Biophys J, May 2000, p. 2590-2596, Vol. 78, No. 5

B800B850 Energy Transfer Mechanism in Bacterial LH2 Complexes Investigated by B800 Pigment Exchange

J. L. Herek,^* N. J. Fraser, T. Pullerits,^* P. Martinsson,^* T. Polívka,^* H. Scheer, R. J. Cogdell, and V. Sundström^*

 ^*Department of Chemical Physics, Lund University, S-22100 Lund, Sweden;  Division of Biochemistry and Molecular Biology, IBLS, University of Glasgow, Glasgow G12 8QQ, United Kingdom; and  Botanisches Institut der Universität München, D-80638 München, Germany

Femtosecond transient absorption measurements were performed on native and a series of reconstituted LH2 complexes from Rhodopseudomonas acidophila 10050 at room temperature. The reconstituted complexes contain chemically modified tetrapyrrole pigments in place of the native bacteriochlorophyll a-B800 molecules. The spectral characteristics of the modified pigments vary significantly, such that within the B800 binding sites the B800 Q_y absorption maximum can be shifted incrementally from 800 to 670 nm. As the spectral overlap between the B800 and B850 Q_y bands decreases, the rate of energy transfer (as determined by the time-dependent bleaching of the B850 absorption band) also decreases; the measured time constants range from 0.9 ps (bacteriochlorophyll a in the B800 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 predicted from Förster spectral overlap calculations, although the experimentally determined rates are ~5 times faster than those predicted by simulations. This discrepancy is attributed to an underestimation of the electronic coupling between the B800 and B850 molecules.

Biophys J, May 2000, p. 2590-2596, Vol. 78, No. 5
© 2000 by the Biophysical Society   0006-3495/00/05/2590/07  $2.00

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