Molecular dynamics simulation of cellulose-coated oil-in-water emulsions

The behaviors of cellulose chains and cellulose mini-crystal in oil-in-water emulsions were studied by molecular dynamics simulations to investigate the coating states and the structural features of cellulose in these emulsions. In oil-in-water emulsion, dispersed cellulose chains gradually assemble during the progress of the simulation, eventually surrounding the octane droplet. In case of a cellulose mini-crystal, the cellulose chain at the corner of the crystal first contacts with the octane droplet through its hydrophobic surface. The other cellulose chains along the crystal plane then gradually move toward the octane molecules. In both emulsions, the cellulose was found to interact with both water and octane surfaces with specific conformations that allow the CH groups of the glucose rings to contact with octane molecules, while the OH groups of these rings contact with water molecules to form hydrogen bonds. The cellulose chains on the octane droplet also contact with each other through lateral hydrogen bonding between chains. These interactions stabilize the emulsion formed by cellulose molecules as surfactants.     

Domain-swapped cytochrome cb 562 dimer and its nanocage encapsulating a Zn–SO4 cluster in the internal cavity

Protein nanostructures have been gaining in interest, along with developments in new methods for construction of novel nanostructures. We have previously shown that c-type cytochromes and myoglobin form oligomers by domain swapping. Herein, we show that a four-helix bundle protein cyt cb ₅₆₂, with the cyt b ₅₆₂ heme attached to the protein moiety by two Cys residues insertion, forms a domain-swapped dimer. Dimeric cyt cb ₅₆₂ did not dissociate to monomers at 4 °C, whereas dimeric cyt b ₅₆₂ dissociated under the same conditions, showing that heme attachment to the protein moiety stabilizes the domain-swapped structure. According to X-ray crystallographic analysis of dimeric cyt cb ₅₆₂, the two helices in the N-terminal region of one protomer interacted with the other two helices in the C-terminal region of the other protomer, where Lys51–Asp54 served as a hinge loop. The heme coordination structure of the dimer was similar to that of the monomer. In the crystal, three domain-swapped cyt cb ₅₆₂ dimers formed a unique cage structure with a Zn–SO₄ cluster inside the cavity. The Zn–SO₄ cluster consisted of fifteen Zn²⁺ and seven SO₄ ^2– ions, whereas six additional Zn²⁺ ions were detected inside the cavity. The cage structure was stabilized by coordination of the amino acid side chains of the dimers to the Zn²⁺ ions and connection of two four-helix bundle units through the conformation-adjustable hinge loop. These results show that domain swapping can be applied in the construction of unique protein nanostructures.     

An instability criterion for activator-inhibitor systems in a two-dimensional ball II

Let B be a two-dimensional ball with radius R. We continue to study the shape of the stable steady states to

     

Rheology and UV-protecting properties of complex suspensions of titanium dioxides and zinc oxides

Ultra-fine particles of titanium dioxide (TiO(2)) and zinc oxides (ZnO) are very attractive as UV-protecting ingredients in cosmetic products. The UV-scattering behavior of complex suspensions in a silicone oil is studied in relation to rheological properties. To control the dispersion stability of suspensions, three polyoxyethylene (POE)-modified silicones of branch-type, (AB)(n)-type, and ABA-type are used as dispersants. Irrespective of molecular structure, the dispersants can stabilize the TiO(2) and ZnO particles and the flow of both single suspensions is Newtonian with low viscosity. However, the Newtonian flow profiles and high dispersion states are maintained only for complex suspensions prepared with ABA-type dispersant. Since the POE groups which are incorporated between terminal silicones groups attach to the particle surfaces, the steric stabilization is responsible for low viscosity and high dispersions. Because the UV scattering of suspensions is determined by the sizes of flocculated structures, the high transmittance in the visible ranges and low transmittance in the UVA and UVB ranges can be achieved in the presence of ABA-type dispersant.     

Synthesis, stabilities, and redox behavior of mono-, di-, and tetracations composed of di(1-azulenyl)methylium units connected to a benzene ring by phenyl- and 2-thienylacetylene spacers. A concept of a cyanine-cyanine hybrid as a stabilized electrochromic system

This paper describes the preparation of two tetracations 4a(4+) and 4b(4+) composed of di(1-azulenyl)methylium units based on a new structural principle of a cyanine-cyanine hybrid for the design of electrochromic materials with two color changes. Di- and monocations 5a(2+), 5b(2+) and 6a+, 6b+ composed of di(1-azulenyl)methylium units were also prepared for the purpose of comparison. The pKR+ values of the tetracations are rather high despite their tetracationic structure, although the stability of these cations decreases with the increase of the number of the existing cation units. The cyclic voltammetry (CV) of these cations revealed the presumed multielectron redox properties. However, the tetracations did not exhibit the idealized electrochemical behavior, in which subsequent two-electron reduction was presumed as the cyanine-cyanine hybrid, probably due to the less effective electrochemical interaction among the positive charges. The scope of the creation of the novel polyelectrochromic materials taking the new structural principle is demonstrated by these examples.     

