Characterization of the light-harvesting polypeptide/bacteriochlorophyll alpha complex isolated from photosynthetic bacteria by the linear dichroism spectra

The linear dichroism (LD) spectra of the light-harvesting polypeptide (LB)/bacteriochlorophyll (BChl a) complex, isolated from photosynthetic bacteria, Rhodospirillum rubrum, were measured to provide insight into the pigment orientation in the LH/BChl a complex. The LD spectrum for the Qy band of BChl a in the complex in a squeezed polyacrylamide gel was positive, while that for the Qx band was negative, indicating that the dipole moment of the Qy band was membrane-parallel while that of the Qx band was membrane-perpendicular. The anisotropy ratio, the LD/A value for the Qy transition of BChl a in the complex, was 0.18+/-0.02 at a 0.5 compression-ratio, consistent with that for the Qy band of a special pair of BChl a in the reaction center (RC) complex isolated from R. rubrum. This result implies that the orientation of the Qy band in the LH/BChl a complex is perpendicular for the transmembrane axis of the LH in the complex as well as that of the special pair of BChl a in the RC. Furthermore, the LD/A spectrum for the Qy band of BChl a in the LH/BChl a complex indicated that the Qy band was geometrically homogeneous in the direction to the transmembrane axis in the complex.