Crystal structure of 3-acetylcoumarin-o-aminobenzoylhydrazone and synthesis, spectral and thermal studies of its transition metal complexes

Transition metal [Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)] complexes of a new Schiff base, 3-acetylcoumarin-o-aminobenzoylhydrazone were synthesized and characterized by elemental analyses, magnetic moments, conductivity measurements, spectral [Electronic, IR, H-1 and C-13 NMR, EPR] and thermal studies. The ligand crystallizes in the monoclinic system, space group P2(1)/n with a = 9.201(5), b = 16.596( 9), c = 11.517(6) angstrom, beta= 101.388(9)degrees, V = 1724.2 (17) angstrom(3) and Z = 4. Conductivity measurements indicated Mn(II) and Co(II) complexes to be 1 : 1 electrolytes whereas Ni(II), Cu(II), Zn(II) and Cd(II) complexes are non-electrolytes. Electronic spectra reveal that all the complexes possess four-coordinate geometry around the metal.

Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties

MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, α, β, γ, δ, and λ structures. Because these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or interlayers with gaps of different magnitudes. Because capacitance properties are due to tercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gaps to accommodate these ions, are expected to be useful for capacitance studies. In order to examine the dependence of capacitance on crystal structure, the present study involves preparation of these various crystal phases of MnO2 in nanodimensions and to evaluate their capacitance properties. Results of α-MnO2 prepared by a microemulsion route (α-MnO2(m)) are also used for comparison. Spherical particles of about 50 nm, nanorods of 30−50 nm in diameter, or interlocked fibers of 10−20 nm in diameters are formed, which depend on the crystal structure and the method of preparation. The specific capacitance (SC) measured for MnO2 is found to depend strongly on the crystallographic structure, and it decreases in the following order: α(m) > α δ > γ > λ > β. A SC value of 297 F g-1 is obtained for α-MnO2(m), whereas it is 9 F g-1 for β-MnO2. A wide (4.6 Å) tunnel size and large surface area of α-MnO2(m) are ascribed as favorable factors for its high SC. A large interlayer separation (7 Å) also facilitates insertion of cations in δ-MnO2 resulting in a SC close to 236 F g-1. A narrow tunnel size (1.89 Å) does not allow intercalation of cations into β-MnO2. As a result, it provides a very small SC.

Structure of polyamidoamide dendrimers up to limiting generations: A mesoscale description

The polyamidoamide (PAMAM) class of dendrimers was one of the first dendrimers synthesized by Tomalia and co-workers at Dow. Since its discovery the PAMAMs have stimulated many discussions on the structure and dynamics of such hyperbranched polymers. Many questions remain open because the huge conformation disorder combined with very similar local symmetries have made it difficult to characterize experimentally at the atomistic level the structure and dynamics of PAMAM dendrimers. The higher generation dendrimers have also been difficult to characterize computationally because of the large size (294852 atoms for generation 11) and the huge number of conformations. To help provide a practical means of atomistic computational studies, we have developed an atomistically informed coarse-grained description for the PAMAM dendrimer. We find that a two-bead per monomer representation retains the accuracy of atomistic simulations for predicting size and conformational complexity, while reducing the degrees of freedom by tenfold. This mesoscale description has allowed us to study the structural properties of PAMAM dendrimer up to generation 11 for time scale of up to several nanoseconds. The gross properties such as the radius of gyration compare very well with those from full atomistic simulation and with available small angle x-ray experiment and small angle neutron scattering data. The radial monomer density shows very similar behavior with those obtained from the fully atomistic simulation. Our approach to deriving the coarse-grain model is general and straightforward to apply to other classes of dendrimers.

Three-dimensional structure of heat shock protein 90 from Plasmodium falciparum: molecular modelling approach to rational drug design against malaria

We have recently implicated heat shock protein 90 from Plasmodium falciparum (PfHsp90) as a potential drug target against malaria. Using inhibitors specific to the nucleotide binding domain of Hsp90, we have shown potent growth inhibitory effects on development of malarial parasite in human erythrocytes. To gain better understanding of the vital role played by PfHsp90 in parasite growth, we have modeled its three dimensional structure using recently described full length structure of yeast Hsp90. Sequence similarity found between PfHsp90 and yeast Hsp90 allowed us to model the core structure with high confidence. The superimposition of the predicted structure with that of the template yeast Hsp90 structure reveals an RMSD of 3.31 angstrom. The N-terminal and middle domains showed the least RMSD (1.76 angstrom) while the more divergent C-terminus showed a greater RMSD (2.84 angstrom) with respect to the template. The structure shows overall conservation of domains involved in nucleotide binding, ATPase activity, co-chaperone binding as well as inter-subunit interactions. Important co-chaperones known to modulate Hsp90 function in other eukaryotes are conserved in malarial parasite as well. An acidic stretch of amino acids found in the linker region, which is uniquely extended in PfHsp90 could not be modeled in this structure suggesting a flexible conformation. Our results provide a basis to compare the overall structure and functional pathways dependent on PfHsp90 in malarial parasite. Further analysis of differences found between human and parasite Hsp90 may make it possible to design inhibitors targeted specifically against malaria.

