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.

Mutations were made in four residues near the bacteriochlorophyll cofactors of the photosynthetic reaction center from Rhodobacter sphaeroides. These mutations, L131 Leu to His and M160 Leu to His, near the dimer bacteriochlorophylls, and M203 Gly to Asp and L177 Ile to Asp, near the monomer bacteriochlorophylls, were designed to result in the placement of a hydrogen bond donor group near the ring V keto carbonyl of each bacteriochlorophyll. Perturbations of the electronic structures of the bacteriochlorophylls in the mutants are indicated by additional resolved transitions in the bacteriochlorophyll absorption bands in steady-state low-temperature and time-resolved room temperature spectra in three of the resulting mutant reaction centers. The major effect of the two mutations near the dimer was an increase up to 80 mV in the donor oxidation-reduction midpoint potential. Correspondingly, the calculated free energy difference between the excited state of the primary donor and the initial charge separated state decreased by up to 55 mV, the initial forward electron-transfer rate was up to 4 times slower, and the rate of charge recombination between the primary quinone and the donor was approximately 30% faster in these two mutants compared to the wild type. The two mutations near the monomer bacteriochlorophylls had minor changes of 25 mV or less in the donor oxidation-reduction potential, but the mutation close to the monomer bacteriochlorophyll on the active branch resulted in a roughly 3-fold decrease in the rate of the initial electron transfer.

The temperature dependence of fluorescence on the picosecond to nanosecond time scale from the reaction centers of Rhodobacter sphaeroides strain R-26 and two mutants with elevated P/P+ midpoint potentials has been measured with picosecond time resolution. In all three samples, the kinetics of the fluorescence decay is complex and can only be well described with four or more exponential decay terms spanning the picosecond to nanosecond time range. Multiexponential fits are needed at all temperatures between 295 and 20 K. The complex decay kinetics are explained in terms of a dynamic solvation model in which the charge-separated state is stabilized after formation by protein conformational changes. Many of these motions have not had time to occur on the time scale of initial electron transfer and/or are frozen out at low temperature. This results in a time- and temperature-dependent enthalpy change between the excited singlet state and the charge-separated state that is the dominant term in the free energy difference between these states. Long-lived fluorescence is still observed even at 20 K, particularly for the high-potential mutants. This implies that the driving force for electron transfer on the nanosecond time scale at low temperature is less than 200 cm-1 (25 meV) in R-26 reaction centers and even smaller on the picosecond time scale or in the high-potential mutants.(ABSTRACT TRUNCATED AT 250 WORDS)

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