Protonation equilibria of transition metal complexes: From model systems toward the Mn-complex in photosystem II

Abstract

We report state-of-the-art pKA computations of manganese model systems. The employed methods can be used to perform pKA computations on the oxygen evolving Mn4CaO5 cluster (OEC) in photosystem II (PSII). These computations are very useful for understanding the function of molecular machines and nano-structures considered for artificial photosynthesis. We first summarize the activities going on in natural and artificial photosynthesis. Next, it is outlined how OEC and manganese model structures can vary with different protonation and oxidation states. We also discuss the role of the dielectric constant of proteins in electrostatic energy computations. Then the results obtained so far on pKA computations of the OEC are summarized. Subsequently, we outline the theoretical background behind performing ab initio pKA value computations. In the application part, we first consider pKA computations for the simple hexa-aqua manganese complex. There it is discussed in detail how computed pKA values depend on atomic radii, charges and the electron leakage effect. A presentation of results follows including a critical discussion of the computed pKA values of di-manganese model complexes. Possible reasons for deviations from measured pKA values and techniques for improvement are discussed and summarized in a final section.