Interlayer structure of iodide intercalated layered double hydroxides (LDHs)

The iodide-containing layered double hydroxides (LDHs) of Mg and Zn with AI crystallize by the inclusion of extensive positional disorder of I- ions in the interlayer region. I- ion given its poor charge to size ratio can neither screen effectively the positive charge nor participate in H-bonding with the metal hydroxide layers. Thereby the I- ions are not stabilized in sites close to the seat of positive charge of the metal hydroxide layers (6c), nor in sites that facilitate H-bonding (3b or 18h). On the other hand, OH- from water can do both and effectively displaces I- from the interlayer. (C) 2010 Elsevier Inc. All rights reserved.

Modular design of synthetic protein mimics. Crystal structure of two seven-residue helical peptide segments linked by .epsilon.-aminocaproic acid

Two seven-residue helical segments, Val-Ala-Leu-Aib-Val-Ala-Leu, were linked synthetically with an epsilon-aminocaproic acid (Acp) linker with the intention of making a stable antiparallel helix-helix motif. The crystal structure of the linked peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Acp-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe (1) shows the two helices displaced laterally from each other by the linker, but the linker has not folded the molecule into a close-packed antiparallel conformation. Two strong intermolecular NH...O = C hydrogen bonds are formed between the top of the lower helix of one molecule and the bottom of the upper helix in a laterally adjacent molecule to give the appearance of an extended single helix. The composite peptide with Boc and OMe end groups, C76H137N15O18.H2O, crystallize in space group P2(1) with a = 8.802 (1) angstrom, b = 20.409 (4) angstrom, c = 26.315 (3) angstrom, and beta = 90.72 (1)degrees; overall agreement R = 7.86% for 5030 observed reflections (\F(o)\ > 3-sigma(F)); resolution = 0.93 angstrom. Limited evidence for a more compact conformation in solution consistent with an antiparallel helix arrangement is obtained by comparison of the HPLC retention times and CD spectra of peptide 1 with well-characterized continuous helices of similar length and sequence.

Peptide-lanthanide interactions. Crystal structure of a europium(III)-triglycine complex

Crystals of Eu-(Gly-Gly-Gly).(H2O)5.(ClO4)3 are triclinic, spacegroup P1BAR with a = 9.123 (2), b = 11.185 (5), c = 11.426 (2) angstrom; alpha = 90.79 (2), beta = 98.08 (1), gamma = 98.57 (2)-degrees; Z = 2. The europium cation is surrounded by four oxygens from three different peptide units and four oxygens from water molecules. The geometry around the metal is a distorted bi-capped trigonal prism. The peptide backbone conformation in this complex is compared with those in the free peptide and in various metal complexes. Considerable differences are observed between Eu(III) and Ca(II) complexes of triglycine. (C) Munksgaard 1994.

Hydration and distortion of peptide helices in crystals. α-Helical structure of a dodecapeptide, Boc-(Ala-Leu-Aib)4-OMe

The dodecapeptide Boc-(Ala-Leu-Aib)(4)-OMe crystallized with two independent helical molecules in a triclinic cell. The two molecules are very similar in conformation, with a 3(10)-helix turn at the N-terminus followed by an alpha-helix, except for an elongated N(7)...O(3) distance in both molecules. All the helices in the crystal pack in a parallel motif. Eleven water sites have been found in the head-to-tail region between the apolar helices that participate in peptide-water hydrogen bonds and a network of water-water hydrogen bonds. The crystal parameters are as follows: 2(C58H104N12O15)+ca. 10H(2)O, space group P1 with a = 12.946(2), b = 17.321(3), c = 20.465(4) Angstrom, alpha = 103.12(2), beta = 105.63(2), gamma = 107.50(2)degrees, Z = 2, R = 10.9% for 5152 data observed > 3 sigma(F), resolution 1.0 Angstrom. In contrast to the shorter sequences [Karle et al. (1988)Proc. Natl. Acad. Sci. USA 85, 299-303] and Boc-(Ala-Leu-Aib)(2)-OMe [Karle et al. (1989) Biopolymers 28, 773-781], no insertion of a water molecule into the helix is observed. However, the elongated N---O distance between Ala(7) NH and Aib(3) CO in both molecules (molecule A, 3.40 Angstrom; molecule B, 3.42 Angstrom) is indicative of an incipient break in the helices. (C) Munksgaard 1994.

