Chelating and bridging diphosphinoamine (PPh2)(2)N(Pr-i) complexes of copper(I)

The ligand bis(diphenylphosphino) isopropylamine (dppipa) has been shown to be a versatile ligand sporting different coordination modes and geometries dictated by copper(I). Most of the molecular structures were confirmed by X-ray crystallography. It is found in a chelating mode, in a monomeric complex when the ligand to copper ratio is 2:1. A tetrameric complex is formed when low ratios of ligand to metal (1: 2) were used. But with increasing ratios of ligand to metal (1: 1 and 2: 1), a trimer or a dimer was obtained depending on the crystallization conditions. Variable temperature P-31{H-1} NMR spectra of these complexes in solution showed that the Cu-P bond was labile and the highly strained 4-membered structure chelate found in the solid state readily converted to a bridged structures. On the other hand, complexes with the ligand in a bridging mode in the solid state did not form chelated structures in solution. The effect of adding tetra-alkylammonium salts to solutions of various complexes of dppipa were probed by P-31{H-1} NMR and revealed the effect of counter ions on the stability of complexes in solution. (C) 2008 Elsevier B.V. All rights reserved.

Conformation of di-n-propylglycine residues (Dpg) in peptides: crystal structures of a type I-turn forming tetrapeptide and an -helical tetradecapeptide

The crystal structures of two oligopeptides containing di-n-propylglycine (Dpg) residues, Boc-Gly-Dpg-Gly-Leu-OMe (1) and Boc-Val-Ala-Leu-Dpg-Val-Ala-Leu-Val-Ala-Leu-Dpg-Val-Ala-Leu-OMe (2) are presented. Peptide 1 adopts a type I-turn conformation with Dpg(2)-Gly(3) at the corner positions. The 14-residue peptide 2 crystallizes with two molecules in the asymmetric unit, both of which adopt -helical conformations stabilized by 11 successive 5 1 hydrogen bonds. In addition, a single 4 1 hydrogen bond is also observed at the N-terminus. All five Dpg residues adopt backbone torsion angles (, ) in the helical region of conformational space. Evaluation of the available structural data on Dpg peptides confirm the correlation between backbone bond angle NCC() and the observed backbone , values. For > 106° , helices are observed, while fully extended structures are characterized by < 106° . The mean values for extended and folded conformations for the Dpg residue are 103.6° ± 1.7° and 109.9° ± 2.6° , respectively.

DMT of Multi-hop Cooperative Networks-Part I: K-Parallel-Path Networks

We consider single-source, single-sink multi-hop relay networks, with slow-fading Rayleigh fading links and single-antenna relay nodes operating under the half-duplex constraint. While two hop relay networks have been studied in great detail in terms of the diversity-multiplexing tradeoff (DMT), few results are available for more general networks. In this two-part paper, we identify two families of networks that are multi-hop generalizations of the two hop network: K-Parallel-Path (KPP) networks and Layered networks. In the first part, we initially consider KPP networks, which can be viewed as the union of K node-disjoint parallel paths, each of length > 1. The results are then generalized to KPP(I) networks, which permit interference between paths and to KPP(D) networks, which possess a direct link from source to sink. We characterize the optimal DMT of KPP(D) networks with K >= 4, and KPP(I) networks with K >= 3. Along the way, we derive lower bounds for the DMT of triangular channel matrices, which are useful in DMT computation of various protocols. As a special case, the DMT of two-hop relay network without direct link is obtained. Two key implications of the results in the two-part paper are that the half-duplex constraint does not necessarily entail rate loss by a factor of two, as previously believed and that, simple AF protocols are often sufficient to attain the best possible DMT.

Simulation of Dust Loaded V-I Characteristics of a Commercial Thermal Power Plant Precipitator

A new approach based on finite difference method, is proposed for the simulation of electrical conditions in a dc energized wire-duct electrostatic precipitator with and without dust loading. Simulated voltage-curren characteristics with and without dust loading were compared with the measured characteristics for analyzing the performance of a precipitator. The simple finite difference method gives sufficiently accurate results with reduced mesh size. The results for dust free simulation were validated with published experimental data. Further measurements were conducted at a thermal power plant in India and the results compares well with the measured ones.

