Generation of new landomycins by combinatorial biosynthetic manipulation of the LndGT4 gene of the landomycin E cluster in S. globisporus

A 3 kb DNA fragment from the Streptomyces globisporus 1912 landomycin E (LaE) biosynthetic gene cluster (lnd) was completely sequenced. Three open reading frames were identified, lndGT4, lndZ4, and lndZ5, whose probable translation products resemble a glycosyltransferase, a reductase, and a hydroxylase, respectively. Studies of generated mutants from disruption and complementation experiments involving the lndGT4 gene allowed us to determine that LndGT4 controls the terminal L-rhodinose sugar attachment during LaE biosynthesis and that LndZ4/LndZ5 are responsible for the unique C11-hydroxylation of the landomycins. Generation of the novel landomycins F, G, and H in the course of these studies provided evidence for the flexibility of lnd glycosyltransferases toward their acceptor substrates and a basis for initial structure-activity relationships within the landomycin family of antibiotics.     

The effects of sample preparation methods on the variability of the electrospray ionization response for model drug compounds

A post-column infusion system was developed in order to analyze suppression of electrospray ionization (ESI) tandem mass spectrometry response in the presence of endogenous plasma interferences. By enabling direct detection of these interfering components, this experimental system was used to analyze the ability of several common extraction procedures to remove endogenous plasma components that cause changes in the ESI response of model drug substances. Methyl-t-butyl ether (MTBE) liquid-liquid, Oasis and Empore solid-phase, and acetonitrile (ACN) protein precipitation sample preparation methods were tested using the post-column infusion system. In all cases, ACN protein precipitation samples showed the greatest amount of ESI response suppression while liquid-liquid extracts demonstrated the least. In addition, the three test compounds, phenacetin, caffeine, and a representative Merck compound, demonstrated that ESI response suppression is compound dependent. Suppression was greatest with caffeine, the most polar analyte, and the smallest for the Merck compound, the least polar analyte. Copyright 1999 John Wiley & Sons, Ltd.     

Gamma-ray spectroscopy with relativistic exotic heavy-ions

Feasibility of gamma-ray spectroscopy at relativistic energies with exotic heavy-ions and new generation of germanium detectors (segmented Clover) is discussed. An experiment with such detector array and radioactive is discussed.     

Percolative transport in the vicinity of charge-order ferromagnetic transition in a hole-doped manganite

We report measurements of non-linear charge transport in epitaxial (La_1−x Pr _x )_0.7Ca_0.3MnO₃ thin films fabricated on (100) oriented SrTiO₃ single crystals by pulsed laser deposition. The end members of this series, namely Pr_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3MnO₃ are canonical charge-ordered (CO) and ferromagnetic manganites, respectively. The onset of the CO state in Pr_0.7Ca_0.3MnO₃ is manifested by a pronounced insulating behavior below ∼ 200 K. The CO state remains stable even when a large (∼ 2×10⁵ V/cm) electric field is applied across the thin film samples. However, on substitution of Pr with La, a crossover from the highly resistive CO state to a state of metallic character is observed at relatively low electric fields. The current-voltage characteristics of the samples at low temperatures show hysteretic and history dependent effects. The electric field driven charge transport in the system is modelled on the basis of an inhomogeneous medium consisting of ferromagnetic metallic clusters dispersed in a CO background.     

Quadratic Hamilton–Poisson Systems on $\mathfrak{se}(1,1)^{*}_{-}$: The Inhomogeneous Case

The blow-up of smooth solution to the isentropic compressible Navier-Stokes-Poisson (NSP) system on \(\mathbb{R}^{d}\) is studied in this paper. We obtain that if the initial density is compactly supported, the spherically symmetric smooth solution to the NSP system on \(\mathbb{R}^{d}\ (d\geq 2)\) blows up in finite time. In the case \(d=1\), if \(2\mu +\lambda >0\), then the NSP system only exits a zero smooth solution on ? for the compactly supported initial density.     

