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Noteworthy Chemistry

September 30, 2013

What makes white wine turbid? Protein aggregation can cause beverages to become turbid. This is a serious problem for white wines because it downgrades their appearance. Although aggregation is often associated with protein unfolding that is induced by high temperatures, other environmental parameters also can be involved.

As part of efforts directed toward identifying the mechanisms of protein aggregation in white wines, C. Poncet-Legrand and coauthors at INRA (Montpellier), Montpellier SupAgro, the University of Montpellier 1, Synchrotron SOLEIL (Gif-sur-Yvette), and URBIA-Nantes (all in France) determined the effects of pH on the conformational stability of white-wine proteins.

The authors first isolated three of the four proteins that they studied from a Sauvignon white wine by using cation-exchange chromatography followed by size-exclusion chromatography. The fourth protein was provided by an Australian colleague. Of the four proteins—an invertase, a thaumatin-like protein, and two chitinases (24.2 and 27.5 kDa)—the first two do not aggregate, and the chitinases are heat-sensitive.

The authors found that the secondary structures of each of the four proteins are almost identical at pH 4.0 and pH 2.5. The pH difference, however, affects the local structures of the chitinases, causing 50% and 16% increases in fluorescence intensity for the 24.2-kDa and 27.5-kDa chitinases, respectively (see figure). The fluorescence of the other two proteins does not change under the same conditions.

Changes in chitinase fluorescence intensity as a result of pH change

The authors believe that pH-induced local conformational changes may reveal the buried hydrophobic sites in chitinases and lead to the formation of aggregates. (Langmuir 2013, 29, 10475–10482 ; Xin Su)

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Convert oximes to chiral acetamides in two efficient, scalable steps. W. Li and co-workers at Boehringer Ingelheim Pharmaceuticals (Ridgefield, CT) improved the reductive acetylation of oximes to N-acetylenamides. The acetylation reaction is followed by an asymmetric hydrogenation of the N-acetylenamide to a chiral acetamide.

Fe(OAc)2 mediates the reductive acetylation reaction, but its commercial availability is limited, and it is unstable in air. The authors generated Fe(OAc)2 in situ by heating iron powder in refluxing HOAc for 3–4 h. Ac2O and the oxime were then added to carry out the reductive acetylation.

The asymmetric reduction uses an extremely efficient [Rh(norbornadiene)(MeO-BIPOP)][BF4] catalyst. Catalyst loadings as low as 0.0005 mol % (turnover number 200,000) give >95:5 er and 100% conversion in 20 h. The authors demonstrated the method on a 3.376-kg scale by producing (S)-N-[1-(4-bromophenyl)ethyl]acetamide in 84% isolated yield and 99.9:0.1 er after crystallization. (Org. Process Res. Dev. 2013, 17, 1061–1065; Will Watson)

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Wholly aromatic polyimides fluoresce efficiently in the solid state. Polymers made from aromatic dianhydrides and diamines are commonly called wholly aromatic polyimides. They are used as engineering plastics in high-tech fields, for example, as thin-film orientation layers in liquid-crystal display (LCD) systems.

If polyimide films can be made to fluoresce efficiently, energy-consuming backlights may not be necessary in LCD devices. Wholly aromatic polyimides in the solid state, however, rarely emit with appreciable fluorescence quantum yields (ΦF).

G.-S. Liou and co-workers at National Taiwan University (Taipei) developed a series of efficient solid-state emitters based on wholly aromatic polyimides (see figure). The polymers are soluble in polar aprotic solvents, macroscopically processible, and thermally stable, with decomposition temperatures as high as 550 ºC. 

Wholly aromatic polyimides that fluoresce in the solid state

The polyimides’ solutions are weakly fluorescent, but they become emissive when they form aggregates in poor solvents or are made into thin films by solution casting. This is yet another example of aggregation-induced emission (AIE). The electrospun nanofibers of the AIE-active polyimides exhibit ΦF values up to 26%, a record emission efficiency for polyimides. (Adv. Opt. Mater. 2013, 1, 668–676; Ben Zhong Tang)

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Antifouling glycopeptoids mimic the cell’s glycocalyx. P. B. Messersmith and colleagues at Northwestern University (Evanston, IL) used glycocalyx glycoprotein–inspired biomimetic design to make glycopeptoids for antifouling applications. Using solid-phase peptide synthesis, they produced a glycocalyx mimic with a polypeptide main chain connected to an anchoring peptide. The chain had oligosaccharide (glucose or β-D-maltose) side groups that were introduced via click chemistry.

