Direct Observation and Hole Burning of the Lowest Exciton Level (B870) of the LH2 Antenna Complex of Rhodopseudomonas acidophila (Strain 10050)

Results of 4.2 K absorption and hole-burning experiments on the B850 absorption band of isolated LH2 (B800−850) complexes from Rhodopseudomonas acidophila are presented for samples that exhibited a B850 absorption band of sufficient narrowness (200 cm-1) to allow for direct observation of the lowest exciton level of the B850 ring of dimers as a weak but distinct shoulder at the red edge of the B850 band. This shoulder correlates perfectly with the zero-phonon hole action spectrum of B870 which has been assigned as the lowest exciton level of B850. The action spectrum reveals that the B870 band carries an inhomogeneous width of 120 ± 10 cm-1, is characterized by weak electron−phonon coupling, and carries 3% of the total intensity of the B850 absorption band. The B870 exciton level lies 200 cm-1 below the B850 band maximum. Based on the X-ray structure of LH2 and under the assumption of perfect C9 symmetry (absence of diagonal and/or off-diagonal energy disorder) for the B850 ring, the B850 maximum should b...