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 C 9 symmetry (absence of diagonal and/or off-diagonal energy disorder) for the B850 ring, the B850 maximum should be determined by the strongly allowed E1 level of the C 9-array of bacteriochlorophyll a dimers and B870 (the A level) should be adjacent, forbidden in absorption and the lowest energy level. However, the location of the A level 200 cm-1 below the E1 level appears to require a coupling between the nearest bacteriochlorophyll a monomers of adjacent dimers that is larger than the coupling between the monomers of the special pair of the reaction center of Rhodobacter sphaeroides. As a result, it is concluded that diagonal and/or off-diagonal energy disorder within the B850 ring must be taken into account in order to understand B870 and the Q y electronic structure of B850. However, the temperature dependence (4.2−270 K) of the LH2 absorption spectrum reveals that the coupling between neighboring B850 molecules strengthens upon formation of the glycerol:H2O glass near 150 K. This is important for electronic structure calculations that utilize the room temperature structure of LH2 and low-temperature spectroscopic data. Finally, data pertaining to the pure dephasing of the B870 exciton level are presented and interpreted in terms of scattering due to imperfections in the B850 ring.