N,N′‐Dicyclohexyl‐N‐[4‐(1H‐indol‐3‐yl)butanoyl]urea

The title compound, C₂₅H₃₅N₃O₂, is a novel urea derivative. Pairs of intermolecular N—H...O hydrogen bonds join the molecules into centrosymmetric R₂²(12) and R₂²(18) dimeric rings, which are alternately linked into one‐dimensional polymeric chains along the [010] direction. The parallel chains are connected via C—H...O hydrogen bonds to generate a two‐dimensional framework structure parallel to the (001) plane. The title compound was also modelled by solid‐state density functional theory (DFT) calculations. A comparison of the molecular conformation and hydrogen‐bond geometry obtained from the X‐ray structure analysis and the theoretical study clearly indicates that the DFT calculation agrees closely with the X‐ray structure.     

Restructuring of colloidal cakes during dewatering

Aqueous suspensions of aggregated silica particles have been dewatered to the point where the colloidal aggregates connect to each other and build a macroscopic network. These wet cakes have been compressed through the application of osmotic pressure. Some cakes offer a strong resistance to osmotic pressure and remain at a low volume fraction of solids; other cakes yield at low applied pressures, achieving nearly complete solid/liquid separation. We used small angle neutron scattering and transmission electron microscopy to determine the processes by which the particles move and reorganize during cake collapse. We found that these restructuring processes follow a general course composed of three stages: (1) at all scales, voids are compressed, with large voids compressed more extensively than smaller ones; the local order remains unchanged; (2) all voids with diameters in the range of 2-20 particle diameters collapse, and a few dense regions (lumps) are formed; and (3) the dense lumps build a rigid skeleton that resists further compression. Depending on the nature of interparticle bonds, some cakes jump spontaneously into stage 3 while others remain stuck in stage 1. To elucidate the relation between bond strength and compression resistance, we have constructed a numerical model of the colloidal network. In this model, particles interact through noncentral forces that are produced by springs attached to their surfaces. Networks made of bonds that break upon stretching evolve through a plastic deformation that reproduces the three stages of restructuring evidenced by the experiments. Networks made of bonds that are fragile jump into stage 3. Networks made of bonds that can be stretched without breaking evolve through elastic compression and restructure only according to stage 1.     

Effect of surface charge on the colloidal stability and in vitro uptake of carboxymethyl dextran-coated iron oxide nanoparticles

An easy and novel routine are reported for the preparation of metallic silver nanoparticles (AgNPs) with controlled morphology, using Na⁺–magadiite swelled with hexadecyltrimethylammonium bromide (CTA⁺–magadiite) and a layered aluminophosphate with kanemite-type structure modified with n-dodecylammonium and n-butylammonium (but,dod-AlPO-kan) as hosts. For the preparation of the metallic AgNPs (Ag⁰) in the interlamellar space, the CTA⁺–magadiite and but,dod-AlPO-kan hosts were dispersed in N,N-dimethylformamide (DMF) solution with different AgNO₃ concentrations. DMF acts as reducing agent of Ag⁺ ions leading to nanoparticles with disk-like morphology of magadiite silicate; these were characterized by TEM and UV–Vis spectroscopy. On the other hand, the AgNPs are intercalated in but,dod-AlPO-kan showing spherical-like morphology. The UV–Vis spectra of the nanocomposites based on Ag⁰ and magadiite silicate show bands at 565 nm that can be attributed to Ag⁰ nanodisks. The Ag-but,dod-AlPO-kan-based nanocomposites present a band at 422 nm attributed to the surface plasmon resonance of Ag⁰ nanospheres. The results of transmission electron microscopy agree very well with XRD and UV–Vis analysis, indicating the formation of AgNPs with different morphologies using the two kinds of lamellar materials. The magadiite host has an important role in the synthesis of Ag nanodisks, because it controls the growth of nanoparticles inside the interlayer region with disk-like morphology due the high interlayer interactions of the silicate, leading to the growth of nanoparticles in only two directions (xy plane). On the other hand, when but,dod-AlPO-kan is used a sphere-like morphology is preferred due the best accommodation of AgNPs between the layers of aluminophosphate host.     

