An anisotropic fibre-network model for mechano-sorptive creep in paper

In this paper a simplified network model for mechano-sorptive creep is presented, which is a further development of an earlier paper [Strömbro, J., Gudmundson, P., 2008. Mechano-sorptive creep under compressive loading – a micromechanical model. International Journal of Solids and Structures 45 (9), 2420–2450.]. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys a constitutive law that is non-linear. Fibre kinks are included in order to capture experimental observations of larger mechano-sorptive effects in compression than in tension. Moisture dependent material parameters and anisotropy in the fibre distribution have been introduced. Theoretical predictions based on the model are compared to experimental results for an anisotropic paper both under tensile and compressive loading at varying moisture content and it is found that the important features in the experiments are captured by the model. Different kinds of drying conditions have also been examined.     

Behaviour and design considerations for continuous flow closed-open-closed liquid microchannels

This paper introduces a method of combining open and closed microchannels in a single component in a novel way which couples the benefits of both open and closed microfluidic systems and introduces interesting on-chip microfluidic behaviour. Fluid behaviour in such a component, based on continuous pressure driven flow and surface tension, is discussed in terms of cross sectional flow behaviour, robustness, flow-pressure performance, and its application to microfluidic interfacing. The closed-open-closed microchannel possesses the versatility of upstream and downstream closed microfluidics along with open fluidic direct access. The device has the advantage of eliminating gas bubbles present upstream when these enter the open channel section. The unique behaviour of this device opens the door to applications including direct liquid sample interfacing without the need for additional and bulky sample tubing.     

Cover Picture

The cover picture shows in the background the whole cell of a methanotrophic bacterium on which are superimposed components of methane monooxygenase (the structure of the hydroxylase component (top), one of the two four-helix bundles that house the catalytic diiron centers (left)) and a schematic diagram of the catalytic cycle by which the enzyme converts dioxygen and methane into methanol and water. More about this unusual enzyme system is reported by Lippard et al. on p. 2782 ff.     

Nickel-Ruthenium Mixed-Metal Carbonyl Clusters Syntheses and Solid State Structures of the Two Tetrahedral Clusters [PPh4][NiRu3(μ3 H)(CO)9(μ-CO)3] and [NEt4]2[NiRu3(CO)8(μ-CO)4]

Redox condensation of [Ru₃H(CO)₁₁]- with Ni(CO)₄, in tetrahydrofuran solution, under a nitrogen atmosphere, yields the tetranuclear anion [NiRu_H(CO)₁₁)-. Subsequent deprotonation with Bu'OK in acetonitrile solution leads to the formation of the related dianion. Both anions have been characterized by spectroscopic techniques, elemental analysis and single crystal X-ray diffraction. [PPh₄][NiRu₃H(CO)₁₂] crystallizes in the triclinic space group PI with unit cell dimensionsof a = 11.842(2) Å,b = 12.335(3) Å, c = 13.3080) Å,a = 91.89(2)°, = 93.35(1)°,y = 96.41(2)°, Z = 2, V= 1926.9(7) Å'. The NiRu₃, metal core of the molecule defines a distorted tetrahedron with nine terminal and three edge bridging carbonyl groups. The hydrido ligand was located by difference Fourier techniques and was found to bridge the NiRu₂ basal triangle at a distance of 0.88(6) A from this plane. Selected average distances and angles are: Ru-Ru = 2.839 Å, Ru-Ni = 2.640 Å, Ru-C, = 1.910 A,Ru-C _b = 2.084 Å, Ni-C _b = 2.022 Å, Ru-H = 1.77 Å, C-0, = 1.135 Å, C-O _b = 1.159 Å, M-C-O, = 176.3°,M-C--O _b = 139.3°;other distances are: Ni-C₁ = l.758(7) Å, Ni-H= 1.85(7) Å. [NEt₄]₂[NiRu₃(CO)₁₂] crystallizes in the orthorhombic space group Pnma (no. 62) with unit cell dimensions ofa=20.247(5) Å,b = 15.038(4)Å,c = 12.079(3) Å, Z=4, V=3678(2) A'. The molecule contains a tetrahedral NiRu₃ core with eight terminal and four edge bridging carbon monoxide groups which bridge the three Ni-Ru and one Ru-Ru bond. Average distances and angles are: Ru -Ru =2.3050A Ru-Ni 2.648 Å, Ru-C _t = 1.878 Å, Ru-C _b 2.045 Å, Ni-C _b = 2.055 Å, C-O _t = 1.145 Å, C-01,=1.157 Å, M-C-O,= 176.9°, M-C-O _b = 138.6°; other distance is: Ni-C _t = 1.754(10) Å,t = terminal,b = bridging.     

