Biosynthesis of novel thermoplastic polythioesters by engineered Escherichia coli

The development of non-petrochemical sources for the plastics industry continues to progress as large multinationals focus on renewable resources to replace fossil carbon. Many bacteria are known to accumulate polyoxoesters as water-insoluble granules in the cytoplasm. The thermoplastic and/or elastomeric behaviour of these biodegradable polymers holds promise for the development of various technological applications. Here, we report the synthesis and characterization of microbial polythioesters (PTEs), a novel class of biopolymers of general technological relevance. Biosynthesis of PTE homopolymers was achieved using a recombinant strain of Escherichia coli that expressed a non-natural pathway consisting of a butyrate kinase, a phosphotransbutyrylase, and a PHA synthase. Different homopolymers were produced, consisting of either 3-mercaptopropionate, 3-mercaptobutyrate, or 3-mercaptovalerate repeating units, if the respective mercaptoalkanoic acids were provided as precursor substrates to the fermentative process. The PTEs contributed up to 30% (w/w) of the cellular dry weight and were identified as hydrophobic inclusions in the cytoplasm. The chemical and stereochemical homogeneity of the purified PTEs were identified by different methods, and the estimated physical properties were compared to the oxypolyester equivalents, revealing low crystalline order and, for the poly(3-mercaptopropionate) improved thermal stability. The ability to produce PTEs through a biosynthetic route opens up new avenues in the field of biomaterials.     

Functional integral approach to the magnetic impurity problem: The superiority of the two-variable method

We discuss the functional integral approach to the magnetic impurity using the symmetric two-variable method. Namely, we avoid using the identityn ²=n for the localized state. We show that this procedure has clear advantages over all other linearizations of the Coulomb interaction in the Anderson Hamiltonian: it satisfies the symmetry with respect to the change of sign ofU in all approximations; it gives the first corrections to the nonmagnetic and magnetic limits already in the static approximation; it allows representation, both in the static approximation and in higher ones, of the partition function as an average overd-state occupation numbers; and calculations are much simplified. We present numerical calculations of the susceptibility for comparison of the results of the two-variable method to Hamann's saddle-point procedure or to the equivalent one-variable approach.     

Is There Really No Extensive Self-Consistent Perturbation Theory?

Many-particle perturbation theory is dominated by the Rayleigh–Schrödinger expansion, because at fixed particle density each term of the power series in the strength of the interaction has the right dependence on the particle number N. The first self-consistent generalization, the Brillouin–Wigner expansion, however, is non-extensive already in 2nd order. In Feenberg's generalization the same feature is found within iterative perturbation theory, but the series can be rearranged in such a way that lower orders are extensive. Applications are suggested.     

Calculation of phase equilibria for AISI M2 high-speed steel

Thermo-calc computations have been made for the iron-rich corner of the system Fe-Cr-Mo-W-V-C to obtain phase equilibria for AISI M2 high-speed steel (German designation S-6-5-2). The calculations describe the main transformation temperatures of this steel with good accuracy. The phases which are present during austenitization and their volume fractions are also correctly accounted for. The solidification sequence of the M2 steel, and in particular the occurrence of M₂C in the ledeburitic eutectic, can be explained if the segregation of alloying elements and carbon in the interdendritic spaces is taken into account. The secondary hardening precipitates can be described in satisfactory approximation on the basis of local two-phase equilibria between each carbide and the matrix, indicating that their compositions are essentially determined by the nucleation process.     

A new parameter for (normalized) evaluation of H-index: countries as a case study

It is known that the H-indexes of individuals, research groups, institutions, scientific journals, and countries strongly depend on the field of study, slowly increase with the number of publications, N, and can be described by empirical power-law functions of the type H?=?C?×?N^a (C and a are constants and depend on the specific field being analyzed). In this paper, we use this function and propose a new index [Montazerian–Zanotto–Eckert (MZE)], which is normalized by the number of publications and typically varies from ??1 to +?1, to characterize the relative standing of a research group, institution, or author to those of his/her peer groups. Due to the rich statistics available, as an example, here we analyzed and tested the new parameter against the citation-related performance (H-index) of countries. We found that the MZE index readily distinguishes between countries that stand above or below the average (for any given number of publications). Generally, publications of countries with a positive MZE index are more interesting or visible than the average. Analyzing publication output in this manner instead of the H-index allows for a less biased comparison between researchers, journals, universities, or countries for any particular combination of H-index and publication output.

     

New approaches to the Anderson lattice system

We present various approaches to the Anderson lattice model, starting from the limit of infinite spatial dimensions d. In this limit the lattice model can be mapped onto the single impurity model (SI) with an effective density of states determined from a selfconsistence condition (SCC). Similar approaches to the lattice in 3d were based on the non crossing approximation (NCA) for the SI, but differed in the SCC used. Relations between the different SCCs and new ones are given, results for transport quantities are discussed. A diagrammatical derivation of the SCC in infinite d is improved by including d ^–1 corrections.     

Structural characterization of non-oxide Chalcogenide Glasses using solid state NMR

In spite of the long-standing importance of non-oxide chalcogenide glasses in infrared optics and semiconductor technology, concepts describing the structural principles governing glass formation in these systems are just emerging. Most recently, modern quantitative solid state NMR techniques have offered new unique insights into the structural organization of these systems. In this review, we discuss the basic principles of various experimental approaches and their application to boron-silicon,- and phosphorus chalcogenide glasses.     

27Al Nuclear Magnetic Resonance of Glassy and Crystalline Zr(1−x)AlxO(2−x/2) Materials Prepared from Solution Precursors

The local environment of the aluminum atoms in a series of metastable Zr_{(1− x )}Al _x O_{(2− x/ 2)} crystalline materials (0.08 ≤ x ≤ 0.57), prepared by diffusion-limited crystallization of amorphous precursors, has been determined by ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). Results show the existence of aluminum in 4-, 5-, and 6-fold coordination in both the amorphous and crystalline states. Although the relative amounts of each type of coordination show no compositional dependence in the amorphous state, the results for the crystalline materials show a systematic decrease in the average aluminum coordination number with increasing aluminum content. Comparisons of MAS NMR results between pure Al₂O₃ precursors and Zr _{(1- x )}Al _x O_{(2- x /2)} crystalline materials processed under similar conditions show a profound effect of ZrO₂ on the coordination environment of the aluminum atom. Both a random distribution model and a model that assumes small-scale clustering of aluminum ions are considered to explain the trends in the type of aluminum coordination as a function of composition.