To distinguish contributions from either of the monomer bacteriochlorophyll cofactors B-L and B-M, we have recorded Raman spectra of reaction centers (RCs) from Rhodobacter sphaeroides, strain R26.1 at low temperature and at resonance with the Q, 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 8 10 nm in the same conditions involved participation from a second, additional cofactor, that we identify with B-M. 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 B-M should strengthen as well, however by about half the enthalpy change of the L-side bond. The absolute wavenumbers, and compared P.+-induced shifts of the stretching modes of the acetyl CO groups of BL and B-M are interpreted as indicating (i) that the acetyl carbonyl of BL is located in a higher permittivity environment than that of B-M and (ii) that the frequency of the acetyl CO vibrator of B-M may accordingly experience a stronger electric field effect from the P.+ radical state than that of B-L. A set of differences observed between BL and B-M spectra specifically concerns their Mg-sensitive modes. These differences are not resulting from a differential field effect, and indicate a sizable difference of conformations between the MgN(4)Nis an element of2(His) groups of BL and B-M. The macroring core sizes of both 131, and 131,1, however, are close to the size of relaxed, five-coordinated BChl a in solution. Finally, comparisons of RR spectra excited at 790, 800, and 810 nm from RCs chemically poised in the P-o or P.+ states revealed a sizable contribution from the neutral state of the primary donor in RR spectra excited at 810 nm only. This confirms that in electronic spectra of reduced RCs the 8 10 nm shoulder should arise in part from the upper excitonic component, Py+, of the primary donor.