Chemical probes for water-oxidation: Synthetic manganese complexes in photoactivation of water splitting complex and as exogenons electron donors to photosystem II

Photoactivation of the water splitting enzyme was performed with 13 differe nt synthetic manganese complexes and characterized by oxygen evolution yiel d, thermoluminescence and chlorophyll fluorescence induction kinetics. The efficiency of different compounds in photoactivation correlated with the ra te of linear electron transport in the presence of these compounds. The org anic ligands, associated with the manganese ions, do not prevent the photoa ctivation of the water splitting complex (WOC). Photoactivation with differ ent manganese complexes depended on the number of the Mn-ions in the comple x, their valence state and the nature of their donor atoms. The most effici ent restorations were achieved by using tetrameric complexes having a dimer +dimer structure, complexes containing Mn(H) ions, and having 4-6 oxygen an d 0-2 nitrogen atoms as donor atoms. Further, the effectiveness of photoact ivation depended largely on the structure of the complexes. Our data suppor t the notion that WOC in intact thylakoids. requires the cooperation and we ll determined arrangement of all four manganese ions, and argue against the hypothesis that two manganese ions are sufficient for water splitting. Pho toactivation by some complexes led to anomalous flash-oxygen patterns, whic h are explained by a modified/perturbed water splitting complex.