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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 Mn4CaO5 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 bacteri- ochlorophyll 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 MnO2, αMn2O3, Mn3O4, CaMn2O4, and Mn3(PO4)2 were tested and compared to MnCl2. 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 αMn2O3 and CaMn2O4 and smallest for Mn3O4 and MnO2. The addition of Mn3(PO4)2 resulted in nearly complete P+ reduc- tion, similar to MnCl2. 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. 

Low-temperature persistent and transient hole-burning (HB) spectra are presented for the triple hydrogen-bonded L131LH + M160LH + M197FH mutant of Rhodobacter sphaeroides. These spectra expose the heterogeneous nature of the P-, B-, and H-bands, consistent with a distribution of electron transfer (ET) times and excitation energy transfer (EET) rates. Transient P+Q − holes are observed for A fast (tens of picoseconds or faster) ET times and reveal strong coupling to phonons and marker mode(s), while the persistent holes are bleached in a fraction of reaction centers with long-lived excited states characterized by much weaker electron−phonon coupling. Exposed differences in electron−phonon coupling strength, as well as a different coupling to the marker mode(s), appear to affect the ET times. Both resonantly and nonresonantly burned persistent HB spectra show weak blue- (∼150 cm−1) and large, red-shifted (∼300 cm−1) antiholes of the P band. Slower EET times from the H- and B-bands to the special pair dimer provide new insight on the influence of hydrogen bonds on mutation-induced heterogeneity.