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Question 1. Marcus theory in photosynthesis. (a) Shown on the next page are the lifetimes (the inverse of the reaction rates) for the first three electron transfer processes that occur in the photosynthetic reaction centers of purple bacteria. The numbers next to the arrows pointed to the right indicate charge transfer reactions that lead to charge separation across the membrane and successful conversion of the photon into a charge separated state. Arrows pointed to the left indicate charge recombination rates. Dashed lines on the vertical axis indicate the energies of the excited state and the three charge-separated states compared to the ground state. These latter rate processes “waste” the solar energy and just produce heat. Use electron transfer theory arguments to rationalize why all of the back reactions are 102 to 104 slower than the forward reactions. It is known that the distance and “tunneling barriers” for the forward and reverse reactions are not very different. (b) Use the energies displayed in the plot to make a rough estimate of the reorganization energy for the bacterial reaction center protein. You can use the data from the “primary” charge separation and recombination rates (1012 s-1 and 108 s-1, respectively) to do this. (c) The energy diagram indicates that the absorbed photon energy is about 1.4 eV, but the voltage stored in the charge-transfer state is only about 0.5 V. Nature thus dissipates over 60% of the photon’s energy in accomplishing the transmembrane electron transfer. What performance benefit does the organism derive from dissipating this much energy? Recall the Marcus theory reorganization energy: Gact = (G + )2/4