Investigations on voltages and currents in the lightning protection system of the Indian satellite launch pad-I during a stroke interception

A reliable protection against direct lightning hit is very essential for satellite launch pads. In view of this, suitable protection systems are generally employed. The evaluation of efficacy of the lightning protection schemes among others requires an accurate knowledge of the consequential potential rise at the struck point and the current injected into soil at the earth termination. The present work has made a detailed effort to deduce these quantities for the lightning protection scheme of the Indian satellite launch pad-I. A reduced scale model of the system with a frequency domain approach is employed for the experimental study. For further validation of the experimental approach, numerical simulations using numerical electromagnetic code-2 are also carried out on schemes involving single tower. The study results on the protection system show that the present design is quite safe with regard to top potential rise. It is shown that by connecting ground wires to the tower, its base current and, hence, the soil potential rise can be reduced. An evaluation of an alternate design philosophy involving insulated mast scheme is also made. The potential rise in that design is quantified and the possibility of a flashover to supporting tower is briefly looked into. The supporting tower is shown to have significant induced currents.

Hanle-Zeeman redistribution matrix. I. Classical theory expressions in the laboratory frame

Polarized scattering in spectral lines is governed by a 4; 4 matrix that describes how the Stokes vector is scattered and redistributed in frequency and direction. Here we develop the theory for this redistribution matrix in the presence of magnetic fields of arbitrary strength and direction. This general magnetic field case is called the Hanle- Zeeman regime, since it covers both of the partially overlapping weak- and strong- field regimes in which the Hanle and Zeeman effects dominate the scattering polarization. In this general regime, the angle-frequency correlations that describe the so-called partial frequency redistribution (PRD) are intimately coupled to the polarization properties. We develop the theory for the PRD redistribution matrix in this general case and explore its detailed mathematical properties and symmetries for the case of a J = 0 -> 1 -> 0 scattering transition, which can be treated in terms of time-dependent classical oscillator theory. It is shown how the redistribution matrix can be expressed as a linear superposition of coherent and noncoherent parts, each of which contain the magnetic redistribution functions that resemble the well- known Hummer- type functions. We also show how the classical theory can be extended to treat atomic and molecular scattering transitions for any combinations of quantum numbers.

Supramolecular Organization in Tetra Aqua (mu-8-Hydroxyquinoline-5-sulfonate) Barium (II) and Ag center dot center dot center dot I Interactions in a Pseudopolymorphic Form of (7-Iodo-8-hydroxyquinoline-5-sulfonate) Silver (I) Monohydrate

The complexes, Ba (HQS) (H2O)(4) (HQS = 8-hydroxyquinoline-5-sulfonic acid) (1) and Ag (HIQS) (H2O) (Ferron = 7-iodo-8-hydroxyquinoline-5-sulfonic acid) (2) have been synthesized and characterized by X-ray diffraction analysis and spectroscopic studies. In compound 1, Ba2+ ion has a nine-coordinate monocapped antiprismatic geometry. In compound 2, Ag+ has distorted tetrahedral coordination and Ag center dot center dot center dot I interactions generate the supramolecular architectures. The complexes have been characterized by FT-IR and UV-Visible measurements. In both the structures, the inversion-related organic ligands are stacked over one another leading to three-dimensional networks.

Perpetuation of immunological memory through common MHC-I binding modes of peptidomimic and antigenic peptides

Understanding the molecular mechanisms of immunological memory assumes importance in vaccine design. We had earlier hypothesized a mechanism for the maintenance of immunological memory through the operation of a network of idiotypic and anti-idiotypic antibodies (Ab2). Peptides derived from an internal image carrying anti-idiotypic antibody are hypothesized to facilitate the perpetuation of antigen specific T cell memory through similarity in peptide-MHC binding as that of the antigenic peptide. In the present work, the existence of such peptidomimics of the antigen in the Ab2 variable region and their similarity of MHC-I binding was examined by bioinformatics approaches. The analysis employing three known viral antigens and one tumor-associated antigen shows that peptidomimics from Ab2 variable regions have structurally similar MHC-I binding patterns as compared to antigenic peptides, indicating a structural basis for memory perpetuation. (C)) 2007 Elsevier Inc. All rights reserved.

Tuning the beta-turn segment in designed peptide beta-hairpins: Construction of a stable type I ' beta-turn nucleus and hairpin-helix transition promoting segments

