Primary processes in plant photosynthesis: photosystem I reaction center

The photosystem I (PSI) pigment-protein complex of plants converts light energy into a transmembrane charge separation, which ultimately leads to the reduction of carbon dioxide. Recent studies on the dynamics of primary energy transfer, charge separation, and following electron transfer of the reaction center (RC) of the PSI prepared from spinach are reviewed. The main results of femtosecond transient absorption and fluorescence spectroscopies as applied to the P700-enchied PSI RC are summarized. This specially prepared material contains only 12–14 chlorophylls per P700, which is a special pair of chlorophyll a and has a significant role in primary charge separation. The P700-enriched particles are useful to study dynamics of cofactors, since about 100 light-harvesting chlorophylls are associated with wild PSI RC and prevent one from observing the elementary steps of the charge separation. In PSI RC energy and electron transfer were found to be strongly coupled and an ultrafast up-hill energy equilibration and charge separation were observed upon preferential excitation of P700. The secondary electron-transfer dynamics from the reduced primary electron acceptor chlorophyll a to quinone are described. With creating free energy differences (ΔG₀) for the reaction by reconstituting various artificial quinones and quinoids, the rate of electron transfer was measured. Analysis of rates versus ΔG₀ according to the quantum theory of electron transfer gave the reorganization energy, electronic coupling energy and other factors. It was shown that the natural quinones are optimized in the photosynthetic protein complexes. The above results were compared with those of photosynthetic purple bacteria, of which the structure and functions have been studied most.