Modeling of Oligomeric-State Dependent Spectral Heterogeneity in the B875 Light-Harvesting Complex of Rhodobacter sphaeroides by Numerical Simulation

Abstract

A series of detergent-isolated light-harvesting 1 (LH1, B875) complexes from Rhodobacter sphaeroides, estimated to range in size from (αβBChl2)4 to (αβBChl2)13, was used to study the combined effects of spectral disorder and excitonic interactions on oligomeric-state dependent optical properties. Numerical simulations of absorption and fluorescence 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 dependence of the polarization spectra was found to be particularly sensitive to the choice of parameters, and vibronic components were included to obtain satisfactory simulations. Good agreement with most experimental features, including the oligomeric-state dependence of the absorption and emission maxima, was obtained only when the inter- and intradimer coupling strengths for adjacent BChls were similar (200−260 cm-1), and the width for the inhomogeneous distribution function (300−400 cm-1) was comparable. The relevance of these findings to existing controversies on the physical origin of spectral heterogeneity observed for the LH1 complex is discussed.