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.

Spectra of individual silicon nanocrystals within porous Si grains are studied by the wide‐field imaging microspectroscopy and their ON–OFF blinking is detected by the confocal single‐photon‐counting microscopy. Observed spectral and blinking properties comprise all features reported before in differently prepared single Si nanocrystals (SiNCs). Former apparently contradictory results are shown to be due to different experimental conditions. When the effect of dark periods (OFF switching) is removed the common ultimate photoluminescence properties of SiO2 passivated SiNCs are found, namely the quantum efficiency (QE) of about 10–20% up to the pumping rate corresponding to one exciton average excitation per quantum dot. At higher pump rates the QE is slowly decreasing as the 0.7th power of excitation. This is most likely due to Auger recombination which, however, seems to be weakened compared with measurements of nanocrystal assemblies. We conclude that SiNCs may be pumped above one exciton occupancy to yield a higher light emission, being advantageous for applications.

Cyanobacteria contain several genes coding for small one-helix proteins called SCPs or HLIPs with significant sequence similarity to chlorophyll a/b-binding proteins. To localize one of these proteins, ScpD, in the cells of the cyanobacterium Synechocystis sp. PCC 6803, we constructed several mutants in which ScpD was expressed as a His-tagged protein (ScpDHis). Using two-dimensional native-SDS electrophoresis of thylakoid membranes or isolated Photosystem II (PSII), we determined that after high-light treatment most of the ScpDHis protein in a cell is associated with PSII. The ScpDHis protein was present in both monomeric and dimeric PSII core complexes and also in the core subcomplex lacking CP43. However, the association with PSII was abolished in the mutant lacking the PSII subunit PsbH. In a PSII mutant lacking cytochrome b 559 , which does not accumulate PSII, ScpDHis is associated with CP47. The interaction of ScpDHis with PsbH and CP47 was further confirmed by electron microscopy of PSII labeled with Ni-NTA Nanogold. Single particle image analysis identified the location of the labeled ScpDHis at the periphery of the PSII core complex in the vicinity of the PsbH and CP47. Because of the fact that ScpDHis did not form any large structures bound to PSII and because of its accumulation in PSII subcomplexes containing CP47 and PsbH we suggest that ScpD is involved in a process of PSII assembly/repair during the turnover of pigment-binding proteins, particularly CP47.

Structure and organisation of Photosystem I and Photosystem II isolated from red alga Cyanidium caldarium was determined by electron microscopy and single particle image analysis. The overall structure of Photosystem II was found to be similar to that known from cyanobacteria. The location of additional 20 kDa (PsbQ') extrinsic protein that forms part of the oxygen evolving complex was suggested to be in the vicinity of cytochrome c-550 (PsbV) and the 12 kDa (PsbU) protein. Photosystem I was determined as a monomeric unit consisting of PsaA/B core complex with varying amounts of antenna subunits attached. The number of these subunits was seen to be dependent on the light conditions used during cell cultivation. The role of PsaH and PsaG proteins of Photosystem I in trimerisation and antennae complexes binding is discussed.