B800 -> B850 energy transfer mechanism in bacterial LH2 complexes investigated by B800 pigment exchange

Femtosecond transient absorption measurements were performed on native and a series of reconstituted LH2 complexes from Rhodopseudomonas acidophila 10 050 at room temperature. The reconstituted complexes contain chemically mod ified tetrapyrrole pigments in place of the native bacteriochlorophyll a-B8 00 molecules. The spectral characteristics of the modified pigments vary si gnificantly, such that within the B800 binding sites the BB00 Q(y) absorpti on maximum can be shifted incrementally from 800 to 670 nm. As the spectral overlap between the 8800 and B850 Q(y) bands decreases, the rate of energy transfer las determined by the time-dependent bleaching of the B850 absorp tion band) also decreases; the measured time constants range from 0.9 ps (b acteriochlorophyll a in the 8800 sites, Q(y) absorption maximum at 800 nm) to 8.3 ps (chlorophyll a in the B800 sites, Q(y) absorption maximum at 670 nm). This correlation between energy transfer rate and spectral blue-shift of the B800 absorption band is in qualitative agreement with the trend pred icted from Forster spectral overlap calculations, although the experimental ly determined rates are similar to 5 times faster than those predicted by s imulations. This discrepancy is attributed to an underestimation of the ele ctronic coupling between the 8800 and B850 molecules.