Crystal structure of dimeric FabZ of Plasmodium falciparum reveals conformational switching to active hexamers by peptide flips

The crystal structure of beta-hydroxyacyl acyl carrier protein dehydratase of Plasmodium falciparum (PfFabZ) has been determined at a resolution of 2.4 angstrom. PfFabZ has been found to exist as a homodimer (d-PfFabZ) in the crystals of the present study in contrast to the reported hexameric form (h-PfFabZ) which is a trimer of dimers crystallized in a different condition. The catalytic sites of this enzyme are located in deep narrow tunnel-shaped pockets formed at the dimer interface. A histidine residue from one subunit of the dimer and a glutamate residue from the other subunit lining the tunnel form the catalytic dyad in the reported crystal structures. While the position of glutamate remains unaltered in the crystal structure of d-PffabZ compared to that in b-PfFabZ, the histidine residue takes up an entirely different conformation and moves away from the tunnel leading to a His-Phe cis-trans peptide flip at the histidine residue. In addition, a loop in the vicinity has been observed to undergo a similar flip at a Tyr-Pro peptide bond. These alterations not only prevent the formation of a hexamer but also distort the active site geometry resulting in a dimeric form of FabZ that is incapable of substrate binding. The dimeric state and an altered catalytic site architecture make d-PfFabZ distinctly different from the FabZ structures described so far. Dynamic light scattering and size exclusion chromatographic studies clearly indicate a pH-related switching of the dimers to active hexamers. (c) 2006 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserv.

Crystal structure of peanut lectin, a protein with an unusual quaternary structure

The x-ray crystal structure of the tetrameric T-antigen-binding lectin from peanut, M(r) 110,000, has been determined by using the multiple isomorphous replacement method and refined to an R value of 0.218 for 22,155 reflections within the 10- to 2.95-A resolution range. Each subunit has essentially the same characteristic tertiary fold that is found in other legume lectins. The structure, however, exhibits an unusual quaternary arrangement of subunits. Unlike other well-characterized tetrameric proteins with identical subunits, peanut lectin has neither 222 (D2) nor fourfold (C4) symmetry. A noncrystallographic twofold axis relates two halves of the molecule. The two monomers in each half are related by a local twofold axis. The mutual disposition of the axes is such that they do not lead to a closed point group. Furthermore, the structure of peanut lectin demonstrates that differences in subunit arrangement in legume lectins could be due to factors intrinsic to the protein molecule and, contrary to earlier suggestions, are not necessarily caused by interactions involving covalently linked sugar. The structure provides a useful framework for exploring the structural basis and the functional implications of the variability in the subunit arrangement in legume lectins despite all of them having nearly the same subunit structure, and also for investigating the general problem of "open" quaternary assembly in oligomeric proteins.

Local structure of Sr(2)FeMoxW1-xO(6) double perovskites studied by EXAFS

Sr2FeMoO6 double perovskits display low field MR at a relatively high temperature and unusual ferromagnetic properties. These compounds depicts metal to insulator transition increasing x above x(c) similar to 0.25. A comparative analysis of the near edge regions (XANES) suggests that iron is Fe3+ in the metallic range. Checking the end compounds, we found that the doped samples can be viewn as inhomogeneous distributions of the end compounds. This could help to distinguish between the two scenarios proposed to explain the metal to insulator transition. Moreover, the local atomic structure of Sr2FeMoxW1-xO6 as a function of composition (0 <= x <= 1) has been investigated by Extended X-ray absorption spectroscopy (EXAFS) a the Fe, Mo, Sr K-edges andW L-III-edge.

Structure of Ferroverdin

FERROVERDIN, a green iron-containing pigment, was isolated in 1955 by Chain, Tonolo and Carilli1 from an unidentified species of Streptomyces. It was at first assigned the formula C30H24O8N2Fe and the iron was shown by measurements of magnetic susceptibility to be in the ferrous state2. Later the ligand present was proved to be the p-vinyl phenyl ester of 3-nitroso-4-hydroxy-benzoic acid3,4. X-ray crystallographic measurements were undertaken to find the atomic arrangement in this unusual complex; they show, in two different crystal structures, that each iron atom is attached to three nitrosophenyl ligands and that the charge is balanced by sodium ions.

Approximate solutions for the structure of shock waves

Approximate solutions of the B-G-K model equation are obtained for the structure of a plane shock, using various moment methods and a least squares technique. Comparison with available exact solution shows that while none of the methods is uniformly satisfactory, some of them can provide accurate values for the density slope shock thickness delta n . A detailed error analysis provides explanations for this result. An asymptotic analysis of delta n for largeMach numbers shows that it scales with theMaxwell mean free path on the hot side of the shock, and that their ratio is relatively insensitive to the viscosity law for the gas.

Dipole moments and structure of some organoselenium compounds

The dipole moments of di-p-tolyl selenide (1.74 D), di-p-tolyl selenide (1.00 D), di-m-tolyl selenide (1.66 D), di-p-anisyl selenide (2.35 D) and di-p-tolyl selenium dichloride (3.69 D) have been determined in benzene at 35°. The results are analysed in terms of mesomeric effects and internal rotation in these systems. The dipole moments of a few aliphatic selenides have been theoretically evaluated.