Photoinduced Electron Transfer in Bis-Porphyrin-Stoppered [2]-Rotaxanes

Abstract

Natural photosynthesis represents the very essence of life since it is the process by which green plants convert light energy into usable chemical energy. The so-called photosynthetic bacteria are also able to perform the same process. In fact, these microorganisms are the simplest natural photosynthetic systems and the mechanism of conversion of photonic energy into chemical energy is now quite well understood. This process takes place in the photosynthetic membrane [1]. As shown schematically in Figure 1, essentially three protein complexes work together: the Reaction Center (RC), a cytochrome b/c1 complex and a cytochrome c2. An overview of their function is as follows: shortly after light excitation of the RC, a hydroquinone (in fact a reduced ubiquinone which has picked up its two protons from the cytoplasma) dissociates from the RC and migrates to the cytochrome b/c1 complex, where it delivers its two protons to the periplasma and two electrons which are reinjected into the RC via a cytochrome c2 shuttle. The released quinone can now reintegrate the ubiquinone pool and feed back the RC. In short, light excitation of the RC results in a cyclic flow of electrons coupled to the development of a proton gradient across the photosynthetic membrane. This proton gradient is chemical energy which is used by the enzyme ATP-synthase in the synthesis of ATP from ADP and inorganic phosphate.