Hole burning with pressure and electric field: a window on the electronic structure and energy transfer dynamics of bacterial antenna complexes

Light-harvesting complexes of cyclic symmetry from purple bacteria are studied using absorption and high pressure- and Stark-hole burning spectroscopies. The B800→B850 energy transfer rate of (~2 ps)-1 determined by hole burning and pump-probe spectroscopies is weakly dependent on B800-B850 energy gap varied by pressure and temperature. This resilience is explained in terms of a modified Forster theory with the spectral overlap provided by the B800 fluorescence origin band and weak vibronic absorption of B850. Explanations for the additional sub-picosecond relaxation channel of B800 with excitation on the blue side are given. Pressure and temperature dependent studies show that the B800 and B850 bacteriochlorophylls are weakly and strongly excitonically coupled, respectively. The B875 molecules are also strongly coupled. It is concluded that electron-exchange, in addition to electrostatic interactions, is important for understanding the strong coupling and electronic structure of the B850 and B875 rings. The high pressure data obtained for the B800 band, which lead to compressibility values, indicate that the packing of the [alpha] , [beta]-polypeptides in the LH2 complex of Rhodobacter sphaeroides, for which a structure is unknown, is looser than for Rhodopseudomonas acidophila whose LH2 structure has been determined. The looser packing is supported by temperature dependent data obtained for the B850 bands. Temperature dependent spectra establish that the subtle and non-denaturing change in LH2 and LH1 structures near 150 K results in a strengthening of the nearest neighbor couplings of B850 and B875 by ~40% for the low temperature structure. Zero-phonon hole action spectroscopy is used to resolve and characterize the lowest exciton level A (B870) of B850 which is found to lie 200 cm-1 below the strongly allowed E1 level. B870, which is forbidden without energy disorder, carries 3-5% of the total absorption intensity of the B850 ring. A novel theory employing symmetry-adapted basis defect patterns (BDP) for analyzing the effects of energy disorder on the excitonic level structure of cyclic arrays is presented and used to interpret the experimental results on B870. It is shown that the effects of energy disorder on the exciton levels can be largely understood in terms of a single e1 BDP. The BDP approach simplifies computations and provides more physical insight than conventional methods. The dipole moment changes associated with S1(Q y