Fluorescence Polarization and Low-temperature Absorption Spectroscopy of a Subunit Form of Light-harvesting Complex I from Purple Photosynthetic Bacteria

Measurements of polarized fluorescence and CD were made on light-harvesting complex 1 and a subunit form of this complex from Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodobacter capsulatus. The subunit form of LH1, characterized by a near-infrared absorbance band at approximately 820 nm, was obtained by titration of carotenoid-depleted LH1 complexes with the detergent n-octyl beta-D-glucopyranoside as reported by Miller et al. (1987) [Miller J. F., Hinchigeri, S. B., Parkes-Loach, P. S., Callahan, P. M., Sprinkle, J. R., & Loach, P. A. (1987) Biochemistry 26, 5055-5062]. Fluorescence polarization and CD measurements at 77 K suggest that this subunit form must consist of an interacting bacteriochlorophyll a dimer in all three bacterial species. A small, local decrease in the polarization of the fluorescence is observed upon excitation at the blue side of the absorption band of the B820 subunit. This decrease is ascribed to the presence of a high-energy exciton component, perpendicular to the main low-energy exciton component. From the extent of the depolarization, we estimate the oscillator strength of the high-energy component to be at most 3% of the main absorption band. The optical properties of B820 are best explained by a Bchl a dimer that has a parallel or antiparallel configuration with an angle between the Qy transition dipoles not larger than 33 degrees. The importance of this structure is emphasized by the results showing that core antennas from three different purple bacteria have a similar structure.(ABSTRACT TRUNCATED AT 250 WORDS)