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A study was conducted to investigate the heat-induced disassembly and subsequent reassembly of the Mn complex by photoactivation and the restoration of the oxygen evolving activity were monitored by polarography and recombination chlorophyll fluorescence. The findings suggest that the binuclear Mn complex formed by heating corresponds to the assembly intermediate connected by a di-[mu]-oxo bridge.

X-ray absorption spectroscopy (XAS) at the Br K-edge in [Cl.sup.-]-depleted and [Br.sup.-]-substituted PSII membrane particles from spinach were used to study the binding of bromide to the [Mn.sub.4]Ca complex of PSII in its dark-stable [S.sub.1] state. The results provided evidence of at least one metal ion, which might be manganese or calcium, at a distance of ~5 Armstrong to [Br.sup.-] and native [Cl.sup.-].

The recent crystallographic structure at 3.0 Å resolution of PSII from Thermosynechococcus elongatus has revealed a cavity in the protein which connects the membrane phase to the binding pocket of the secondary plastoquinone QB. The cavity may serve as a quinone diffusion pathway. By fluorescence methods, electron transfer at the donor and acceptor sides was investigated in the same membrane-free PSII core particle preparation from T. elongatus prior to and after crystallization; PSII membrane fragments from spinach were studied as a reference. The data suggest selective enrichment of those PSII centers in the crystal that are intact with respect to O2 evolution at the manganese–calcium complex of water oxidation and with respect to the integrity of the quinone binding site. One and more functional quinone molecules (per PSII monomer) besides of QA and QB were found in the crystallized PSII. We propose that the extra quinones are located in the QB cavity and serve as a PSII intrinsic pool of electron acceptors. [Copyright &y& Elsevier]

We applied precise densimetry and ultrasound velocimetry methods to study the interaction of a synthetic alpha-helical transmembrane peptide, acetyl-K(2)-L(24)-K(2)-amide (L(24)), with model bilayer lipid membranes. The large unilamellar vesicles (LUVs) utilized were composed of a homologous series of n-saturated diacylphosphatidylcholines (PCs). PCs whose hydrocarbon chains contained from 13 to 16 carbon atoms, thus producing phospholipid bilayers of different thicknesses and gel to liquid-crystalline phase transition temperatures. This allowed us to analyze how the difference between the hydrophobic length of the peptide and the hydrophobic thickness of the lipid bilayer influences the thermodynamical and mechanical properties of the membranes. We showed that the incorporation of L(24) decreases the temperature and cooperativity of the main phase transition of all LUVs studied. The presence of L(24) in the bilayer also caused an increase of the specific volume and of the volume compressibility in the gel state bilayers. In the liquid crystalline state, the peptide decreases the specific volume at relatively higher peptide concentration (mole ratio L(24):PC=1:50). The overall volume compressibility of the peptide-containing lipid bilayers in the liquid-crystalline state was in general higher in comparison with pure membranes. There was, however, a tendency for the volume compressibility of these lipid bilayers to decrease with higher peptide content in comparison with bilayers of lower peptide concentration. For one lipid composition, we also compared the thermodynamical and mechanical properties of LUVs and large multilamellar vesicles (MLVs) with and without L(24). As expected, a higher cooperativity of the changes of the thermodynamical and mechanical parameters took place for MLVs in comparison with LUVs. These results are in agreement with previously reported DSC and (2)H NMR spectroscopy study of the interaction of the L(24) and structurally related peptides with phosphatidylcholine bilayers. An apparent discrepancy between (2)H NMR spectroscopy and compressibility data in the liquid crystalline state may be connected with the complex and anisotropic nature of macroscopic mechanical properties of the membranes. The observed changes in membrane mechanical properties induced by the presence of L(24) suggest that around each peptide a distorted region exists that involves at least 2 layers of lipid molecules.