Energetics of cofactors in photosynthetic complexes: Relationship between protein-cofactor interactions and midpoint potentials

In photosynthetic organisms, solar energy drives electron and proton transfer reactions across cell membranes in order to create energy-rich compounds. These reactions are performed by pigment-protein complexes, including bacterial reaction centers and photosystem II. In this chapter we discuss how electron transfer is determined by the transition energies and oxidation-reduction midpoint potentials of the cofactors and how protein environments can alter the energetics of these cofactors, in particular the primary electron donors, the bacteriochlorophyll dimer of reaction centers and P680 of photosystem II. A Hückel model is presented that provides an accurate description of the electronic structure of the bacteriochlorophyll dimer, including why specific protein interactions, namely, electrostatic and hydrogen bonding interactions, alter not only the oxidation-reduction midpoint potentials but also the electron spin distribution. A special focus is placed on how protein environments can create strong oxidants, including the ability of photosystem II to perform the highly oxidizing reactions needed to oxidize water and the involvement of the Mn₄Ca cluster in this process.