Resonant emission from the B870 exciton state and electron–phonon coupling in the LH2 antenna chromoprotein

Fluorescence of the peripheral LH2 antenna complex from the photosynthetic bacterium Rhodobacter sphaeroides has been studied at 5 K upon selective excitation into the B870 exciton absorption band. Heterogeneous nature of the LH2 emission band has been confirmed by a double spectral selection technique simultaneously utilizing hole burning and fluorescence line narrowing spectroscopy. The phonon structure of the spectra related to the B870 band has been analyzed resulting in a genuine single-exciton state absorption/emission profile. The total electron–phonon coupling strength S=0.8±0.2, larger than previously assumed, has been determined.