Exciton delocalization length in the B850 antenna of Rhodobacter sphaeroides

The properties of elementary excited states in the B850 band of the peripheral light-harvesting antenna (LH2) of the photosynthetic purple bacterium Rhodobacter sphaeroides has been studied at room temperature by means of femtosecond transient absorption experiments combined with computer simulations. Polarized pump-probe kinetics have a fast component of 100 and 65 fs for the anisotropic and isotropic decays, respectively. Direct numerical simulations show that for incoherent hopping-like excitation transfer in the B850 ring of 18 Bchl a molecules at room temperature the fast component of the anisotropy decay is 3 times longer than the corresponding component of the isotropic decay, strongly suggesting that delocalized exciton states are involved in the observed dynamics. To estimate the coherence length of the exciton we have measured absorption difference spectra of LH2 from 810 to 880 nm 2 ps after the excitation into the B800 band with 75 fs laser pulses. Exciton calculations where also monomeric doubly excited states are included give a good fit to the experimental spectra for a coherence length of 4 +/- 2 bacteriochlorophyll monomers. The relatively big error limits are due to the lack of detailed enough information about the doubly excited state of Bchl a.