Solvation and aggregation of n,n'-dialkylimidazolium ionic liquids: a multinuclear NMR spectroscopy and molecular dynamics simulation study

The solvation and aggregation of the ionic liquid (IL) 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl) in water and dimethylsulfoxide (DMSO) were examined by analysis of (1)H and (35/37)Cl chemical shift perturbations and molecular dynamics (MD) simulations. Evidence of aggregation of the IL n-butyl chains in aqueous environments at IL concentrations of 75-80 wt% was observed both in the NMR experiments and in the MD simulations. The studies also show that [C4mim]Cl behaves as a typical electrolyte in water, with both ions completely solvated at low concentrations. On the other hand, the data reveal that the interactions between the [C4mim](+) and Cl(-) ions strengthen as the DMSO content of the solutions increases, and IL-rich clusters persist in this solvent even at concentrations below 10 wt%. These results provide an experimentally supported atomistic explanation of the effects that these two solvents have on some of the macroscopic properties of [C4mim]Cl. The implications that these findings could have on the design of IL-based solvent systems are briefly discussed.     

Bonding in the metallic molecular solid α-Gallium

ABSTRACT

Solid, liquid and alloyed phases of gallium play a role in a variety of important technological applications. While many of the gallium phases involved in these applications are metallic, some have been proposed or are known to contain covalently bound Ga dimers. Thus, understanding the nature of bonding in Ga is crucial to the development of Ga-based materials. The solid phase of gallium at ambient conditions, α-Ga, is metallic and composed of molecular dimers, and can serve as a testing ground for studying gallium bonding with electronic structure calculations. We use density functional theory-based molecular dynamics simulations in conjunction with maximally localised Wannier functions to examine the nature of chemical bonding in α-Ga. We propose a geometric criterion for defining various bonding environments, which enables the quantification of covalent and weak bonds in solid gallium. We additionally connect the bonding structure of α-Ga to its phonon density of states and discuss similarities and differences with diatomic halogen crystals.     

Self-chemical ionization of diethylzinc

The self-chemical ionization of diethylzinc is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and semiempirical molecular orbital calculations. Electron impact of diethylzinc neutral produces the radical cation, C(4)H(15)Zn(+) (m/z x 122), which reacts further with the neutral (C(2)H(5))(2)Zn to give the following product ions: Zn(+) (m/z x 64), C(2)H(5)Zn(+) (m/z x 93), C(4)H(9)Zn(+) (m/z x 121), C(4)H(11)Zn(2)(+) (m/z x 187), and C(6)H(15)Zn(2)(+) (m/z x 215). To determine the structure and pathways for production of these ions, monoisotopic (12)C(4)H(15)(64)Zn(+), (64)Zn(+) and (12)C(2)H(5)(64)Zn(+) were individually isolated and reacted with the neutral background. We also performed semiempirical molecular orbital calculations (ZINDO/1). The molecular orbital calculations and experimental data are consistent in predicting that the ethyl group on the diethylzinc cation carries the positive charge. Copyright 1999 John Wiley & Sons, Ltd.     

The dynamic structure of jadomycin B and the amino acid incorporation step of its biosynthesis

Jadomycin B, an antifungal antibiotic with a unique 8H-benz[b]oxazolo[3,2-f]phenanthridine pentacyclic skeleton produced by the bacterium Streptomyces venezuelae ISP 5230, exists in a dynamic equilibrium of two diastereomers differing in the configuration of C-3a. Several novel jadomycins with various amino acid-derived 1-side chains could be generated, by replacing isoleucine in the production medium of S. venezuelae with other amino acids. These two findings led to the conclusion that a nonenzymatic reaction with the amino acid followed by a likewise nonenzymatic cyclization cascade are crucial for its late biosynthesis.     

Deconstructing Classical Water Models at Interfaces and in Bulk

Using concepts from perturbation and local molecular field theories of liquids we divide the potential of the SPC/E water model into short and long ranged parts. The short ranged parts define a minimal reference network model that captures very well the structure of the local hydrogen bond network in bulk water while ignoring effects of the remaining long ranged interactions. This deconstruction can provide insight into the different roles that the local hydrogen bond network, dispersion forces, and long ranged dipolar interactions play in determining a variety of properties of SPC/E and related classical models of water. Here we focus on the anomalous behavior of the internal pressure and the temperature dependence of the density of bulk water. We further utilize these short ranged models along with local molecular field theory to quantify the influence of these interactions on the structure of hydrophobic interfaces and the crossover from small to large scale hydration behavior. The implications of our findings for theories of hydrophobicity and possible refinements of classical water models are also discussed.