Noteworthy Chemistry

October 21, 2013

 

A Vietnamese starfish produces anti-inflammatory compounds. Starfish historically have been used in Vietnamese folk medicines and medicinal foods. There are no reports, however, of the chemical constituents and biological activity of the common edible starfish Astropecten monacanthus. A recent investigation uncovered a series of asterosaponins from this starfish that have significant anti-inflammatory activity.

C. V. Minh, Y. H. Kim, and coauthors at the Vietnam Academy of Science and Technology (Hanoi), Chungnam National University (Daejeon, Korea), Jeju National University (Korea), and Nhatrang Institute of Technology Research and Application (Vietnam) extracted A. monacanthus with MeOH. They then suspended the extract in water and successively partitioned it with EtOAc and n-BuOH. They chromatographically separated the n-BuOH fraction to produce the six asterosaponin compounds shown in the figure.

Six asterosaponins from a Vietnamese starfish

The previously known compounds psilasteroside (4) and marthasteroside (6) were identified by their physical and spectroscopic properties. The authors characterized compounds 1, 2, 3, and 5 by using Fourier transform cyclotron resonance MS and 1H and 13C NMR. Compounds 1, 2, and 4 contain the same oligosaccharide chain. Compounds 14 have the same steroidal nucleus but different side chains. Compounds 5 and 6 have similar steroidal nuclei and pentasaccharide chains but different side chains.

The authors evaluated the anti-inflammatory effects of the MeOH extract and n-BuOH fractions by measuring the production of the pro-inflammatory cytokines IL-12 p40, IL-6, and TNF-α in lipopolysaccharide-stimulated bone marrow–derived dendritic cells, with the following results:

  • Compound 5 has potent inhibitory effects on all three cytokines.
  • Compounds 1 and 6 significantly inhibit IL-6 production.
  • Compound 3 shows moderate inhibitory effects on IL-6 production.
  • Compounds 2 and 4 do not significantly reduce any of the three pro-inflammatory cytokines.

In vivo testing is required to ascertain whether the asterosaponin compounds retain their bioactivity after oral ingestion or if they are inactivated by the rapid cleavage of the oligosaccharide chains. (J. Nat. Prod. 2013, 76, 1764–1770; Nancy McGuire)

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It’s all about location for light-activated colloidal particles. A. J. C. Kuehne* and co-workers at RWTH Aachen University (Germany) synthesized photoswitchable, monodisperse colloidal particles by using the Suzuki–Miyaura dispersion polymerization of a fluorenediboronic acid ester with 3,5-dibromoazobenzene units.

The authors examined the effects of placing the azobenzene unit along the polymer main chain (P1), perpendicular to the main chain (P2), or delocalized from the main chain via a hexyl linker (P3). Precipitation of the conjugated polymeric particles occurs at a critical molecular weight, which varies from flat platelets with columnar packing (P1) to amorphous, spherical colloids (P2 and P3) and depends on side chain flexibility and the placement of the azobenzene derivative. Below a threshold concentration, the particle sizes of the P1 and P2 polymers are tunable; and low polydispersity is maintained.

Radiation-induced switchability between the trans and cis conformations of the azo groups is considerably faster for P1 compared with P2 and P3 because the azo functionalities are incorporated along the backbone. Light stimulation also elongates the P1 platelets irreversibly. In contrast, the spherical P2 and P3 colloids maintain their shapes upon irradiation. The conformational switching of P3 occurs at higher energies than the others because of its decoupled electronic state.

The authors believe that their findings have applications in such diverse fields as coatings, imaging, and electronics. (ACS Macro Lett. 2013, 2, 766–769; LaShanda Korley)

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“Draw” gas sensors on paper with PENCIL. PENCILs, short for process-enhanced nanocarbons for integrated logic, are materials composed of nanostructured carbon and chemical selectors that sense specific analytes. T. M. Swager and colleagues at MIT (Cambridge, MA) used PENCILs to make paper-based chemical resistor gas sensors by a rapid, solvent-free process.

Selectors used in PENCIL- and paper-based VOC sensors

The authors chose nanotubes and graphite as the carbon sources and selectors 14 for their PENCILs to target common volatile organic compounds (VOCs). Selectors 14 exhibit diverse conductivity responses to various gaseous analytes because of their distinct chemical properties.

Sensor preparation consists of two steps:

  1. Nanostructured carbon is ball-milled with one or more selectors, and the mixture is compressed into the shape of a conventional pencil "lead".
  2. The PENCIL is mechanically abraded to deposit the nanocomposites onto the surface of cellulose-based paper chips that are equipped with gold electrodes (the "drawing" step).

This process can be completed in <15 min.

Sensors based on various nanocarbons and selector 1 can be used to differentiate and quantify vapors of acetone, THF, and dimethyl methanephosphonate (a flame retardant) at the ppm level. Cross-reactive arrays made from a combination of selectors (e.g., 14) can sense a variety of gases and VOCs. This protocol may open a pathway to next-generation, customizable gas sensors and monitors. (Proc. Natl. Acad. Sci. U.S.A. 2013, 110, Early Edition; Xin Su)

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Control structural color in cellulose nanocrystal–resin composites. M. J. MacLachlan and coauthors at the University of British Columbia and FPInnovations (both in Vancouver) prepared cellulose nanocrystal (CNC)–amino resin composites with the goal of varying structural coloring by controlling the composite formulation. Specifically, they mixed a basic solution of melamine–urea–formaldehyde (MUF) resin precursor with an aqueous CNC dispersion derived from wood pulp to produce flexible, iridescent CNC composites (≈60 wt%) in a chiral nematic arrangement within the MUF framework.

When the authors increased the ionic strength of the composites by adding NaCl, the composites’ color shifted from red to blue. They attribute this phenomenon to changes in the helical pitch of the CNC assembly. They also emphasize that the flexibility of the composites provides another option for manipulating their photonic behavior. For example, mechanical pressing induces a change from red to blue as observed in UV–vis spectra.

Curing the pressed sample produces a dimensionally stable, blue-shifted CNC–MUF composite film with an intact chiral structure that has a smaller helical pitch. The authors also showed that chiral nematic patterns can be imprinted into composite films by using a stamp. They believe that this technique can be used in value-added markets such as security and sensing. (ACS Macro Lett. 2013, 2, 818–821; LaShanda Korley)

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Copper wires promote fog collection. Artificial fog collection may be an effective way to convert water vapor to bulk liquid water for alleviating water shortages. Fog collection technology relies primarily on materials with different surface wettabilities and needs external forces to function properly. Environmental fluctuations can compromise the efficiency of these systems.

Inspired by the moisture-collecting mechanism of cacti, L. Jiang and coauthors at the Chinese Academy of Sciences (Beijing) and Jilin University (China) developed conical copper wires (CCWs) that efficiently and continuously collect water from fog.

The authors prepared CCWs by using the gradient electrochemical corrosion of cylindrical copper wires. The CCWs are plated with a thin layer of gold nanoparticles and then modified with a gradient of hydrophobic to hydrophilic alkanethiols so that the cones are increasingly wettable from the tip to the base.

The CCWs’ performance is faster and more efficient than purely hydrophobic or purely hydrophilic surfaces. Mechanistically, the hydrophobic tips of the CCWs promote the growth of water droplets, and the conical shape and wettability gradient accelerate the directional transport of water because of Laplace forces and the chemical driving force, respectively. (Adv. Mater. 2013, 25, Early View; Xin Su)

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