A COMPARISON OF THE LH2 ANTENNA COMPLEX OF 3 PURPLE BACTERIA BY HOLE-BURNING AND ABSORPTION SPECTROSCOPIES

The Light harvesting 2 or B800-B850 complexes of Rhodospirullum molish ianum, Rhodopseudomonas acidophila(strain 10050) and Rhodobacter sphae roides are compared on the basis of thermal broadening and shifting of the B800 and B850 absorption bands, the B800-->B850 energy transfer t ime and B870 which is the lowest exciton level of the B850 ring of BCh l 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, rep orted for the first time. The properties of the complexes of Rs; molis chiamum and Rps. acidophila bear a close resemblance. LH2 of Rb. sphae roides is distinct, e.g. the thermal shifting of its B850 band is sign ificantly weaker, consistent with weaker excitonic couplings in the B8 50 ring. For all species, these couplings strengthen upon glass format ion near 150 K. Thermal broadening of B850 is interpreted in terms of inter-exciton level relaxation.