Dimer-of-dimers model for the oxygen-evolving complex of photosystem II. Synthesis and properties of [MnIV4O5(terpy)4(H2O)2](ClO4)6

A dimer-of-dimers model compound for the oxygen-evolving complex of photosystem II, [[(H(2)O)(terpy)Mn(IV)(micro-O)(2)Mn(IV)(terpy)](2)(micro-O)](ClO(4))(6) (terpy = 2,2':6',2' '-terpyridine), has been prepared and characterized by X-ray crystallography and ESI-MS. Low pH was found to promote the disproportionation of [Mn(III/IV)(2)O(2)(terpy)(2)(OH(2))(2)](3+) to Mn(2+) and a Mn(IV/IV)(2)O(2)(terpy)(2) species; the latter complex slowly dimerizes to form the title complex. Protonation of a micro-oxo bridge is proposed to initiate the disproportionation, based on analogy with the [Mn(III/)(IV)(2)O(2)(bpy)(4)](3+) system.     

New linear high-valent tetranuclear manganese-oxo cluster relevant to the oxygen-evolving complex of photosystem II with oxo, hydroxo, and aqua coordinated to a single Mn(IV)

An unprecedented atom connectivity, MnIV(mu-O)MnIV(mu-O)2MnIV(mu-O)MnIV, is found in the complex [MnIV4O4(EtO-terpy)4(OH)2(OH2)2](ClO4)(6).8H2O (EtO-terpy=4'-ethoxyl-2,2':6',2' '-terpyridine), which has been characterized by X-ray crystallography, X-ray powder diffraction, EPR spectroscopy, and magnetic studies. This complex is the first example of a compound where a MnIV ion is coordinated by all three types of water-derived ligands: oxo, hydroxo, and aqua. Bond distances and angles for this complex are consistent with a MnIV4 oxidation state assignment. The di-mu-oxo- and mono-mu-oxo-bridged Mn-Mn distances are 2.80 and 3.51 A, respectively. The variable-temperature magnetic susceptibility data for this complex, in the range of 10-300 K, are consistent with an S=0 ground state and were fit using the spin Hamiltonian HHDvV=-J1S2S1-J2S1S1A-J1S1AS2A (S1=S1A=S2=S2A=3/2) with J1=-432 cm-1 and J2=-164 cm-1 (where J1 and J2 are exchange constants through the mono-mu-oxo and the di-mu-oxo bridges, respectively). The first excited spin state of this tetramer is a spin triplet state at 279 cm-1 above the diamagnetic ground state. The next spin states are the S=1 and S=2 levels at about 700 and 820 cm-1 above the S=0 ground state, respectively. These large energy gaps are consistent with the absence of an EPR signal for this complex, even at high temperature.     

Rapamycin selectively inhibits interleukin-2 activation of p70 S6 kinase.

The macrolide rapamycin induces cell cycle G1 arrest in yeast and in mammalian cells, which suggests that an evolutionarily conserved, rapamycin-sensitive pathway may regulate entry into S phase. In mammals, rapamycin inhibits interleukin-2 receptor-induced S phase entry and subsequent T-cell proliferation, resulting in immunosuppression. Here we show that interleukin-2 selectively stimulates the phosphorylation and activation of p70 S6 kinase but not the erk-encoded MAP kinases and rsk-encoded S6 kinases. Rapamycin completely and rapidly inhibits interleukin-2-induced phosphorylation and activation of p70 S6 kinase at concentrations comparable to those blocking S phase entry of T cells (0.05-0.2 nM). The structurally related macrolide FK506 competitively antagonizes the actions of rapamycin, indicating that these effects are mediated by FKBP, which binds the transition-state mimic structure common to both rapamycin and FK506 (refs 4, 6, 9-11). The selective blockade of the p70 S6 kinase activation cascade by the rapamycin-FKBP complex implicates this signalling pathway in the regulation of T cell entry into S phase.     

Intramolecular alkyne hydroalkoxylation and hydroamination catalyzed by iridium hydrides

[reaction: see text] Iridium(III) hydrides prove to be air-stable active catalysts for intramolecular hydroalkoxylation and hydroamination of internal alkynes with proximate nucleophiles. The cyclization follows highly selective 6-endo-dig regiochemistry when regioselectivity is an issue.     

Vortex flow and transverse flux screening at the bose glass transition

We study the vortex phase diagram in untwinned YBCO crystals with columnar defects. These randomly distributed defects are expected to induce a "Bose glass" phase of localized vortices exhibiting a vanishing resistance and Meissner effect for magnetic fields H( perpendicular) transverse to the columns. We directly observe the transverse Meissner effect using a Hall probe array. As predicted, the Meissner state breaks down at temperatures T(s) that decrease linearly as H( perpendicular) increases. However, T(s) lies far below the conventional melting temperature T(m) determined by a vanishing resistivity, suggesting a regime where vortices are effectively localized even when rotated off the columnar defects.     

Optical coupling experiments with tapered (horn shaped) channel waveguides

Arrays of horn shaped waveguides having different tapered section length L and different taper angle were fabricated. Optical coupling experiments indicate the presence of a coupling efficiency minimum at a characteristic length L_min, which is independent of taper angle.     

The flexible engineer

Summary It is argued that an engineer has to be flexible, in the sense of being adaptable, by using illustrations of a wide range of research topics with which the author was associated during 20 years in the Aerodynamics Department of the Royal Aircraft Establishment.The examples given include industrial (or non-aircraft) aerodynamics, magnetogasdynamics, combustion in high-speed air flows, sonic boom, and reduction of aircraft noise by airframe shielding and vortex refraction.

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Zusammenfassung Ein Ingenieur muss die Eigenschaft haben, ganz verschiedene Probleme angreifen zu können. Dies wird mit einem breiten Spektrum von Problemen illustriert, die der Verfasser während seiner zwanzigjährigen Tätigkeit am Royal Aircraft Establishment bearbeitete.Die Beispiele umfassen Industrieaerodynamik, Magnetogasdynamik, Verbrennung bei grossen Strömungsgeschwindigkeiten, Ueberschallknall und Fluglärmeduktion durch Abschirmen mit der Zelle und durch den Einfluss der Randwirbel.

     

Two-band superconductivity in MgB2

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at Delta(1)=2.3 meV and Delta(2)=7.1 meV at 4.2 K. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multiband system. Temperature evolution of the tunneling spectra follows the BCS scenario [Phys. Rev. Lett. 3, 552 (1959)]] with both gaps vanishing at the bulk T(c). The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [Phys. Rev. Lett. 87, 087005 (2001)]].