Modeling of oligomeric-state dependent spectral heterogeneity in the B875 light-harvesting complex of Rhodobacter sphaeroides by numerical simulation

A series of detergent-isolated light-harvesting 1 (LH1, B875) complexes fro m Rhodobacter sphaeroides, estimated to range in size from (alpha beta BChl (2))(4) to (alpha beta BChl(2))(13), was used to study the combined effects of spectral disorder and excitonic interactions on oligomeric-state depend ent optical properties. Numerical simulations of absorption and fluorescenc e emission, excitation, and polarization spectra, based on the structure of the related LH2 complex, were compared to spectra measured experimentally at 77 K (Westerhuis and Niederman, in preparation). The aggregation-state d ependence of the polarization spectra was found to be particularly sensitiv e to the choice of parameters, and vibronic components were included to obt ain satisfactory simulations. Good agreement with most experimental feature s, including the oligomeric-state dependence of the absorption and emission maxima, was obtained only when the inter- and intradimer coupling strength s for adjacent BChls were similar (200-260 cm(-1)), and the width for the i nhomogeneous distribution function (300-400 cm(-1)) was comparable. The rel evance of these findings to existing controversies on the physical origin o f spectral heterogeneity observed for the LH1 complex is discussed.