Diastereoselective production of homoallylic alcohols bearing quaternary centers from gamma-substituted allylic indiums and ketones

Highly stereoselective In-employed addition of gamma-substituted allylic halides (cyclohexenyl halides, cinnamyl halides, and ethyl 4-bromocrotonate) to ketones is established to produce homoallyl alcohols bearing quaternary centers. The reactivity patterns and relative stability of allylic indiums were studied. The addition of water characteristically affected the reactions. Cyclohexenyl indium addition was completely disturbed, but a clear reaction was observed in the cinnamyl and crotonate-indium addition. In the case of ethyl 4-bromocrotonate, an interesting conversion of a gamma-adduct into an alpha-adduct was observed in anhydrous conditions.     

Total synthesis of (±)-debromoflustramine B and E and (±)-debromoflustramide B based on one-pot intramolecular Ullmann coupling and Claisen rearrangement

Total syntheses of (±)-debromoflustramine B and E and (±)-debromoflustramide B were accomplished using a common and versatile intermediate spirocyclic oxindole 14, which was concisely prepared through intramolecular Ullmann coupling and Claisen rearrangement from iodoindole 13 on a multigram scale.     

Structural requirements and reaction pathways in dimethyl ether combustion catalyzed by supported Pt clusters

The identity and reversibility of the elementary steps required for catalytic combustion of dimethyl ether (DME) on Pt clusters were determined by combining isotopic and kinetic analyses with density functional theory estimates of reaction energies and activation barriers to probe the lowest energy paths. Reaction rates are limited by C-H bond activation in DME molecules adsorbed on surfaces of Pt clusters containing chemisorbed oxygen atoms at near-saturation coverages. Reaction energies and activation barriers for C-H bond activation in DME to form methoxymethyl and hydroxyl surface intermediates show that this step is more favorable than the activation of C-O bonds to form two methoxides, consistent with measured rates and kinetic isotope effects. This kinetic preference is driven by the greater stability of the CH3OCH2* and OH* intermediates relative to chemisorbed methoxides. Experimental activation barriers on Pt clusters agree with density functional theory (DFT)-derived barriers on oxygen-covered Pt(111). Measured DME turnover rates increased with increasing DME pressure, but decreased as the O2 pressure increased, because vacancies (*) on Pt surfaces nearly saturated with chemisorbed oxygen are required for DME chemisorption. DFT calculations show that although these surface vacancies are required, higher oxygen coverages lead to lower C-H activation barriers, because the basicity of oxygen adatoms increases with coverage and they become more effective in hydrogen abstraction from DME. Water inhibits reaction rates via quasi-equilibrated adsorption on vacancy sites, consistent with DFT results indicating that water binds more strongly than DME on vacancies. These conclusions are consistent with the measured kinetic response of combustion rates to DME, O2, and H2O, with H/D kinetic isotope effects, and with the absence of isotopic scrambling in reactants containing isotopic mixtures of 18O2-16O2 or 12CH3O12CH3-13CH3O13CH3. Turnover rates increased with Pt cluster size, because small clusters, with more coordinatively unsaturated surface atoms, bind oxygen atoms more strongly than larger clusters and exhibit lower steady-state vacancy concentrations and a consequently smaller number of adsorbed DME intermediates involved in kinetically relevant steps. These effects of cluster size and metal-oxygen bond energies on reactivity are ubiquitous in oxidation reactions requiring vacancies on surfaces nearly saturated with intermediates derived from O2.     

Determination of the absolute configuration of sialic acids in gangliosides from the sea cucumber Cucumaria echinata

Enantiomeric pairs of sialic acid, D- and L-NeuAc (N-acetylneuraminic acid), were converted to D- and L-arabinose, respectively, by chemical degradation. Using this method, the absolute configuration of the sialic acid residues, NeuAc and NeuGc (N-glycolylneuraminic acid), in the gangliosides from the sea cucumber Cucumaria echinata was determined to be the D-form. Although naturally occurring sialic acids have been believed to be the D-form on the basis of biosynthetic evidence, this is the first report of the determination of the absolute configuration of the sialic acid residues in gangliosides using chemical methods.