Bandwidth of excitons in LH2 bacterial antenna chromoproteins

The bandwidth of the exciton manifold in LH2 peripheral antenna complexes from the photosynthetic bacterium Rhodobacter sphaeroides has been determined using polarized fluorescence excitation spectroscopy and model simulations. It is shown that the fluorescence anisotropy spectra reveal a hidden structure of the exciton band that is correlated with its boundaries. The estimated exciton coupling energy and exciton bandwidth in the native complex is ∼360 and ∼1620 cm−1, respectively. The corresponding numbers in a mutant with the B800 bacteriochlorophyll molecules absent are somewhat larger (∼420 and ∼1890 cm−1), probably due to tightening of the protein structure.