Poly(ε-caprolactone) macroligands with β-diketonate binding sites: synthesis and coordination chemistry

Dibenzoylmethane (dbm) initiators with one and two alcohol sites were used to generate dbm end-functionalized and dbm-centered poly(ε-caprolactone) macroligands (dbmPCL and dbmPCL₂) with low polydispersities (∼1.1). Chelation of polymeric ligands to metal ions (Eu³⁺, Fe³⁺, Ni²⁺ and Cu²⁺) produced metal-centered star polymers, which were characterized by UV-vis and fluorescence spectroscopy, as well as gel permeation chromatography.     

Protective effects of cyanidin-3-O-beta-glucopyranoside against UVA-induced oxidative stress in human keratinocytes

Ultraviolet-A (UVA) radiation causes significant oxidative stress because it leads to the generation of reactive oxygen species (ROS), leading to extensive cellular damage and eventual cell death either by apoptosis or necrosis. We evaluated the protective effects of cyanidin-3-O-beta-glucopyranoside (C-3-G) against UVA-induced apoptosis and DNA fragmentation in a human keratinocyte cell line (HaCaT). Treatment of HaCaT cells with C-3-G before UVA irradiation inhibited the formation of apoptotic cells (61%) and DNA fragmentation (54%). We also investigated antioxidant properties of C-3-G in HaCaT cells against ROS formation at apoptotic doses of UVA; C-3-G inhibited hydrogen peroxide (H2O2) release (an indicator of cellular ROS formation) after UVA irradiation. Further confirmation of the potential of C-3-G to counteract UVA-induced ROS formation comes from our demonstration of its ability to enhance the resistance of HaCaT cells to the apoptotic effects of both H2O2 and the superoxide anion (O2*-), two ROS involved in UVA-oxidative stress. Furthermore, in terms of Trolox Equivalent Antioxidant Activity, C-3-G treatment led to a greater increase in antioxidant activity in the membrane-enriched fraction than in the cytosol (55% vs 19%). The protective effects against UVA-induced ROS formation can be attributed to the higher membrane levels of C-3-G incorporation. These encouraging in vitro results support further research into C-3-G (and other anthocyanins) as novel agents for skin photoprotection.     

N-Heterocyclic carbene–carbodiimide (NHC–CDI) betaine adducts: synthesis,characterization, properties,and applications

N-Heterocyclic carbenes (NHCs) are an important class of reactive organic molecules used as ligands, organocatalysts, and σ-donors in a variety of electroneutral ylide or betaine adducts with main-group compounds. An emerging class of betaine adducts made from the reaction of NHCs with carbodiimides (CDIs) form zwitterionic amidinate-like structures with tunable properties based on the highly modular NHC and CDI scaffolds. The adduct stability is controlled by the substituents on the CDI nitrogens, while the NHC substituents greatly affect the configuration of the adduct in the solid state. This Perspective is intended as a primer to these adducts, touching on their history, synthesis, characterization, and general properties. Despite the infancy of the field, NHC–CDI adducts have been applied as amidinate-type ligands for transition metals and nanoparticles, as junctions in zwitterionic polymers, and to stabilize distonic radical cations. These applications and potential future directions are discussed.

N-heterocyclic carbene-carbodiimide betaine adducts are zwitterionic amidinate-like structures with tunable properties that have applications as ligands, junctions in supramolecular polymers, and stabilizers for radical cations.     