The authors investigated fouling behavior on a TiO2 substrate that was functionalized by grafting the peptide anchor with the peptoid mimics. The uniform ≈30-μm thickness was independent of the type of saccharide. The functionalized substrate was less hydrophilic than the original.

The authors found that the glycocalyx-inspired peptoid significantly reduced fibrinogen adsorption and overall cell and bacterial adhesion for up to 7 days. They believe that an interfacial water layer provides a barrier against protein adsorption. This was confirmed by atomistic molecular dynamic simulations, which illustrated the influence of the large number of hydrogen-bonding sites.

The authors show that the lifetime of the hydrogen bonding between the saccharide moieties and water is greatly increased by the antifouling peptoids. Control experiments using acetylated glycopeptoids suggest that a link exists between hydrogen-bonding sites, hydration layer stability, and antifouling capabilities. The authors believe that complementary steric repulsion is also a key component in the design of protein-resistant mimics. (J. Am. Chem. Soc. 2013, 135, 13015–13022 ; LaShanda Korley)

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“Legacy” mercury limits benefits from closing down smelters. For 80 years, Manitoba's Flin Flon copper smelter (see figure) was Canada's largest point source of mercury emissions. The smelter was closed in 2010, but its legacy lives on in the air, water, and soil.

Map of Flin Flon, MB, copper smelter

C. S. Eckley, M. T. Parsons, and colleagues at Environment Canada (Toronto; Edmonton, AB; and Dartmouth, NS), the US Environmental Protection Agency (Seattle, WA), and the University of Alberta (Edmonton) measured total gaseous mercury (TGM) in the air, mercury in precipitation, and ancillary parameters in the area around the smelter to assess any lingering effects on the environment.

The Flin Flon smelter is an ideal case study because data could be collected before and after the smelter closed and because the nearest industrial releases of any significance are >450 km to the southwest in Saskatoon, SA. While the smelter was operating, TGM levels (4.1 ± 3.7 ng/m3) in the air near the smelter were ≈3-fold greater than those at other Canadian monitoring stations. They decreased 20% during the year after it closed and have remained at twice the background levels since then. Concentrations before and after closure were highly variable, and there were frequent concentration peaks. Similar trends were observed for mercury in precipitation.

Evidence indicates that contaminated soil and tailings, which were the largest contributors to local TGM concentrations during and after smelter operations, are re-emitting mercury into the air. During smelter operations, stack emissions were a more significant contribution to the regional and global mercury pool.

After the smelter closed, the absolute magnitude of surface-air TGM flux decreased, but it contributed more on a percentage basis to the TGM in the air. The authors make no estimate of how long the soil and tailings will continue to re-emit mercury into the air, and they recommend further investigation. (Environ. Sci. Technol. 2013, 47, 10339–10348; Nancy McGuire)

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Image molecular orbitals in single-molecule magnets. Single-molecule magnets (SMMs) are a class of materials with unique quantum magnetic properties that are useful in electromagnetic applications. As SMMs have become increasingly popular, the importance of obtaining detailed structural information about them has likewise increased.

K. Park, J. Wang and coauthors at Southwest University (Chongqing, China), Virginia Tech (Blacksburg), and Tsinghua University (Beijing) observed molecular orbitals (MOs) in a Mn12 SMM via a tip-deposition method that uses low-temperature scanning tunneling microscopy (STM).

The authors used a tungsten STM tip to load Mn12 [complete formula Mn12O12(OAc)16(H2O)4] SMMs that consist of Mn12O12 cores surrounded by acetate groups. The figure shows top-down (left) and side-on (right) views of the molecule’s structure. 

Formation of ligand 3 that leads to self-sorting

When the authors applied a bias 5-V pulse at 78 K, the Mn12 SMMs were deposited individually on a semimetallic Bi(111) surface. The spaces between Mn12 SMMs ranged from a few to tens of nm. The SMMs adopted flat-lying (50%), side-lying (20%), and tilted (30%) orientations.

The HOMO–LUMO gap in a flat-lying M12 SMM was 0.40 eV, with HOMO and LUMO energy levels of –0.37 and 0.03 eV, respectively, consistent with DFT+U simulations. (DFT is density functional theory.) The gap was significantly lower than that of a Mn12 crystal, which the authors attribute to asymmetric charge transfer caused by Mn12–Bi interactions. This protocol allows the direct characterization of local electronic structures of SMMs on semimetallic surfaces. (ACS Nano 2013, 7, 6825–6830; Xin Su)

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