Nature of Disorder and Inter-Complex Energy Transfer in LH2 at Room Temperature:  A Three Pulse Photon Echo Peak Shift Study

Three Pulse Photon Echo Peak Shift data are reported for the B850 band of the peripheral light-harvesting complex (LH2) of Rb. sphaeroides at room temperature. The data are obtained for both the detergent solubilized and purified complex, and the native membrane samples containing LH2 as the sole bacteriochlorophyll-protein complex. Both samples reveal an ultrafast decay on a 100−200 fs time scale that is attributed to exciton relaxation dynamics within individual aggregates. The samples in native membranes also contain a ∼5 ps decay component which is assigned to inter-complex, LH2−LH2 energy transfer. The observation of the 5 ps component in the membrane samples strongly suggests the presence of two levels of energetic disorder in the system.