Diffraction structural biology--introductory overview

An introductory overview to the special issue papers on diffraction structural biology in this issue of the journal.     

Deriving the ultrastructure of α-crustacyanin using lower-resolution structural and biophysical methods

The low-resolution structure of α-crustacyanin has been determined to 30 ? resolution using negative-stain electron microscopy (EM) with single-particle averaging. The protein, which is an assembly of eight β-crustacyanin dimers, appears asymmetrical and rather open in layout. A model was built to the EM map using the X-ray crystallographic structure of β-crustacyanin guided by PISA interface analyses. The model has a theoretical sedimentation coefficient that matches well with the experimentally derived value from sedimentation velocity analytical ultracentrifugation. Additionally, the EM model has similarities to models calculated independently by rigid-body modelling to small-angle X-ray scattering (SAXS) data and extracted in silico from the β-crustacyanin crystal lattice. Theoretical X-ray scattering from each of these models is in reasonable agreement with the experimental SAXS data and together suggest an overall design for the α-crustacyanin assembly.     

Structural, spectroscopic and redox properties of uranyl complexes with a maleonitrile containing ligand

The reaction of uranyl nitrate hexahydrate with the maleonitrile containing Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (salmnt((Et(2)N)(2))H(2)) in methanol produces [UO(2)(salmnt((Et2N)2))(H(2)O)] (1) where the uranyl equatorial coordination plane is completed by the N(2)O(2) tetradentate cavity of the (salmnt((Et(2)N)(2)))(2-) ligand and a water molecule. The coordinated water molecule readily undergoes exchange with pyridine (py), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and triphenylphosphine oxide (TPPO) to give a series of [UO(2)(salmnt((Et(2)N)(2)))(L)] complexes (L = py, DMSO, DMF, TPPO; 2-5, respectively). X-Ray crystallography of 1-5 show that the (salmnt((Et(2)N)(2)))(2-) ligand is distorted when coordinated to the uranyl moiety, in contrast to the planar structure observed for the free protonated ligand (salmnt((Et(2)N)(2))H(2)). The Raman spectra of 1-5 only display extremely weak bands (819-828 cm(-1)) that can be assigned to the typically symmetric O=U=O stretch. This stretching mode is also observed in the infrared spectra for all complexes 1-5 (818-826 cm(-1)) predominantly caused by the distortion of the tetradentate (salmnt((Et(2)N)(2)))(2-) ligand about the uranyl equatorial plane resulting in a change in dipole for this bond stretch. The solution behaviour of 2-5 was studied using NMR, electronic absorption and emission spectroscopy, and cyclic voltammetry. Complexes 2-5 exhibit intense absorptions in the visible region of the spectrum due to intramolecular charge transfer (ICT) transitions and the luminescence lifetimes (< 5 ns) indicate the emission arises from ligand-centred excited states. Reversible redox processes assigned to the {UO(2)}(2+)/{UO(2)}(+) couple are observed for complexes 2-5 (2: E(1/2) = -1.80 V; 3,5: E(1/2) = -1.78 V; 4: E(1/2) = -1.81 V : vs. ferrocenium/ferrocene {Fc(+)/Fc}, 0.1 M Bu(4)NPF(6)) in dichloromethane (DCM). These are some of the most negative half potentials for the {UO(2)}(2+)/{UO(2)}(+) couple observed to date and indicate the strong electron donating nature of the (salmnt((Et(2)N)(2)))(2-) ligand. Multiple uranyl redox processes are clearly seen for [UO(2)(salmnt((Et(2)N)(2)))(L)] in L (L = py, DMSO, DMF; 2-4: 0.1 M Bu(4)NPF(6)) indicating the relative instability of these complexes when competing ligands are present, but the reversible {UO(2)}(2+)/{UO(2)}(+) couple for the intact complexes can still be assigned and shows the position of this couple can be modulated by the solvation environment. Several redox processes were also observed between +0.2 and +1.2 V (vs. Fc(+)/Fc) that prove the redox active nature of the maleonitrile-containing ligand.     

