Energy transfer in the B800 rings of the peripheral bacterial light-harvesting complexes of Rhodopseudomonas acidophila and Rhodospirillum molischianum studied with photon echo techniques

Eenergy transfer in the B800 ring of the LH2 antenna of the purple bacteria Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rs.) molischianum was studied at room temperature using three-pulse echo peak shift (3PEPS) and transient grating (TG) techniques. From the transient grating experiments, we found the B800 → B850 energy transfer rates to be 600−700 fs for both species. The anisotropy of the TG signal decays in about 1 ps for both species, which is ascribed to B800 → B800 energy transfer. The occurrence of B800 → B800 energy transfer was further substantiated by 3PEPS experiments. When measured over the whole B800 band, the initial peak shift of about 30 fs exhibited a fast <100 fs decay to about 10 fs due to the coupling to protein phonons, followed by a slow phase of about 1 ps, during which the peak shift decayed to 1−3 fs. Polarized 3PEPS experiments systematically resulted in smaller peak shift values for the third pulse polarized perpendicular to the first two than for the third pulse par...