Ultrafast pump-probe spectroscopy of the P700- and F_x-containing Photosystem I core protein from Synechococcus sp. PCC 6301 (Anacystis nidulans)

Time-resolved absorption difference profiles were obtained using one-color and two-color pump-probe techniques on the P700-containing core protein from the blue-green bacterium Synechococcus sp. PCC 6301, which contains approx. 100 chlorophyll (Chl) a antenna pigments per reaction center. Excitation wavelengths in the two-color studies were 665 and 680 nm; probe wavelengths ranged from 650 to 676 nm. The isotropic absorption difference spectra show evidence for rapid equilibration among Chl a spectral forms in the core antenna. In addition, they exhibit a dynamic blue shift with lifetime ∼ 9-14 ps, which occurs irrespective of excitation wavelength. Analyses of isotropic profiles for different combinations of pump and probe wavelengths yield four families of lifetime components: < 5 ps, which is attributed to spectral equilibration; 9-14 ps; several tens of ps (observed at wavelengths longer than 665 nm); and at least several hundred ps (observed at probe wavelengths 650 and 657 nm under 665 nm excitation). Anisotropic decays observed with identical pump and probe wavelengths exhibit large-amplitude components with easily resolved lifetimes (∼ 15 ps and ∼ 7 ps at 665 and 670 nm, respectively). The initial anisotropies are < 0.4, indicating that much faster depolarization processes also occur in these one-color experiments. The two-color anisotropy functions tend to be dominated by unresolvably fast decay, suggesting that mainstream energy transfer steps in the core antenna occur on a subpicosecond time scale.