An LD-ADMR study on reaction centers of the LH(L131) and LH(M160) hydrogen-bonding mutants of Rhodobacter sphaeroides

Abstract

Reaction centers of the LH(L131) and LH(M160) mutants of Rhodobacter sphaeroides, which have a leucine near the primary donor, a bacteriochlorophyll dimer, changed into a histidine, have been investigated with linear-dichroic absorbance-detected magnetic resonance, and the results are compared with those obtained for native reaction centers. The microwave-induced triplet-minus-singlet absorbance-difference spectra of the three reaction centers upon triplet formation of the primary donor show small differences in band shifts in the Q Y-region of the accessory bacteriochlorophylls. The orientations of the Q Y-transition moments with respect to the dipolar axes of the triplet state of the primary 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. From this we conclude that the electronic composition of the triplet state and the Q Y transitions are basically the same for the mutant and native reaction centers.