Insertion of chlorophyll a derivatives into the binding sites of B800 bacteriochlorophyll a in light-harvesting complex 2 from the purple photosynthetic bacterium Rhodoblastus acidophilus

The LH2 protein has two types of bacteriochlorophyll (BChl) a termed B800 and B850 BChl a. Substitution of B800 BChl a with chlorophyll (Chl) a derivatives in this protein has considerable attention from the viewpoints of elucidation of the energy transfer mechanism and developments of artificial photosynthetic devices based on the LH2 structure. Herein we examine reconstitution properties of Chl a and 13²-demethoxycarbonyl Chl a (pyroChl a) into the B800 binding sites in LH2 from the purple photosynthetic bacterium Rhodoblastus (Rbl.) acidophilus. Both the Chl a derivatives were successfully inserted into the B800 binding sites in the LH2 protein whose B800 BChl a was removed (B800-free LH2), and the energy transfer from these derivatives to B850 BChl a was observed in the reconstituted LH2 proteins. Extraction of chlorophyllous pigments from the reconstituted LH2 proteins allows us to estimate the occupancy of Chl a and pyroChl a in the nine B800 binding sites from the molar ratio of (pyro)Chl a to residual B850 BChl a: the occupancy of Chl a was 72% and was 1.2-times larger than that of pyroChl a. These results indicate that Chl a and pyroChl a can be accommodated in the B800 binding sites in the LH2 protein derived from this bacterium, but the affinity of pyroChl a would be slightly smaller than that of Chl a. The binding properties of Chl a derivatives into the B800 binding sites in LH2 will be useful for manipulation of light-harvesting abilities in photosynthetic antenna proteins.