here the structure-function relationships of this uniquely efficient process. First, the RC is discussed in relation to the overall photosynthetic apparatus. Second, we highlight the X-ray diffraction analysis of two bacterial RCs, discussing the problems associated with the analysis of a non-water-soluble protein of molecular weight over 120 kDa. The structure of the polypeptide chains in the RC-protein, and the configuration of the cofactors non-covalently bound to the polypeptide scaffolding, is reviewed in detail. Third, we present a detailed account of investigations on the functioning of the RC with a number of optical and magnetic resonance spectroscopic techniques, with emphasis on the relation between structure and function. The results for RCs of purple bacteria, green bacteria and the two plant photosystems are compared, and discussed in the framework of current theories of photosynthetic electron transport.