Mimicking Photosynthetic Electron Transfer
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MIMICKING PHOTOSYNTHETIC ELECTRON TRANSFER DEVENS GUST, THOMAS A. MOORE, AND ANA L. MOORE Department of Chemistry and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona, 85287, USA.
ABSTRACT The photosynthetic reaction centers of plants and bacteria are photovoltaic devices on the molecular scale which convert light energy into chemical potential energy in the form of longlived, energetic charge separated states. It is now possible to prepare synthetic multicomponent molecules which mimic important aspects of this process. For example, one of the keys to reaction center function is a multistep electron transfer strategy. In this paper, two general types of multistep electron transfer, sequential and parallel, are described and illustrated with several synthetic triad and pentad molecules.
INTRODUCTION In photosynthetic organisms, the conversion of light energy into useful potential energy takes place in a structure known as the reaction center. The reaction center is actually a photovoltaic device which operates at the molecular level. It uses the energy of a photon to transfer an electron across the thickness of a lipid bilayer membrane and generate an energetic, long-lived charge separated state. The potential energy of this state is then exploited by the organism in a number of ways. Photosynthesis is an extremely successful solar energy harvesting process, and as a result the design, synthesis and study of artificial photosynthetic systems which mimic some aspects of the natural process is an active field of research. The knowledge gleaned from such studies can not only tell us more about how natural photosynthesis works, but also contribute to the design of man-made solar energy conversion systems and molecular electronic devices. Mimicry of the photosynthetic reaction center would at first appear to be a formidable task, as the typical reaction center from a photosynthetic bacterium contains thousands of atoms. However, most of the mass of the reaction center is associated with protein material, whereas the basic photochemistry is carried out by a few relatively small organic cofactors. A major role of the protein is to hold the organic cofactors in the spatial arrangement and environment necessary for photosynthetic electron and energy transfer. One approach to artificial photosynthesis is to use synthetic organic pigments, electron donors, and acceptors similar to those found in natural reaction Mat. Res. Soc. Symp. Proc. Vol. 218. ©1991 Materials Research Society
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centers, but to replace the structural role of the protein with covalent chemical linkages. This is the approach which will be discussed below.
NATURAL PHOTOSYNTHETIC ELECTRON TRANSFER The organic cofactors found in the reaction centers of photosynthetic bacteria include two bacteriochlorophyll molecules in a "special pair," two accessory bacteriochlorophylls, two bacteriopheophytins (bacteriochlorophylls in which the central magnesium atom is replaced by hydrogens), two quinone molecules, an
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