Orientation of the bacteriochlorophyll triplet and the primary ubiquinone acceptor of Rhodospirillum rubrum in membrane multilayers determined by ESR spectroscopy (I).

Chromatophores from Rhodospirillum rubrum were oriented as multilayers on quartz slides under reducing conditions. Irradiation of these multilayers in the resonance cavity of an ESR spectrometer at 6 K yielded the spectrum of the bacteriochlorophyll dimer triplet. The relative intesities of the main six lines of the triplet were dependent on the angle subtended by the direction of the external magnetic field with plane of the multilayers. The angular dependence of the intensities of these transitions can best be interpreted in terms of one of the principal axes of the triplet lying along the plane of the membrane while the other two axes are titled 10--20 degrees away from the parallel to and normal to the membrane directions. If we assume the porphyrin planes of the dimer to be parallel and the largest splitting of the triplet transitions to correspond to those transitions in a direction normal to this plane, then these data imply that the dimer planes are nearly perpendicular to the membrane plane. Purified iron-depleted phototrap complexes were similarly oriented in reconstituted phosphatidylcholine multilayers and the angular dependence of the light-induced spectrum recorded at room temperature. A computer analysis of this angular dependence suggests that the plane of the primary ubiquinone acceptor molecule is parallel to the plane of the membrane and therefore perpendicular to the donor.