Performance of low-pressure thermionic converters is evaluated

Experiments, evaluating the performance of low-pressure thermionic converters, were conducted with cesium, potassium, and sodium-metal vapors. The results of the investigation are useful in the selection of favorable conditions for the design of thermionic reactor fuel elements, including RF output for special applications

CASCADE INVESTIGATION OF COOLING CHARACTERISTICS OF A CORRUGATEDINSERT AIR-COOLED TURBINE BLADE FOR USE IN A TURBOPROP ENGINE

Heat transfer of air-cooled turbine blade for turboprop engin

Site-saturation studies of β-lactamase: Production and characterization of mutant β-lactamases with all possible amino acid substitutions at residue 71

A mutagenic technique that "saturates" a particular site in a protein with all possible amino acid substitutions was used to study the role of residue 71 in β-lactamase (EC 3.5.2.6). Threonine is conserved at residue 71 in all class A β-lactamases and is adjacent to the active site Ser-70. All 19 mutants of the enzyme were characterized by the penam and cephem antibiotic resistance they provided to Escherichia coli LS1 cells. Surprisingly, cells producing any of 14 of the mutant β-lactamases displayed appreciable resistance to ampicillin; only cells with mutants having Tyr, Trp, Asp, Lys, or Arg at residue 71 had no observable resistance to ampicillin. However, the mutants are less stable to cellular proteases than wild-type enzyme is. These results suggest that Thr-71 is not essential for binding or catalysis but is important for stability of the β-lactamase protein. An apparent change in specificity indicates that residue 71 influences the region of the protein that accommodates the side chain attached to the β-lactam ring of the substrate

Structure-function relations in phosphorylcholine-binding mouse myeloma proteins

The binding site interactions between the phosphorylcholine (phosphocholine)-binding mouse myeloma proteins TEPC 15, W3207, McPC 603, MOPC 167, and MOPC 511 and the isotopically substituted hapten phosphoryl-[methyl-13C]choline have been investigated using 13C and 31P nuclear magnetic resonance (NMR) spectroscopy. Each protein exhibits a unique NMR pattern, but extensive similarities in chemical shift parameters upon binding of hapten to immunoglobulin suggest a significant degree of conservation of important hapten-binding site interactions. Moreover, independent binding studies, in conjunction with the NMR data, allow construction of a simple model of the binding sites of these antibodies, analyzed in terms of the relative strength of interaction between hapten and two main subsites. The NMR evidence supports the view that the heavy chains of these proteins dominate in interacting with bound phosphorylcholine; the various subspecificities of these proteins for phosphorylcholine analogues can be accounted for by amino acid changes in the hypervariable regions of the heavy chains

Ionization behavior of the histidine residue in the catalytic triad of serine proteases

α-Lytic protease is a homologue of the mammalian serine proteases such as trypsin, chymotrypsin, and elastase, and its single histidine residue belongs to the Asp-His-Ser catalytic triad. This single histidine residue has been selectively enriched in the C-2 carbon with 13C. Magnetic resonance studies of the chemical shift and coupling constant (1Jch) behavior of this nucleus as a function of pH suggest that the imidazole ring is neutral above pH 5 and therefore that the group which is known to ionize with pKa near 6.7 must be the aspartic acid residue. Implications of these new pKa assignments for the catalytic mechanism of serine proteases are discussed and include the absence of any need to separate charge during catalysis. The histidine residue plays two roles. (a) It insulates the aspartic acid from an aqueous environment and accordingly raises its pKa. (b) It serves as a bidentate base to accept a proton from the serine at one of its nitrogens and concertedly transfer a proton from its other nitrogen to the buried carboxylate anion during formation of the tetrahedral intermediate

Comparative Genomics of a Parthenogenesis-Inducing Wolbachia Symbiont.

Wolbachia is an intracellular symbiont of invertebrates responsible for inducing a wide variety of phenotypes in its host. These host-Wolbachia relationships span the continuum from reproductive parasitism to obligate mutualism, and provide a unique system to study genomic changes associated with the evolution of symbiosis. We present the genome sequence from a parthenogenesis-inducing Wolbachia strain (wTpre) infecting the minute parasitoid wasp Trichogramma pretiosum The wTpre genome is the most complete parthenogenesis-inducing Wolbachia genome available to date. We used comparative genomics across 16 Wolbachia strains, representing five supergroups, to identify a core Wolbachia genome of 496 sets of orthologous genes. Only 14 of these sets are unique to Wolbachia when compared to other bacteria from the Rickettsiales. We show that the B supergroup of Wolbachia, of which wTpre is a member, contains a significantly higher number of ankyrin repeat-containing genes than other supergroups. In the wTpre genome, there is evidence for truncation of the protein coding sequences in 20% of ORFs, mostly as a result of frameshift mutations. The wTpre strain represents a conversion from cytoplasmic incompatibility to a parthenogenesis-inducing lifestyle, and is required for reproduction in the Trichogramma host it infects. We hypothesize that the large number of coding frame truncations has accompanied the change in reproductive mode of the wTpre strain

The effect of parallel static and microwave electric fields on excited hydrogen atoms

Motivated by recent experiments we analyse the classical dynamics of a hydrogen atom in parallel static and microwave electric fields. Using an appropriate representation and averaging approximations we show that resonant ionisation is controlled by a separatrix, and provide necessary conditions for a dynamical resonance to affect the ionisation probability. The position of the dynamical resonance is computed using a high-order perturbation series, and estimate its radius of convergence. We show that the position of the dynamical resonance does not coincide precisely with the ionisation maxima, and that the field switch-on time can dramatically affect the ionisation signal which, for long switch times, reflects the shape of an incipient homoclinic. Similarly, the resonance ionisation time can reflect the time-scale of the separatrix motion, which is therefore longer than conventional static field Stark ionisation. We explain why these effects should be observed in the quantum dynamics. PACs: 32.80.Rm, 33.40.+f, 34.10.+x, 05.45.Ac, 05.45.MtComment: 47 pages, 20 figure

Outer-Sphere Effects on Reduction Potentials of Copper Sites in Proteins: The Curious Case of High Potential Type 2 C112D/M121E Pseudomonas aeruginosa Azurin

Redox and spectroscopic (electronic absorption, multifrequency electron paramagnetic resonance (EPR), and X-ray absorption) properties together with X-ray crystal structures are reported for the type 2 Cu^(II) C112D/M121E variant of Pseudomonas aeruginosa azurin. The results suggest that Cu^(II) is constrained from interaction with the proximal glutamate; this structural frustration implies a “rack” mechanism for the 290 mV (vs NHE) reduction potential measured at neutral pH. At high pH (~9), hydrogen bonding in the outer coordination sphere is perturbed to allow axial glutamate ligation to Cu^(II), with a decrease in potential to 119 mV. These results highlight the role played by outer-sphere interactions, and the structural constraints they impose, in determining the redox behavior of transition metal protein cofactors

A projection method for statics and dynamics of lattice spin systems

A method based on Monte Carlo sampling of the probability flows projected onto the subspace of one or more slow variables is proposed for investigation of dynamic and static properties of lattice spin systems. We illustrate the method by applying it, with projection onto the order-parameter subspace, to the three-dimensional 3-state Potts model in equilibrium and to metastable decay in a three-dimensional 3-state kinetic Potts model.Comment: 4 pages, 3 figures, RevTex, final version to appear in Phys. Rev. Let