Spectral Properties of the Rhodobacter Sphaeroides Mutant Photo-Reaction Center with Double Amino Acid Substitution I(L177)H+H(L173)L

In the reaction center of purple bacterium Rhodobacter sphaeroides histidine L173 serves as the fifth ligand of Mg atom of the bacteriochlorophyll PA. It is known that after substitution of this histidine by leucine the bacteriochlorophyll loses its central Mg atom and turns into bacteriopheophytin. Our data show that double mutation I(L177)H + H(L173)L results in considerable changes of spectral properties of the reaction center: distinct QY absorption band of the dimer bacteriochlorophyll P disappears, and QY bacteriochlorophyll band near 800 nm broadens and shifts by 6 nm to the red. The pigment content of the mutant RC and the value of Em P/P+ remain unaltered indicating secure PA Mg atom coordination. Our data show that among other potential ligands located close enough to the center of the PA macrocycle, histidine L177 seems to be the most probable. According to the obtained results the dislocation of the fifth ligand of the PA Mg atom from L173 to L177 position has little effect on the stability and overall photochemistry of the mutant RC, changing mainly the excitonic interaction between PA and PB, which leads to a considerable blue shift of the P QY band.