Application of computational chemistry to understanding the structure and mechanism of the Mn catalytic site in photosystem II - A review

Applications of Density Functional Theory (DFT) computational techniques to studies of the molecular structure and mechanism of the oxygen evolving, water oxidising Mn₄/Ca catalytic site in Photosystem II are reviewed. We summarise results from the earlier studies (pre 2000) but concentrate mainly on those developments which have occurred since publication of several PS II crystal structures of progressively increasing resolution, starting in 2003. The work of all computational groups actively involved in PS II studies is examined, in the light of direct PS II structural information from X-ray diffraction crystallography and EXAFS on the metals in the catalytic site. We further address the consistency of the various computational models with results from a range of spectroscopic studies on the PS II site, in all of those functionally intermediate states (S-states) amenable to study. Experimental data considered include Mn K-edge XANES studies, hyperfine coupling of Mn nuclei and various ligand nuclei (including those from substrate water) seen by several EPR techniques applied to the net spin half intermediates, S₀ and S₂, at low temperatures. Finally we consider proposed catalytic mechanisms for the O-O bond formation step, from two groups, in the light of the available experimental evidence bearing on this process, which we also summarise.