A reaction center of photosystem II was isolated from Pisum sativum by using immobilized metal affinity chromatography. This reaction center is photochemically active and has a room temperature Qy chlorophyll (Chl) absorption band peaking at 677.5 nm. From HPLC analysis, the pigment stoichiometry was suggested to be 5 Chls per 1 (3-carotene per 2 pheophytins. Low-temperature absorption measurements at 77 K were consistent with the removal of one of the Chls associated with the usual form of the reaction center isolated by using ion-exchange chromatography. Transient absorption spectroscopy on the picosecond time scale indicated that the Chl removed belongs to a pool of Chl absorbing at -670 nm (C6701I) that transfers energy relatively slowly to the primary donor P680 in support of our recently proposed model. The results also support the previous conclusion that radical pair formation is largely associated with a 21-ps time constant when P680 is directly excited and that the identity of C6701 is likely to be peripherally bound ChIs possibly ligated to conserved His residues at positions 118 on the Dl and D2 proteins.that have been assigned to energy transfer from accessory Phs and Chls to the primary donor P680 (17,(19)(20)(21)(22)(23)(24). It has been further suggested that the slow energy transfer processes are associated with two additional Chls bound to the periphery of the reaction center complex (21, 24). Moreover, the presence of these slow energy transfer processes has made the determination of the time constants for primary charge separation more difficult, leading to controversy (16,21,(24)(25)(26)(27)(28). It is therefore particularly interesting to compare the primary photochemistry of the 6-Chl-containing PSII reaction center with those in which 1 or 2 of the peripheral Chl molecules have been removed. In this paper, we report a procedure for isolating the reaction center of PSII by using immobilized metal affinity chromatography. HPLC analysis indicated that the complex contained 5 Chls per 2 Phs and, relative to that of the 6-Chl-containing preparation, showed a spectral shift indicative of the removal of an accessory Chl absorbing on the blue side of the main Qy band. The role of this Chl as an accessory pigment was confirmed by time-resolved picosecond absorption spectroscopy.

PhotosystemIH reaction centers have been studied by femtoend trent absorption spectroseopy. We demonstrate that it is possible to achieve good photoselectivity between the primary electron donor P680 and the majority of the accessory chlorins. Energy tanser can be observed in both directions between P680 and these accessory chlorins depending on which Is initily excited. This oxidizing potential is used to drive water splitting, which gives rise to oxygen evolution. The primary electron donor of PSII is thought to correspond to a spectral feature at 680 nm and is referred to as P680 (3), while a pheophytin (Ph) molecule functions as an electron acceptor (4-6). Studies of PSII core complexes binding 60 and 80 antenna chlorophylls have suggested that the primary radical pair P680+Ph-is formed at a rate of 4100 psfollowing the absorption ofa photon by antenna pigments (7).A similar conclusion was reached from photovoltage studies of larger PSII complexes (8). A kinetic model, in which there is a rapid (--1 ps) equilibration of excitation energy between the antenna pigments and P680, followed by the observed trapping of the excitation energy by radical pair formation in ""100 ps, has been proposed for this process (refs. 9 and 10; reviewed in ref. 2). This so-called trapping limited model (11) is valid when the rate of electron transfer from the primary electron donor is slower than energy transfer back to the antenna pigments. This model has also been applied to other photosynthetic antenna/reaction center complexes (12)(13)(14).However, previous studies have not been able to time resolve the energy-transfer processes that are predicted to cause the equilibration of excitation energy between the antenna and primary electron donor pigments prior to radical pair formation.We report here a study ofexcitation energy equilibration in the isolated Dl/D2/cytochrome b559 complex, which is the reaction center of PSII. This complex is much smaller than the isolated PSII core complexes discussed above, binding only six chlorophyll a and two Ph a pigments (15, 16). While several of these pigments are presumably involved in electron-transfer processes, these pigments will also function in an energy-transferring capacity before charge separation. Time-resolved fluorescence studies have determined that at least 94% of the chlorin pigments in our PSI1 reaction center preparation are able to transfer excitation energy to P680, resulting in a near unity quantum yield of the primary radical pair (17,18). In a previous study, we determined that when P680 is directly excited, Ph reduction occurs primarily at a rate of 21 ps-1 in isolated PSII reaction centers at room temperature (4).There have been several discussions of the similarities between the PSII reaction center of higher plants and the reaction center of purple bacteria (19,20). However, these two complexes are likely to be very different in terms oftheir energy-transfer kinetics when isolated from their antenna systems. The lowest SO-S, opticil transition for the pr...

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Transient absorption spectroscopy has been used to study sub-picosecond energy transfer processes in isolated photosystem II (PS II) reaction centers. As reported previously [Durrant, J. R.; et al. Proc. Natl. Acad. Sci. U.S.A. 1992 , 89, 11632−11636], using long wavelength (694 nm) excitation, spectral evolution of the isotropic Q y band bleach/stimulated emission is dominated by energy transfer processes with a 100 ± 50 fs time constant. In contrast, depolarization of this signal occurs with a time constant of 400 ± 30 fs, from an initial anisotropy of ∼0.4 to a value of ∼0.15 at 1.5 ps. This decay of the anisotropy is attributed to energy transfer between at least two degenerate states contributing to reaction center absorption circa 680 nm, with these states having approximately orthogonal transition dipoles. The transient anisotropy barely changes between 1.5 and 60 ps, indicating that under these excitation conditions equilibration of the excitation energy between reaction center excited states occurs on a sub-picosecond time scale. Transient data collected for pheophytin Q x absorption bands indicate that pheophytin molecules are included in the 100 fs equilibration process. These results are discussed in the context of the PS II multimer model and are shown to be in good agreement with this model.

Abstract. Following recent observations of molecular iodine (I2) in the coastal marine boundary layer (MBL) (Saiz-Lopez and Plane, 2004), it has become important to determine the absolute absorption cross-section of I2 at reasonably high resolution, and also to evaluate the rate of photolysis of the molecule in the lower atmosphere. The absolute absorption cross-section (σ) of gaseous I2 at room temperature and pressure (295K, 760Torr) was therefore measured between 182 and 750nm using a Fourier Transform spectrometer at a resolution of 4cm-1 (0.1nm at λ=500nm). The maximum absorption cross-section in the visible region was observed at λ=533.0nm to be σ=(4.24±0.50)x10-18cm2molecule-1. The spectrum is available as supplementary material accompanying this paper. The photo-dissociation rate constant (J) of gaseous I2 was also measured directly in a solar simulator, yielding J(I2)=0.12±0.03s-1 for the lower troposphere. This is in excellent agreement with the value of 0.12±0.015s-1 calculated using the measured absorption cross-section, terrestrial solar flux for clear sky conditions and assuming a photo-dissociation yield of unity. A two-stream radiation transfer model was then used to determine the variation in photolysis rate with solar zenith angle (SZA), from which an analytic expression is derived for use in atmospheric models. Photolysis appears to be the dominant loss process for I2 during daytime, and hence an important source of iodine atoms in the lower atmosphere.