A Comparison of the LH2 Antenna Complex of Three Purple Bacteria by Hole Burning and Absorption Spectroscopes

The light harvesting 2 or B800-B850 complexes of Rhodospirullum molishianum, Rhodopseudomonas acidophila (strain 10050) and Rhodobacter sphaeroides are compared on the basis of thermal broadening and shifting of the B800 and B850 absorption bands, the B800→B850 energy transfer time and B870 which is the lowest exciton level of the B850 ring of BChl a dimers. The existence of B870 for Rs. molischianum is established and its B800→B850 energy transfer time, 1.9 ± 0.2 ps at 4.2 K, reported for the first time. The properties of the complexes of Rs. molischianum and Rps. acidophila bear a close resemblance. LH2 of Rb. sphaeroides is distinct, e.g. the thermal shifting of its B850 band is significantly weaker, consistent with weaker excitonic couplings in the B850 ring. For all species, these couplings strengthen upon glass formation near 150 K. Thermal broadening of B850 is interpreted in terms of inter-exciton level relaxation