Photosynthetic reaction centers.

The capture of solar radiation and the conversion of its free energy into chemical energy involves a sequence of reactions that occur within a physical structure called the photosynthetic RC. Following the initial capture of a photon by antenna pigments, the photon is transferred to the RC pigments, where it gives rise to a separation and stabilization of charge across the photosynthetic membrane. Figure 1 depicts this process and illustrates the time scales typically involved. One feature of the photochemistry is that all photosynthetic RCs undergo charge separation with a quantum yield approaching unity, which makes them marvelous molecular machines. The first notions of the operation of a photosynthetic RC originated with the photosynthetic unit experiments of Emerson and Arnold (9), which demonstrated that approximately 2,500 chlorophyll molecules were involved in the release of just one molecule of O2. Thus, a photosynthetic unit contains numerous pigments but the photochemically active chromophores are present in much lower concentration. This pioneering concept led to the distinction of two types of pigments: the light-harvesting, but photochemically inactive, antenna chromophores; and the photochemically active RC pigments. The antenna pigments physiologically increase the absorption cross section of the RC dramatically. Moreover, they ensure that the potentially reactive intermediates containing unpaired electron spins (e.g. semiquinones) generated by single photon photochemistry are efficiently converted by a second photochemical event to products (e.g. hydroquinones) that contain only paired spins. For efficient energy transfer between the antenna and the RC, the RC absorbs at longer wavelengths, effectively forming a trap for excitation energy. Despite these conceptual advances, more than 35 years passed before the first physical isolation of a pigment protein RC complex was reported (17). Since that time, many other RCs have been isolated and characterized biophysically and biochemically. STRUCTURAL AND OPERATIONAL INSIGHTS