High-resolution Mn EXAFS of the oxygen-evolving complex in photosystem II: structural implications for the Mn4Ca cluster

The biological generation of oxygen by the oxygen-evolving complex in photosystem II (PS II) is one of nature's most important reactions. The recent X-ray crystal structures, while limited by resolutions of 3.2-3.5 A, have located the electron density associated with the Mn4Ca cluster within the multiprotein PS II complex. Detailed structures critically depend on input from spectroscopic techniques, such as EXAFS and EPR/ENDOR, as the XRD resolution does not allow for accurate determination of the position of Mn/Ca or the bridging and terminal ligand atoms. The number and distances of Mn-Mn/Ca/ligand interactions determined from EXAFS provide important constraints for the structure of the Mn4Ca cluster. Here, we present data from a high-resolution EXAFS method using a novel multicrystal monochromator that show three short Mn-Mn distances between 2.7 and 2.8 A and, hence, the presence of three di-mu-oxo-bridged units in the Mn4Ca cluster. This result imposes clear limitations on the proposed structures based on spectroscopic and diffraction data and provides input for refining such structures.     

The quadrupole interaction at iodine impurities in a tellurium matrix derived from β-γ TDPAC experiments

Time differential perturbed angular correlation measurements on the implated single crystal system and , using an allowed - cascade, are reported. The information obtained, determines the orientation of the principal axes system of the electric fieldgradient and its value at room temperature. The results are compared with earlier Mössbauer effect measurements and the binding of an iodine impurity in tellurium is discussed.     

A theoretical analysis of micelle-mediated transport through porous membranes

A novel method is proposed for the purpose of controlled release of a sparingly water soluble compound. The solubility of a sparingly water soluble compound can be increased by addition of a sufficient amount of surfactant to form micelles. The flux of the compound across a porous membrane can be enhanced if the membrane has pores larger than the micelle size so that the compound-loaded micelles can diffuse simultaneously, and micelle-mediated transport occurs. The membrane permeability of the micelle is a monotonically decreasing function of the ratio of the size of the micelle to the membrane pore size (R_m/R_p). However, the solubilizing capacity of the micelle increases with increasing size of the micelle. These opposing effects influence the transport and may result in an optimum flux of the solubilizate at a particular size of the micelle. In the determination of the optimum surfactant molecule, the concept of the hydrophilic-lipophilic balance (HLB) is employed in order to have stable aqueous solutions of the surfactants. For a family of nonionic surfactants solubilizing the hydrophobic and hydrocarbon substance n-heptane, it is shown that there exists a maximum flux of the solubilizate at a value of R_m/R_p within the limitation of the HLB. The release rates over a long period of time are nearly constant for micelles close to the optimum size for a given pore size.     

Oxide eines neuen Formeltyps: Zur Kenntnis von K3Ni2O4 und K3Pt2O4

Oxides of a New Type of Formula: On the Knowledge of K₃Ni₂O₄ and K₃Pt₂O₄ K₃Ni₃O₄, greyblack single crystals, obtained by heating K₂O and NaNiO₂ [K:Ni = 2:1, 500°C, 7d, Ag-cylinders], crystallizes orthorhombic with a = 6.04₄, b = 9.04₉, c = 10.56₇ Å, Z = 4, space group Cmcm (d_rö = 3.43, d_pyk = 3.32 g · cm^?3). Due to four cycle diffractometer data (374 hkl, MoKα, R = 8.6percnt;) a new type of structure is found with wavebands of [NiO_4/2] \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^{\rm 1} $\end{document} exhibits the C.N. 6, \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^{\rm 2} $\end{document} the C.N. 4. Effective Coordination Numbers, ECoN, calculated by means of Mean Fictive Ionic Radii, MEFIR, the Madelung Part of Lattice Energy, MAPLE and the magnetic properties are discussed. K₃Pt₂O₄, black single crystals with metallic lustre, obtained by heating KO_x (x = 1.55 or 1.88) and Pt (powder) [K:Pt = 4:1,1000°C, lh, 600°C, 2d, Pt-capsules] is isotypic to K₃Ni₂O₄ (four cycle diffractometer data: 458 hkl, MoKα, R = 12percnt;). Cellparameters are a = 6.15, b = 9.27, c = 11.51 Å (single crystal data), d_rö = 5.79 and d_pyk = 5.77 g · cm^?3.