Tungsten(VI)-gluconic acid complexes: Polarimetric and13C-N.m.r. Studies in an excess of tungsten(VI)

Summary A polarimetric study of the tungsten (VI)-gluconic acid system in an excess of metal reveals the formation of four stable complexes: two monomers with 1 : 2 and 2: 1 stoichiometries and two dimers of 2:2 composition. The pH ranges of these species, the amount of acid equivalents their formation requires, and their conditional stability constants have also been calculated.The probable coordination of the organic ligand to the metallic centre in solutions containing different metal : ligand ratios has been investigated by¹³C-n.m.r. spectroscopy. The results confirm the formation of different complexes depending on the reagent which is in excess. The behaviour of the system when the metal is in excess is related to that of other polyhydroxylic ligands such as mannitol and sorbitol.     

Acidity of a Cu-bound histidine in the binuclear center of cytochrome C oxidase

Cytochrome c oxidase (CcO) is a crucial enzyme in the respiratory chain. Its function is to couple the reduction of molecular oxygen, which takes place in the Fea3-CuB binuclear center, to proton translocation across the mitochondrial membrane. Although several high-resolution structures of the enzyme are known, the molecular basis of proton pumping activation and its mechanism remain to be elucidated. We examine a recently proposed scheme (J. Am. Chem. Soc. 2004, 126, 1858; FEBS Lett. 2004, 566, 126) that involves the deprotonation of the CuB-bound imidazole ring of a histidine (H291 in mammalian CcO) as a key element in the proton pumping mechanism. The central feature of that proposed mechanism is that the pKa values of the imidazole vary significantly depending on the redox state of the metals in the binuclear center. We use density functional theory in combination with continuum electrostatics to calculate the pKa values, successively in bulk water and within the protein, of the Cu-bound imidazole in various Cu- and Cu-Fe complexes. From pKas in bulk water, we derived a value of -266.34 kcal.mol(-1) for the proton solvation free energy (Delta). This estimate is in close agreement with the experimental value of -264.61 kcal.mol(-1) (J. Am. Chem. Soc. 2001, 123, 7314), which reinforces the conclusion that Delta is more negative than previous values used for pKa calculations. Our approach, on the basis of the study of increasingly more detailed models of the CcO binuclear center at different stages of the catalysis, allows us to examine successively the effect of each of the two metals' redox states and of solvation on the acidity of imidazole, whose pKa is approximately 14 in bulk water. This analysis leads to the following conclusions: first, the effect of Cu ligation on the imidazole acidity is negligible regardless of the redox state of the metal. Second, results obtained for Cu-Fe complexes in bulk water indicate that Cu-bound imidazole pKa values lie within the range of 14.8-16.6 throughout binuclear redox states corresponding to the catalytic cycle, demonstrating that the effect of the Fe oxidation states is also negligible. Finally, the low-dielectric CcO proteic environment shifts the acid-base equilibrium toward a neutral imidazole, further increasing the corresponding pKa values. These results are inconsistent with the proposed role of the Cu-bound histidine as a key element in the pumping mechanism. Limitations of continuum solvation models in pKa calculations are discussed.     

Propagation of melting cooperativity along the phosphodiester backbone of DNA

The role of the DNA phosphodiester backbone in the transfer of melting cooperativity between two helical domains was experimentally addressed with a helix-bulge-helix DNA model, in which the bulge consisted of a varying number of either conformationally flexible propanediol or conformationally constrained bicyclic anucleosidic phosphodiester backbone units. We found that structural communication between two double helical domains is transferred along the DNA backbone over the equivalent of ca. 12-20 backbone units, depending on whether there is a symmetric or asymmetric distribution of the anucleosidic units on both strands. We observed that extension of anucleosidic units on one strand only suffices to disrupt cooperativity transfer in a similar way as if extension occurs on both strands, indicating that the length of the longest anucleosidic inset determines cooperativity transfer. Furthermore, conformational rigidity of the sugar unit increases the distance of coopertivity transfer along the phosphodiester backbone. This is especially the case when the units are asymmetrically distributed in both strands.