EXCITON COHERENCE AND ENERGY-TRANSFER IN THE LH2 ANTENNA COMPLEX OF RHODOPSEUDOMONAS-ACIDOPHILA AT LOW-TEMPERATURE

Excited-state dynamics and exciton delocalization in the isolated LH2 antenna complex (B800-850) of the photosynthetic bacterium Rhodopseudo monas acidophila were studied at low temperature by means of femtoseco nd transient absorption spectroscopy. By comparing the contribution by stimulated emission to the absorbance difference signal of B850 with the bleaching of the primary donor P of reactions centers of Rhodobact er sphaeroides R26, we found that at 5 K the thermalized B850 transiti on has a 1.85 times larger oscillator strength than P. This correspond s to an oscillator strength equivalent to 3.4 reaction center pigments . With the in vivo extinction coefficients for the reaction center and LH2 from the literature, we arrive at an oscillator strength equivale nt to 2.3 antenna pigments. Comparison of the stimulated emission band of B850 with the optical signal of B800 yields an oscillator strength equivalent with that of 2.7 B800 pigments. Our results strongly suppo rt a model in which the exciton states are partly localized as a resul t of static disorder. Energy-selective steady-state fluorescence measu rements indeed indicate inhomogeneity of the B850 band. Energy transfe r from B800 to B850 takes place with a time constant of 1.8 ps at 10 K .