Designed octapeptides Boc-Leu-Val-Val-Aib-(D)Xxx-Leu- Val-Val-OMe ((D)Xxx = (D)Ala, 3a; (D)Val, 3c and (D)Pro, 5a) and Boc-Leu-Phe-Val-Aib-DAla-Leu-Phe-Val-OMe (3b) have been investigated to construct models of a stable type I' beta-turn nucleated hairpin and to generate systems for investigating helix-hairpin conformational transitions. Peptide 5a, which contains a central Aib-(D)Pro segment, is shown to adopt a stable type I' beta-turn nucleated hairpin structure, stabilized by four cross-strand hydrogen bonds. The stability of the structure in diverse solvents is established by the observation of all diagnostic NOEs expected in a beta-hairpin conformation. Replacement of (D)Pro5 by (D)Ala/(D)Val (3a-c) results in sequences that form beta-hairpins in hydrogen bonding solvents like CD3OH and DMSO-d(6). However, in CDCl3 evidence for population of helical conformations is obtained. Peptide 6b (Boc-Leu-Phe-Val-Aib-Aib-Leu-Phe-Val-OMe), which contains a centrally positioned Aib-Aib segment, provides a clear example of a system, which exhibits a helical conformation in CDCl3 and a significant population of both helices and hairpins in CD3OH and DMSO-d(6). The coexistence of multiple conformations is established by the simultaneous observation of diagnostic NOEs. Control over stereochemistry of the central beta-turn permits generation of models for robust beta-hairpins and also for the construction of systems that may be used to probe helix-hairpin conformational transitions. (c) 2006 Wiley Periodicals, Inc.

Conformation of di-n-propylglycine residues (Dpg) in peptides: crystal structures of a type I 'beta-turn forming tetrapeptide and an alpha-helical tetradecapeptide

The crystal structures of two oligopeptides containing di-n-propylglycine (Dpg) residues, Boc-Gly-Dpg-Gly-Leu-OMe (1) and Boc-Val-Ala-Leu-Dpg-Val-Ala-Leu-Val-Ala-Leu-Dpg-Val-Ala-Leu-OMe (2) are presented. Peptide 1 adopts a type I' beta-turn conformation with Dpg(2)-Gly(3) at the corner positions. The 14-residue peptide 2 crystallizes with two molecules in the asymmetric unit, both of which adopt alpha-helical conformations stabilized by 11 successive 5 -> 1 hydrogen bonds. In addition, a single 4 -> 1 hydrogen bond is also observed at the N-terminus. All live Dpg residues adopt backbone torsion angles (phi, psi) in the helical region of conformational space. Evaluation of the available structural data on Dpg peptides confirm the correlation between backbone bond angle N-C-alpha-C' (tau) and the observed backbone phi,psi values. For tau > 106 degrees, helices are observed, while fully extended structures are characterized by tau < 106 degrees. The mean r values for extended and folded conformations for the Dpg residue are 103.6 degrees +/- 1.7 degrees and 109.9 degrees +/- 2.6 degrees, respectively. Copyright (C) 2007 European Peptide Society and John Wiley & Sons, Ltd.

The syntheses, characterizations, X-ray crystal structures and properties of Cu(I) complexes of a bis-bidentate schiff base ligand

Bis-bidentate Schiff base ligand L and its two mononuclear complexes [CuL(CH3CN)(2)]ClO4 (1)and [CuL(PPh3)(2)]ClO4 (2)have been prepared and thoroughly characterized by elemental analyses, IR, UV-Vis, NMR spectroscopy and X-ray diffraction analysis. In both the complexes the metal ion auxiliaries adopt tetrahedral coordination environment. Their reactivity, electrochemical and photophysical behavior have been studied. Complex 1 shows reversible Cu-II/I couple with potential 0.74 V versus Ag/AgCl in CH2Cl2. At room temperature L is weakly fluorescent in CH2Cl2, however in Cu(I)complexes 1 and 2 the emission in quenched. (C) 2009 Elsevier B. V. All rights reserved.

Studies of the polarographic and coulometric behaviour of aromatic nitro-compounds: I. Nitrobenzene in ethanol

A detailed polarographic (a.c. and d.c.) and coulometric investigation of nitrobenzene has been made at various pH values in the presence of different concentrations of ethanol. Below pH 4.7, two waves are apparent but above this pH, the second wave does not appear. Coulometric evidence indicates that the first and second waves correspond to the four-and two-electron processes, respectively. The coulometric method was not applicable in sodium hydroxide and sodium acetate solutions. When the diffusion coefficients (from the diaphragm cell) are used in the Ilkovic equation, no reliable conclusions can be reached for the number of electrons involved in the reduction process in alkaline solutions. The a.c. polarographic method gives evidence for the formation of species such as: C6H5NO2H22+, C6H5NO2− and C6H5NO22−. Analysis of d.c. polarographic data by Delahay's treatment of irreversible waves, indicates that the number of electrons involved in the rate-determining step is 2. In sodium hydroxide solutions, however, the first main wave is split indicating more than one rate-determining step. The results presented in this paper indicate that the first wave in the reduction of nitrobenzene is a four-electron process at all pH values. The second wave, which appears below pH 4.7, corresponds to a two-electron process irrespective of wave heights. The difference in the a.c. polarographic behaviour in acid and alkaline solutions has given evidence for the formation of species like C6H5NO2H2, C6H5NO2−, and C6H5NO22.