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The excitation transport and trapping kinetics of core antenna-reaction center complexes from photosystem I of wild-type Synechocystis sp. PCC 6803 were investigated under annihilation-free conditions in complexes with open and closed reaction centers. For closed reaction centers, the long-component decay-associated spectrum (DAS) from global analysis of absorption difference spectra excited at 660 nm is essentially flat (maximum amplitude <10(-5) absorbance units). For open reaction centers, the long-time spectrum (which exhibits photobleaching maxima at approximately 680 and 700 nm, and an absorbance feature near 690 nm) resembles one previously attributed to (P700(+) - P700). For photosystem I complexes excited at 660 nm with open reaction centers, the equilibration between the bulk antenna and far-red chlorophylls absorbing at wavelengths >700 nm is well described by a single DAS component with lifetime 2.3 ps. For closed reaction centers, two DAS components (2.0 and 6.5 ps) are required to fit the kinetics. The overall trapping time at P700 ( approximately 24 ps) is very nearly the same in either case. Our results support a scenario in which the time constant for the P700 --> A(0) electron transfer is 9-10 ps, whereas the kinetics of the subsequent A(0) --> A(1) electron transfer are still unknown.

The fluorescence kinetics of the nitrobenzoxadiazole (NBD) chromophore were studied at low concentrations in solvents with varying polarity and hydrogen-bonding donor strength. The emission decay was essentially single exponential in all solvents studied. While the absorption and fluorescence solvatochromism is determined largely by the solvent polarity, the S1 state decay kinetics are strongly modulated by the solvent H-bonding capacity. The NBD emission lifetime, generally approximately 7-10 ns in the aprotic solvents, is reduced to 0.933 ns in water. The solvent deuterium isotope effect on the fluorescence decay is substantial in D2O and in methanol-d4, but is insignificant in DMSO-d6. These results are consistent with acceleration of S1----S0 internal conversion through an accepting vibrational mode created by intermolecular hydrogen-bonding of the NBD chromophore to an H atom-donating solvent. This work bears on the practically of using NBD as a fluorophore in assays for estrogen and progesterone receptors.

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Ultrafast primary processes in the trimeric photosystem I core antenna-reaction center complex of the cyanobacterium Synechocystis sp. PCC 6803 have been examined in pump-probe experiments with approximately 100 fs resolution. A global analysis of two-color profiles, excited at 660 nm and probed at 5 nm intervals from 650 to 730 nm, reveals 430 fs kinetics for spectral equilibration among bulk antenna chlorophylls. At least two lifetime components (2.0 and 6.5 ps in our analysis) are required to describe equilibration of bulk chlorophylls with far red-absorbing chlorophylls (>700 nm). Trapping at P700 occurs with 24-ps kinetics. The multiphasic bulk left arrow over right arrow red equilibration kinetics are intriguing, because prior steady-state spectral studies have suggested that the core antenna in Synechocystis sp. contains only one red-absorbing chlorophyll species (C708). The disperse kinetics may arise from inhomogeneous broadening in C708. The one-color optical anisotropy at 680 nm (near the red edge of the bulk antenna) decays with 590 fs kinetics; the corresponding anisotropy at 710 nm shows approximately 3.1 ps kinetics. The latter may signal equilibration among symmetry-equivalent red chlorophylls, bound to different monomers within trimeric photosystem I.

X-ray specular reflectivity at the liquid/gas interface of dihexadecyl phosphate (DHDP) on pure H2O and on a series of pigment-containing aqueous solutions are reported along with visible absorption spectra of corresponding monomolecular Langmuir−Blodgett films on quartz substrates. Molecular level interpretation of the reflectivity from DHDP on pure water reveals that at large surface pressure (>10 mN/m), the film is closely packed with practically untilted hydrocarbon chains and hydrated phosphate headgroups. On solutions containing either water-soluble cationic tetraazaphthalocyanines or tetrapyridylporphyrins, significant changes in the organization of the lipid with respect to that on pure water are found. Total film thicknesses are larger and consistent with the adsorption of a single pigment layer contiguous to the headgroup, whereas the hydrocarbon tail region is shorter, suggestive of tilted alkyl chains. In addition, film thicknesses for phthalocyanine-containing films suggest formation of an iodide counterion layer underneath the plane containing the pigments. This sharply contrasts the interfacial profile obtained for porphyrin-containing films, in which the iodide counterions appear to exist within the pigment plane. Visible absorption spectra of all transferred films indicate a closely packed single pigment layer, consistent with the reflectivity results. The optical spectra of the pigment are preserved (in relation to the aqueous solution monomer spectra) in the transferred film, indicating a suppression of pigment aggregation. Reflectivity measurements at large molecular areas on pure water indicate that DHDP forms an inhomogeneous film, suggestive of phase segregation; on the pigment-containing solutions, DHDP induces (through attractive Coulomb interactions) the adsorption of a homogeneous monopigment layer. The existence of a complete pigment monolayer over the measured surface pressure−molecular area (π−A) isotherms has been evidenced by both X-ray reflectivity and visible optical studies. Preservation of pigment functionality has been demonstrated through the process of Coulomb association of the chromophores with charged lipid monolayer headgroups at the air/water interface. The potential for applications as model photosynthetic antennae will be discussed.

The charge separation P700*A(0) --> P700(+)A(0)(-) and the subsequent electron transfer from the primary to secondary electron acceptor have been studied by subtracting absorption difference profiles for cyanobacterial photosystem I (PS I) complexes with open and closed reaction centers. Samples were excited at 660 nm, which lies toward the blue edge of the core antenna absorption spectrum. The resulting PS I kinetics were analyzed in terms of the relevant P700, P700(+), A(0), and A(0)(-) absorption spectra. In our kinetic model, the radical pair P700(+)A(0)(-) forms with 1.3 ps rise kinetics after creation of electronically excited P700*. The formation of A(1)(-) via electron transfer from A(0)(-) requires approximately 13 ps. The kinetics of the latter step are appreciably faster than previously estimated by other groups (20--50 ps).

We describe simulations of absorption difference spectra in strongly coupled photosynthetic antennas. In the presence of large resonance couplings, distinctive features arise from excited-state absorption transitions between one- and two-exciton levels. We first outline the theory for the heterodimer and for the general N-pigment system, and we demonstrate the transition between the strong and weak coupling regimes. The theory is applied to Fenna-Matthews-Olson (FMO) bacteriochlorophyll a protein trimers from the green photosynthetic bacterium Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from the green bacterium Chlorobium tepidum.