Nanocomposites: Industrial opportunity or challenge?

Polymer nanocomposites based on organically modified layered silicates are an area of substantial scientific interest and of emerging industrial practice. Despite the proven benefits of nanocomposites such as mechanical properties, barrier properties and contribution to fire retardancy, polymer nanocomposites are used today only in niche applications. The reasons for the limited growth of nanocomposites are explained through the availability of alternative solutions, processing and dispersion challenges and inferior oxidative and photooxidative stability. Recent developments show the improved dispersion of unmodified nanoclays in polyolefins with the help of selected copolymer structures. The (photo)oxidative instability of nanocomposites is compensated with adjusted stabilizer systems.     

Graphene: Safe or Toxic? The Two Faces of the Medal

Graphene is considered the future revolutionary material. For its development, it is of fundamental importance to evaluate the safety profile and the impact on health. Graphene is part of a bigger family which has been identified as the graphene family nanomaterials (GFNs). Clarifying the existence of multiple graphene forms allows better understanding the differences between the components and eventually correlating their biological effects to the physicochemical characteristics of each structure. Some in vitro and in vivo studies clearly showed no particular risks, while others have indicated that GFNs might become health hazards. This Minireview critically discusses the recent studies on the toxicity of GFNs to provide some perspective on the possible risks to their future development in materials and biomedical sciences.     

Prediction of the activity of β-lactam antibiotics

The pharmacological properties of penam, 3-cepheme, and semisynthetic penicillins were predicted by means of the ORAKUL automated system. A comparative evaluation of the similarity between the structures of these compounds and the structures of 8800 biologically active substances in the data base of the system made it possible to uncover the high probability of the manifestation of anti-inflammatory, analgesic, antitumorigenic, antiallergic, and anticoagulant activity by structural analogs of -lactam antibiotics.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 555–564, April, 1992.     

Antagonism of chemical genetic interaction networks resensitize MRSA to β-lactam antibiotics

Antibiotic drug resistance among hospital and community acquired methicillin resistant Staphylococcus aureus (MRSA) has dramatically eroded the efficacy of current therapeutics. We describe a chemical genetic strategy using antisense interference to broadly identify new drug targets that potentiate the effects of existing antibiotics against both etiological classes of MRSA infection. Further, we describe the resulting chemical genetic interaction networks and highlight the prominent and overlapping target sets that restore MRSA susceptibility to penicillin, cephalosporins, and carbapenems. Pharmacological validation of this approach is the potent synergy between a known inhibitor to a member of this genetic potentiation network (GlmS) and a broad set of β-lactam antibiotics against methicillin resistant Staphylococci. Developing drug-like leads to these targets may serve as rational and effective combination agents when paired with existing β-lactam antibiotics to restore their efficacy against MRSA.     

Confinement or the nature of the interface? Dynamics of nanoscopic water

The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (nonionic head group) in 50/50 wt % cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time-dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, are very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.     

The critical re-evaluation of the aromatic/antiaromatic nature of Ti3(CO)3: a missed opportunity?

The nature of bonding and aromaticity of Ti(3)(CO)(3), a mill-shaped metal-carbonyl complex, is studied carefully. A unique bonding mechanism between metal and carbonyl groups is found in this species. Ti(3)(CO)(3) is an example of a metal-carbonyl complex with prominent metal to carbonyl donation. Moreover, it is proven that not only is Ti(3)(CO)(3) not an antiaromatic complex but also it is the first synthesized example of d-block, σ+π aromatic species. A quick survey among the first row of transition metals in the periodic table shows that other local minima with similar structures and aromaticity are present and Ti(3)(CO)(3) is the first synthesized species of an unknown family.     

8-AminoBODIPYs: cyanines or hemicyanines? The effect of the coplanarity of the amino group on their optical properties

The role of the amino group twisting ability in the BODIPY photophysics for nonsterically hindered and constrained molecular structures was studied. When a coplanar disposition of the amino and the BODIPY core is feasible, a hemicyanine-like delocalized π-system gives rise to novel blue and efficient BODIPY laser dyes. The key role of such rotamer is confirmed by newly synthesized derivatives where the amino and the BODIPY core are electronically decoupled by steric repulsions.     

The Binding Pocket at the Interface of Multimeric Telomere G-quadruplexes: Myth or Reality?

