Structural information on the light-harvesting complex II of green plants that can be deciphered from polarized absorption characteristics

The atomic model of light-harvesting complex II of green plants (LHCII) reveals a densely packed arrangement of 12 chlorophylls and two carotenoids. At the current resolution of 3.4 Angstrom chlorophylls can only be modeled as ''naked'' tetrapyrrole rings. Consequently, definitive assignments of the identities of the chlorophylls (chlorophyll a or chlorophyll b) and the directions of the transition dipole moments are obstructed. These uncertainties lead to a large number of possible configurations, and a detailed understanding of the structure-function relationship is obscured. It is demonstrated that a large reduction in the number of possible configurations and a considerable amount of additional structural information can be obtained by deciphering global features of the polarized absorption spectra within the context of exciton calculations. It is shown that only a limited number of configurations are able to explain the global features of the linear and circular dichroism spectra of LHCII. Assuming that the preliminary assignment of the identities of the 12 chlorophylls by Kuhlbrandt and co-workers is correct, it is possible to deduce the most likely orientations for most of the chlorophylls. The information presented in this study on the most likely orientations will be important for a detailed understanding of the relation between the structure and spectroscopy.