Interactions between chromophores in reaction centers of purple bacteria. A reinterpretation of the triplet-minus-singlet spectra of Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis

The orientations of the optical transition moments contributing to the Q(Y)-region of the triplet-minus-singlet absorbance-difference spectrum of the reaction center of Rhodobacter sphaeroides R26 have been determined with respect to the triplet axes of the primary donor with linear-dichroic absorbance-detected magnetic resonance, The orientation of the S-1 <-- S-0 transition moment of the reaction center relative to the triplet x- and y-axis was found to be 85 +/- 5 degrees and 18 +/- 3 degrees, respectively. A comparison of the (linear-dichroic) triplet-minus-singlet absorbance-difference spectra is made with those obtained previously for the reaction center of Rhodopseudomonas viridis (E.J. Lous and A.J. Hoff (1987) Proc. Natl. Acad, Sci. USA 84, 6147-6151). For both types of reaction centers the transitions in the absorption region of the accessory bacteriochlorophylls appear to shift to lower energies upon triplet formation on the primary donor. The orientations of the transition moments, however, do not change upon excitation of the primary donor to its triplet state, from which we conclude that the band shifts in the triplet-minus-singlet spectrum cannot be explained by a change of dipolar interactions between the transitions of the dimer and the adjacent bacteriochlorophylls.