Human telomeric DNA with hundreds of repeats of the 5’-TTAGGG-3’ motif plays a crucial role in several biological processes. It folds into G-quadruplex (G4) structures and features a pocket at the interface of two contiguous G4 blocks. Up to now no structural NMR and crystallographic data are available for ligands interacting with contiguous G4s. Naphthalene diimide monomers and dyads were investigated as ligands of a dimeric G4 of human telomeric DNA comparing the results with those of the model monomeric G4. Time-resolved fluorescence, circular dichroism, isothermal titration calorimetry and molecular modeling were used to elucidate binding features. Ligand fluorescence lifetime and induced circular dichroism unveiled occupancy of the binding site at the interface. Thermodynamic parameters confirmed the hypothesis as they remarkably change for the dyad complexes of the monomeric and dimeric telomeric G4. The bi-functional ligand structure of the dyads is a fundamental requisite for binding at the G4 interface as only the dyads engage in complexes with 1 : 1 stoichiometry, lodging in the pocket at the interface and establishing multiple interactions with the DNA skeleton. In the absence of NMR and crystallographic data, our study affords important proofs of binding at the interface pocket and clues on the role played by the ligand structure.     

Are natural protein networks similar to synthetic elastomeric networks? Structural implications of the characteristic pentapeptide repeat from the elastic protein network of wool

The elasticity of the cross-linked matrix of wool fibers is not well understood. The primary structure of high sulfur proteins of the matrix is known to be dominated by the pentapeptide repeat CysGln²Pro³Thr⁴Cys⁵. The linear proteins containing the repeat are cross-linked by multiple CysCys disulfide bonds to form an elastic network. The existing hypothesis proposes that the disulfide bonds are formed predominantly between Cys¹ and Cys⁵ of the same repeat, which is facilitated by the tight turn induced by the repeat. To elucidate the structural implications of the repeat, we applied solution ¹H NMR to model peptides containing the repeat and also dynamic light scattering and electron microscopy to the polymeric products of their oxidation. The NMR measurements revealed that the repeat in the model peptides does not induce tight turns (water, DMSO). The NOE patterns indicate that to form the Cys¹Cys⁵ intra-repeat disulfide bond, the Gln²-Pro³ peptide bond of the repeat changes its configuration from trans to cis. A substantial energy barrier associated with this transition should decrease the propensity of the repeat for the formation of the Cys¹Cys⁵ bonds. The NMR measurements in DMSO-d₆ revealed that for the repeat with a cycle closed by the Cys¹Cys⁵ bond, a significant population of the conformations adopt a rare type VIa tight turn with cis-Pro³ in position i +2 of the turn. These results suggest that a modified model of the matrix is needed and that the nature of elasticity of the cross-linked polypeptide network from wool fiber might be similar to that of synthetic polymer networks.     

Conductive metal-organic frameworks and networks: fact or fantasy?

Electrical conduction is well understood in materials formed from inorganic or organic building blocks, but their combination to produce conductive hybrid frameworks and networks is an emerging and rapidly developing field of research. Self-assembling organic-inorganic compounds offer immense potential for functionalising material properties for a wide scope of applications including solar cells, light emitters, gas sensors and bipolar transparent conductors. The flexibility of combining two distinct material classes into a single solid-state system provides an almost infinite number of chemical and structural possibilities; however, there is currently no systematic approach established for designing new compositions and configurations with targeted electronic or optical properties. We review the current status in the field, in particular, the range of hybrid systems reported to date and the important role of materials modelling in the field. From theoretical arguments, the Mott insulator-to-metal transition should be possible in semiconducting metal-organic frameworks, but has yet to be observed. The question remains as to whether electro-active hybrid materials will evolve from chemical curiosities towards practical applications in the near term.     

A theoretical study of beta-sheet models: is the formation of hydrogen-bond networks cooperative?

The cooperativity in terms of enthalpy contribution for beta-sheet formation of polyglycine models in a vacuum has been studied theoretically by using a repeating unit approach. No cooperativity is found in the parallel direction for both the parallel and antiparallel beta-sheets. Cooperativity in the perpendicular direction is dependent upon the residue number (m) in each beta-strand. While there is large cooperativity in the acetamide hydrogen-bond chain (m = 0), the cooperativity is not large in beta-sheet networks (m > 0). SCIPCM solvent model calculations also significantly reduce the cooperativity in hydrogen-bond chains. It is concluded that cooperativity is mainly due to long-range electrostatic interactions and not due to the resonance effect.     

The disarming effect of the 4,6-acetal group on glycoside reactivity: torsional or electronic?

