Chlorophyll dichroism of three-dimensional crystals of the light-harvesting chlorophyll a/b-protein complex

AbstractThin three-dimensional crystals of the light-harvesting chlorophyll a/b-protein complex from pea chloroplast membranes were investigated by microspectrophotometry. Measurements of tilted crystals revealed pronounced linear dichroism in a direction perpendicular to the crystal plane but no observable dichroism in the crystal plane. Qy transition moments of chlorophylls were rotationally isotropic with respect to the crystal plane and partially aligned in this plane. Qx transition moments showed no preferential orientation and appeared to be wholly isotropic. The orientation of transition moments in the crystal was consistent with three-fold symmetry of the light-harvesting complex indicated by the crystal structure. The linear dichroism of the crystals reflected the optical properties of the complex in the photosynthetic membrane, indicating a highly symmetrical arrangement of antenna chlorophylls