Bound manganese oxides capable of reducing the bacteriochlorophyll dimer of modified reaction centers from Rhodobacter sphaeroides

A biohybrid model system is described that interfaces synthetic Mn-oxides with bacterial reaction centers to gain knowledge concerning redox reactions by metal clusters in proteins, in particular the Mn 4 CaO 5 cluster of photosystem II. The ability of Mn-oxides to bind to modified bacterial reaction centers and transfer an electron to the light-induced oxidized bacteriochlorophyll dimer, P + , was characterized using optical spectroscopy. The environment of P was altered to obtain a high P/P + midpoint potential. In addition, different metal-binding sites were introduced by substitution of amino acid residues as well as extension of the C-terminus of the M subunit with the C-terminal region of the D1 subunit of photosystem II. The Mn-compounds MnO 2 , αMn 2 O 3 , Mn 3 O 4 , CaMn 2 O 4 , and Mn 3 (PO 4 ) 2 were tested and compared to MnCl 2 . In general, addition of the Mn-compounds resulted in a decrease in the amount of P + while the reduced quinone was still present, demonstrating that the Mn-compounds can serve as secondary electron donors. The extent of P + reduction for the Mn-oxides was largest for αMn 2 O 3 and CaMn 2 O 4 and smallest for Mn 3 O 4 and MnO 2 . The addition of Mn 3 (PO 4 ) 2 resulted in nearly complete P + reduction, similar to MnCl 2 . Overall, the activity was correlated with the initial oxidation state of the Mn-compound. Transient optical measurements showed a fast kinetic component, assigned to reduction of P + by the Mn-oxide, in addition to a slow component due to charge recombination. The results support the conjecture that the incorporation of Mn-oxides by ancient anoxygenic phototrophs was a step in the evolution of oxygenic photosynthesis.