Exciton (de)localization in the LH2 antenna of Rhodobacter sphaeroides as revealed by relative difference absorption measurements of the LH2 antenna and the B820 subunit

We report the results of relative difference absorption measurements for the LH2 antenna of Rhodobacter sphaeroides and the B820 subunit of Rhodospirillum I rubrum at room temperature. It is shown that significant differences between shapes and amplitudes of photoinduced, absorption changes reflect a different degree of exciton delocalization in the intact antenna compared with the dimeric subunit, Using the exciton model in the presence of static disorder, we have obtained a consistent and quantitative fit of the amplitudes and shapes of the pump-probe spectra of the LH2 antenna and the B820 subunit. We estimate that the nearest-neighbors interaction energy in the antenna is about 400 cm(-1) and the diagonal disorder is about 450 cm(-1). For these values the coherence length (FWHM) of the steady-state exciton wavepacket corresponds to 5 BChl molecules at room temperature. The amplitude of the difference absorption reflects a cooperative behavior within at least 12 BChls of the B850 antenna. The calculations suggest that the dimeric subunit is characterized by a decrease of the interaction energy to 300 cm(-1) together with an increase of the disorder value to about 600.