An evaluation of whether the well-known deactivating effect of a 4,6-acetal protection group on glycosyl transfer is caused by torsional or an electronic effect from fixation of the 6-OH in the tg conformation was made. Two conformationally locked probe molecules, 2,4-dinitrophenyl 4,8-anhydro-7-deoxy-2,3,6-tri-O-methyl-beta-D-glycero-D-gluco-octopyranoside (18R) and the L-glycero-D-gluco isomer (18S), were prepared, and their rate of hydrolysis was compared to that of the flexible 2,4-dinitrophenyl 2,3,4,6-tetra-O-methyl-beta-D-glucopyranoside (21) and the locked 2,4-dinitrophenyl 4,6-O-methylidene-2,3-di-O-methyl-beta-D-glucopyranoside (26). The rate of hydrolysis at pH 6.5 was 21 > 18R > 18S > 26, which showed that the deactivating effect of the 4,6-methylene group is partially torsional and partially electronic. A comparison of the rate of acidic hydrolysis of the corresponding methyl alpha-glycosides likewise showed that the probe molecules 17S and 17R hydrolyzed significantly slower than methyl tetra-O-methyl-glucoside 19, confirming a deactivating effect of locking the saccharide in the (4)C(1) conformation. The experiments showed that the hydroxymethyl rotamers deactivate the rate of glycoside hydrolysis in the order tg > gt > gg.     

Analysis of the π-electronic structure of infinitely large networks

Summary Applying the Coulson and Longuet-Higgins integral method to polycyclic aromatics, the analytical solutions of bond orders, -electronic energy and benzene character for the infinitely large cyclic polyacene and polyphenanthrene with various modes of bond alternation are obtained in the HMO scheme. Most of the results are explicitly and newly expressed in terms of three kinds of the elliptic integrals. Judging from the magnitudes of bond orders and benzene character the most probable modes of the bond alternation for these two networks are discussed with their Kekulé structures. It was shown that if bond alternation is properly taken into consideration, HMO calculation can fairly well reproduce the results obtained by more sophisticated methods.     

Purification or contamination? The effect of sorbents on ionic liquids

Commonly used for purification, alumina and silica are found to contaminate ionic liquids with particles; these particles cannot be removed easily and can have a non-negligible impact on the electrochemical, spectroscopic and physical properties of an ionic liquid, including its nucleation and crystallisation kinetics.     

The application of Bimetallic metal–organic frameworks for antibiotics adsorption

Fe/Zr-base metal–organic frameworks(Fe/Zr-MOFs) were prepared using a solvothermal method from 1,3,5-phthalic acid (H₃BTC, 98 %) as the organic chain and ferrous heptahydrate (FeSO₄·7H₂O) and zirconium acetate Zr(CH₃COO)₄] as the metal ions. The resulting material was used to remove Doxycycline hydrochloride (DC). The experimental results showed that when the concentration of DC was 10 ppm and the mass of Zr/Fe-MOFs was 100 mg, the maximum removal rate after 5 h was 87.5 %. The results showed that the correlation coefficients (R²) of the pseudo-second-order kinetics model and Freundlich isotherm model of Zr/Fe-MOFs adsorption of DC were greater than 0.99, indicating good consistency. The results showed that the adsorption process of DC by Zr/Fe-MOFs was endothermic and spontaneous. Fe/Zr-MOFs had a high adsorption capacity for DC removal and good application prospects.     

Can instrumental neutron activation analysis keep pacewith the needs of the high purity materials industry?

The University of Missouri Research Reactor Center (MURR) has been one of the premier providers of neutron activation analysis (NAA) to the high purity materials industry for the past 20 years. Over the last two decades, significant advances in contamination control in the manufacturing process and the development of alternate analytical techniques have challenged the NAA community to keep pace. This paper presents an overview of the High Purity Materials Analysis Program at MURR. Specifically we present trends in the trace element concentrations that we have observed in our laboratory over the past 10 years and compare our experience with the relevant literature. The prospects for the future success of NAA and the methodological changes required for satisfying the industry's need will be discussed.     

Nanoscale heterogeneities in radiation-cured diacrylate networks: Weakness or asset?

The crosslinking polymerization of multifunctional monomers is known to yield brittle matrices, therefore limiting the development of this technique for the production of high performance composite materials. Among the various possible causes of the brittleness, the spontaneous formation of nanoheterogeneities during radiation-initiated polymerization is supported by atomic force microscopy imaging and by calorimetric analyses. The controlled polymerization-induced phase separation of nanosized clusters of polyethersulfone was evaluated as a means for alleviating the inherent tendency of the diacrylate materials to fragile failure. Various homogeneous formulations including the aromatic diacrylate monomers, and polyethersulfone together with a compatible reactive diluent were prepared and polymerized by electron beam irradiation. The resulting toughened materials show optimized critical stress intensity factor (K_Ic) over 2 MPa m^0.5, whereas the K_Ic value is about 1 MPa m^0.5 for the unmodified reference resin.