Computer-aided study of the stability of parallel incompressible flows in a co-linear magnetic field

Summary The analysis is based on the differential equations governing infinitesimal disturbances of the form of three-dimensional waves [ e ^{im + ia(z–ct)}] superimposed on a parallel, plane or axisymmetric, laminar flow of viscous incompressible fluid in a uniform magnetic field aligned with the basic flow. A method which makes use of the fact that the governing vector eigenvalue equations for the perturbation quantities are solvable analytically in the case of constant basic-flow velocity, was employed to obtain approximate solutions of the problem. After testing this technique in the cases of the two-dimensional sech ² r profile and the Blasius profile, numerical results in form of curves of neutral stability were obtained for the three-dimensional (1 + r ²)^–2 profile with m = 1 (see Fig. 5).

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Übersicht Ausgangspunkt der Arbeit sind die Differentialgleichungen für infinitesimale Störgr;oßen von der Form dreidimensionaler Wellen [ e ^{im + ia(z–ct)}], die der parallelen, ebenen oder axisymmetrischen, Laminarströmung einer inkompressiblen Flüssigkeit nichtverschwindender Viskosität in einem zur Strömungsrichtung parallelen und im ganzen Raum konstanten Magnetfeld überlagert sind. Eine Methode, die davon Gebrauch macht, daß die Vektor-Eigenwertgleichungen für die Störgr;oßen im Falle räumlich konstanter Geschwindigkeit der Grundströmung exakt lösbar sind, wurde zur näherungsweisen Behandlung des Problems herangezogen. Nachdem diese Methode an den Testbeispielen des zweidimensionalen sech ² r-Profils und des Blasius-Profils erprobt worden war, wurden numerische Ergebnisse in Form von Stabilitätskurven für das dreidimensionale (1 + r ²)^–2-Profil mit m = 1 gewonnen (s. Abb. 5).

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This research was supported by the German Academic Exchange Service and by the National Science Foundation (Grant No. GK-72). The computations were performed at the Computation Center of the Massachusetts Institute of Technology, Cambridge, Mass., USA.     

X-ray crystal structures of reduced rubrerythrin and its azide adduct: a structure-based mechanism for a non-heme diiron peroxidase

Rubrerythrin (Rbr) is a 44-kDa homodimeric protein, found in many air-sensitive bacteria and archaea, which contains a unique combination of a rubredoxin-like [Fe(SCys)(4)] site and a non-sulfur, oxo/dicarboxylato-bridged diiron site. The diiron site structure resembles those found in O2-activating diiron enzymes. However, Rbr instead appears to function as a hydrogen peroxide reductase (peroxidase). The diferrous site in all-ferrous Rbr (Rbr(red)) shows a much greater reactivity with H2O2 than does the diferric site in all-ferric Rbr (Rbr(ox)), but only the latter structure has been reported. Here we report the X-ray crystal structures of the recombinant Rbr(red) from the sulfate reducing bacterium, Desulfovibrio vulgaris, as well as its azide adduct (Rbr(red)N3). We have also redetermined the structure of Rbr(ox) to a higher resolution than previously reported. The structural differences between Rbr(ox) and Rbr(red) are localized entirely at the diiron site. The most striking structural change upon reduction of the diferric to the diferrous site of Rbr is a 1.8-A movement of one iron away from a unique glutamate carboxylate ligand and toward a trans-disposed histidine side chain, which replaces the glutamate as a ligand. This movement increases the inter-iron distance from 3.3 to 4 A. Rbr(red)N(3) shows this same iron movement and His-->Glu ligand replacement relative to Rbr(ox), and, in addition, an azide coordinated to the diiron site in a cis mu-1,3 fashion, replacing two solvent ligands in Rbr(red). Relative to those in O2-activating enzymes, the bridging carboxylate ligation of the Rbr diiron site is less flexible upon diferric/diferrous interconversion. The diferrous site is also much more rigid, symmetrical, and solvent-exposed than those in O2-activating enzymes. On the basis of these unique structural features, a mechanism is proposed for facile reduction of hydrogen peroxide by Rbr involving a cis mu-eta(2) H2O2 diferrous intermediate.     

Modifikationen des Silbersalz-Persulfatverfahrens zur Bestimmung gr?sserer Manganmengen

Ohne Zusammenfassung     

Near-infrared absorptions of monomethylhydrazine

The peak absorption coefficients for two near-infrared absorptions of monomethylhydrazine, CH₃-N₂H₃, (MMH) were measured. Absorption bands located at 1.524 μm (6560 cm^-1), 1.557 μm (6423 cm^-1), and 1.583 μm (6316 cm^-1) are assigned to the υ = 2 overtones of the infrared N-H stretching fundamentals at 3317, 3245 and 3177 cm^-1. An absorption band located at 1.04 μm (9620±100 cm^-1) is assigned to the υ = 3 overtone of one of these fundamentals. The peak absorption coefficients (₁₀) at 1.524 μm (6560±20 cm^-1) and 1.04 μm (9620±100 cm^-1) are 31 × 10^-3 and 0.97 × 10^-3 (cm atm)^-1, respectively. Uncertainties in these coefficients were estimated to be less than ±20% due primarily to uncertainties in the partial vapor pressure of MMH.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.