Chromium(III) stars and butterflies: synthesis, structural and magnetic studies of tetrametallic clusters

We report the synthesis, structures and magnetic properties of a series of chromium(III) metal-centered triangle (or "star") clusters, [Cr(4){RC(CH(2)O)(3)}(2)(4,4'-R'(2)-bipy)(3)Cl(6)] [R = Et, R' = H (2); R = HOCH(2), R' = H (3); R = Et, R' = (t)Bu (4)], prepared by two-step solvothermal reactions starting from [CrCl(3)(thf)(3)]. The product of the first stage of this reaction is the salt [Cr(bipy)(2)Cl(2)](2)[Cr(2)Cl(8)(MeCN)(2)] (1). In the absence of the diimine, a different family of tetrametallics is isolated: the butterfly complexes [Cr(4){EtC(CH(2)O)(3)}(2){NH(C(R)NH)(2)}(2)Cl(6)] (R = Me (5), Et (6), Ph (7)] where the chelating N-acetimidoylacetamidine NH(C(R)=NH)(2) ligands are formed in situ via condensation of the nitrile solvents (RCN) under solvothermal conditions. Magnetic measurements show the chromium stars to have an isolated S = 3 ground state, arising from antiferromagnetic coupling between the central and peripheral metal ions, analogous to the well-known Fe(III) stars. Bulk antiferromagnetic ordering is observed at 0.6 K. The butterfly complexes have a singlet ground state, with a low-lying S = 1 first excited state, due to dominant wing-body antiferromagnetic coupling.     

Short-Time Existence for Some Higher-Order Geometric Flows

We establish short-time existence and regularity for higher-order flows generated by a class of polynomial natural tensors that, after an adjustment by the Lie derivative of the metric with respect to a suitable vector field, have strongly parabolic linearizations. We apply this theorem to flows by powers of the Laplacian of the Ricci tensor, and to flows generated by the ambient obstruction tensor. As a special case, we prove short-time existence for a type of Bach flow.     

Crystal Structures of a Series of 1,1-Spiro-bis(1,2,3,4-tetrahydronaphthalene)-Based Derivatives

Abstract  

The structures of four spirobisnaphthalenes based monomers 1, 2, 3a and 3b are reported. Each compound represents a methoxylated precursor which after deprotection led to the formation of a monomer successfully used for the synthesis of Polymers of intrinsic microporosity. The spiro-centre represents the site of contortion that, since its rigidity, leads to inefficient packing in the solid state inducing microporosity in the final polymer. Compound 1 crystallized in the monoclinic P2/c space group with unit cell parameters a = 8.1659(19) ?, b = 7.5298(18) ?, c = 19.130(5) ?, β = 97.935(4)°, V = 1165.0(5) ?³, Z = 2, D = 1.210 Mg m⁻³. Compound 2 crystallized in the monoclinic P2₁/n space group with unit cell parameters a = 12.6940(9) ?, b = 7.7267(6) ?, c = 19.9754(15) ?, β = 97.220(1)°, V = 1943.7(3) ?³, Z = 4, D = 1.355 Mg m⁻³. Compound 3a crystallized in the monoclinic P2₁/c space group with unit cell parameters a = 16.8137(14) ?, b = 10.5577(9) ?, c = 31.344(3) ?, β = 103.618(1)°, V = 5407.5(8) ?³, Z = 8, D = 1.308 Mg m⁻³. Compound 3b crystallized in the monoclinic Pc space group with unit cell parameters a = 15.594 ?, b = 12.564 ?, c = 18.339 ?, β = 90.224(1)°, V = 3593.0 ?³, Z = 4, D = 1.236 Mg m⁻³.