Structure-Based Calculations of the Optical Spectra of the LH2 Bacteriochlorophyll-Protein Complex from Rhodopseudomonas acidophila

Abstract— The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein antenna complex from the purple non-sulfur photosynthetic bacterium Rhodopseudomonas acidophila , strain 10050 provides the positions and orientations of the 27 bacteriochlorophyll (BChl) molecules in the complex. Our structure-based model calculations of the distinctive optical properties (absorption, CD, polarization) of LH2 in the near-infrared region use a point-monopole approximation to represent the BChl Q_y transition moment. The results of the calculations support the assignment of the ring of 18 closely coupled BChl to B850 (BChl absorbing at 850 nm) and the larger diameter, parallel ring of 9 weakly coupled BChl to B800. All of the significantly allowed transitions in the near infrared are calculated to be perpendicular to the C9 symmetry axis, in agreement with polarization studies of this membrane-associated complex. To match the absorption maxima of the B800 and B850 components using a relative permittivity (dielectric constant) of 2.1, we assign different site energies (12 500 and 12260 cm⁻¹, respectively) for the Q_y transitions of the respective BChl in their protein binding sites. Excitonic coupling is particularly strong among the set of B850 chromophores, with pairwise interaction energies nearly 300 cm between nearest neighbors, comparable with the experimental absorption bandwidths at room temperature. These strong interactions, for the full set of 18 B850 chromophores, result in an excitonic manifold that is 1200 cm⁻¹ wide. Some of the upper excitonic states should result in weak absorption and perhaps stronger CD features. These predictions from the calculations await experimental verification.