Structural Asymmetry of Bacterial Reaction Centers: A Qy Resonant Raman Study of the Monomer Bacteriochlorophylls†

To distinguish contributions from either of the monomer bacteriochlorophyll cofactors BL and BM, we have recorded Raman spectra of reaction centers (RCs) from Rhodobacter sphaeroides, strain R26.1 at low temperature and at resonance with the Qy electronic band at 800 nm. Spectra excited from ferricyanide-treated RCs at 800 nm involved a single BChl species, that we identify with the BL cofactor. Spectra excited at 810 nm in the same conditions involved participation from a second, additional cofactor, that we identify with BM. The present, selective RR data on BL fully confirm an earlier interpretation of difference, Soret resonant Raman spectra (Robert, B.; Lutz, M. Biochemistry 1988, 27, 5108−5114), according to which a H-bond engaged with water by the keto group of BL should significantly strengthen upon formation of the P•+ radical state of the primary donor. The present data also indicate that the equivalent H-bond engaged by the keto group of BM should strengthen as well, however by about half the e...