Absorption and CD spectra of a photosynthetic bacterial antenna complex are calculated on the basis of the crystal structure of the LH2 (B800-850) complex from Rhodopseudomonas acidophila. This complex contains a ring of 18 tightly coupled bacteriochlorophylls (B850) and a ring of 9 more weakly coupled bacteriochlorophylls (B800). Molecular orbitals for bacteriochlorophylls with the three different geometries seen in the crystal structure are obtained by semiempirical quantum mechanical calculations (QCFF/PI). Exciton and charge-transfer interactions are introduced at the level of configuration interactions. Particular attention is paid to the dependence of these interactions on the interatomic distances and on dielectric screening. Absorption band shapes are treated with the aid of vibronic parameters and homogeneous line widths that have been measured by hole burning (Reddy, N. R. S., et al., Photochem. Photobiol. 1993, 57, 35-39). Inhomogeneous broadening due to diagonal disorder in the monomeric and charge-transfer transition energies is included by a Monte Carlo method. The calculations successfully reproduce the main features of measured absorption and CD spectra of the complex. The results support the view that the excited states of the B850 bacteriochlorophylls are extensively delocalized over the ring of pigments while the excited states of the B800 bacteriochlorophylls are much more localized.

The absorption (OD) and circular dichroism (CD) spectra of LH2 complexes from various purple bacteria have been measured and modeled. Based on the lineshapes of the spectra we can sort the LH2 complexes into two distinguishable groups: "acidophila"-like (type 1) and "molischianum"-like (type 2). Starting from the known geometric structures of Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rsp.) molischianum we can model the OD and CD spectra of all species by just slightly varying some key parameters: the interaction strength, the energy difference of alpha- and beta-bound B850 bacteriochlorophylls (BChls), the orientation of the B800 and B850 BChls, and the (in)homogeneous broadening. Although the ring size can vary, the data are consistent with all the LH2 complexes having basically very similar structures.

The photosynthetic bacterium Rhodobacter sphaeroides contains a light-harvesting antenna complex (LH2) with a ring of interacting bacteriochlorophyll molecules (B850). Excitation of membrane-bound LH2 complexes with low-intensity, femtosecond pulses causes changes in absorption and stimulated emission that initially depend on the excitation wavelength but relax to a quasiequilibrium with a time constant of 100 ± 20 fs. Excitation on the blue side of the B850 absorption band is followed by a shift of the signals to longer wavelengths and a decrease in amplitude, whereas the relaxations following excitation on the red side consist mainly of a decrease in amplitude. The signals have an apparent initial anisotropy of approximately 0.5 when the complex is excited with broadband pulses, and 0.35−0.4 with narrower pulses. The anisotropy decays to 0.1 with a time constant of about 30 fs. The anisotropies are similar at wavelengths on either side of the absorption band and are relatively insensitive to the excitation wavelength. Contributions of coherent pump−probe coupling and perturbed free induction decay to the measured anisotropies are considered. Pump−probe coupling could increase the initial anisotropy but cannot account for the decay kinetics. Using a density-matrix formalism, we show that the initial light-induced signals are consistent with coherent excitation of multiple exciton levels in an inhomogeneous ensemble of LH2 complexes and that the main features of the spectral relaxations and the anisotropy can be explained by electronic dephasing and thermal equilibration within the manifold of exciton levels.

The effects of charge-charge interactions on the midpoint reduction potential (E(m)()) of the primary electron donor (P) in the photosynthetic reaction center of Rhodobacter sphaeroides were investigated by introducing mutations of ionizable amino acids at selected sites. The mutations were designed to alter the electrostatic environment of P, a bacteriochlorophyll dimer, without greatly affecting its structure or molecular orbitals. Two arginine residues at homologous positions in the L and M subunits [residues (L135) and (M164)], Asp (L155), Tyr (L164), and Cys (L247) were changed independently. Arginine (L135) was replaced by Lys, Leu, Gln, or Glu; Arg (M164), by Leu or Glu; Asp (L155), by Asn; Tyr (L164), by Phe; and Cys (L247), by Lys or Asp. The R(L135)E/C(L247)K double mutant also was made. The shift in the E(m)() of P/P(+) was measured in each mutant and was compared with the effect predicted by electrostatics calculations using several different computational approaches. A simple distance-dependent dielectric screening factor reproduced the effects remarkably well. By contrast, microscopic methods that considered the reaction field in the protein and solvent but did not include explicit counterions overestimated the changes in the E(m)() considerably. Including counterions for the charged residues reduced the calculated effects of the mutations in molecular dynamics calculations. The results show that electrostatic interactions of P with ionizable amino acid residues are strongly screened, and suggest that counterions make major contributions to this screening. The screening also could reflect penetration of water or other relaxations not taken into account because of incomplete sampling of configurational space.