AN LD-ADMR STUDY ON REACTION CENTERS OF THE LH(L131) AND LH(M160) HYDROGEN-BONDING MUTANTS OF RHODOBACTER-SPHAEROIDES

Reaction centers of the LH(L131) and LH(M160) mutants of Rhodobacter s phaeroides, which have a leucine near the primary donor, a bacteriochl orophyll dimer, changed into a histidine, have been investigated with linear-dichroic absorbance-detected magnetic resonance, and the result s are compared with those obtained for native reaction centers. The mi crowave-induced triplet-minus-singlet absorbance-difference spectra of the three reaction centers upon triplet formation of the primary dono r show small differences in band shifts in the Q(Y)-region of the acce ssory bacteriochlorophylls. The orientations of the Q(Y)-transition mo ments with respect to the dipolar axes of the triplet state of the pri mary donor were found to be equal for all reaction centers, indicating that the orientation of the triplet axes with respect to the reaction center coordinate frame is the same for all three reaction centers. F rom this we conclude that the electronic composition of the triplet st ate and the Q(Y) transitions are basically the same for the mutant and native reaction centers.