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Noteworthy Chemistry

May 25, 2015

 

Zeolite nanocrystals inhibit palm oil oxidation. Palm oil represents 30% of global vegetable oil output, but it has low thermal and oxidative stability, and it is prone to hydrolysis. Oxidation products from heated cooking oils such as palm oil may pose a health risk.

Nanosized zeolite crystals prepared without a templating agent are nontoxic and suitable for use as food additives. They are used to filter out impurities and oxidation products from oils so that they can be reused. This study shows that zeolites can also retard oxidation.

S. Mintova at the University of Caen (France). E.-P. Ng at the University of Science, Malaysia (Penang), and coauthors in Malaysia, France, Indonesia, Singapore, and Bulgaria studied palm oil to which 0.5 wt% zeolite M-X (M= Li+, Na+, K+, or Ca2+) was added. They stirred the mixtures at 150 ºC and drew samples at 100, 200, 300, and 400 h to follow the oxidative evolution of each oil sample with several analytical techniques.

A reference oil that contained no zeolite changed color from yellow to black and developed a strong smell over the course of the experiment. The discoloration rates of the zeolite-containing oil samples were inversely correlated with the polarizability of the zeolite counterion: fastest to slowest, Li+ > Na+ > Ca2+ > K+. NMR spectroscopy showed that zeolites with highly basic cations were better at slowing the formation of aldehydes and carbonyl compounds. The figure shows the color changes for each oil sample from (a) 100 h to (d) 400 h.

Color change for each oil–zeolite sample over time

Oxidation products with polar acidic groups attract other organic molecules to form bulkier molecules, which increase the oil’s viscosity. Zeolite counterions with low charge densities (K+ and Na+) were better at protecting the oils from viscosity changes by slowing the formation of polymeric oxidized compounds.

Zeolites that contained Ca2+ and K+ were best at hindering the formation of hydroxyl compounds, and all of the zeolites slowed the generation of water during oxidation. Ca2+ was best at stabilizing hydroperoxides, the primary oxidation product, which slowed the formation of secondary oxidation products (alcohols, aldehydes, ketones, carboxylic acids, and esters). Divalent Ca2+ also was most effective in adsorbing oxidation products from the oils; and it formed salts with the carboxylic acids to reduced oil acidity.

Periodic density functional theory calculations showed that the zeolite counterions interacted preferentially with hydroperoxide oxidation products rather than the C=C bonds in the oil molecules. The calculations predicted that Ca2+ would be most effective at hindering oxidation, but the experiments placed it behind K+. (J. Agric. Food Chem. DOI: 10.1021/acs.jafc.5b00380; Nancy McGuire)

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A long noncoding RNA regulates cell proliferation. Coding genes are patently significant, but researchers are learning that noncoding RNAs are also important. Two-thirds of the mammalian genome is transcribed, yet <2% of the DNA encodes for proteins, suggesting that noncoding RNAs have a functional role.

Noncoding transcripts longer than 200 nucleotides are known as long noncoding RNAs (lncRNAs). Recently researchers have learned that lncRNAs affect various biological processes; and accordingly, their dysregulation is associated with cancer and other diseases.

R. Chen and collaborators at the Chinese Academy of Sciences, the University of the Chinese Academy of Sciences, the Research Network of Computational Biology, and Chinese PLA General Hospital (all in Beijing); and Hebei North University (Zhangjiakou, China) identified and characterized Lnc_bc060912, a nuclear-retained lncRNA with increased expression in four tumor types.

The researchers conducted experiments in a lung cancer epithelial cell line and found that the overexpression of Lnc_bc060912 results in a low rate of apoptosis. Also, Lnc_bc060912 knockdown significantly reduces cell viability, which indicates that the lncRNA suppresses apoptosis.

The authors then looked at the association between the lncRNA and the tumor suppressor p53 and discovered that p53 overexpression distinctly decreases Lnc_bc060912 expression. Diminished p53 levels, however, induce a marked increase in Lnc_bc060912. This inverse relationship indicates that p53 represses Lnc_bc060912 expression.

The authors used a new chromatin isolation by RNA purification (ChIRP) method in which formaldehyde cross-links proteins that interact with lncRNA and antisense probes to capture in vivo lncRNAs. They identified two direct Lnc_bc060912 binding partners: the antiapoptotic DNA damage repair proteins poly(ADP-ribose) polymerase and nucleophosmin. This physical interaction suggests a mechanism for Lnc_bc060912 apoptotic repression. (Biochemistry DOI: 10.1021/acs.biochem.5b00259; Abigail Druck Shudofsky)

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Form π-conjugated covalent networks on surfaces by using acetyl cyclotrimerization. On-surface synthesis can create well-defined, robust molecular structures on solid surfaces with covalent connections. Two-dimensional (2-D) conjugated polymers have attracted much attention because they are similar to graphene sheets and may have useful materials properties.

Cyclotrimerization reactions are seldom used to form 2-D frameworks on surfaces, although they are extensively used to prepare covalent networks in solution. But recently Q. Li, L. Chi, and coauthors at Soochow University (Suzhou, China) and Linköping University (Sweden) developed a surface-assisted cyclotrimerization reaction for making 2-D covalent networks on surfaces.

The authors formed the cyclic structure by coupling three acetyl groups (see figure). On a metal (e.g., silver) surface, the reaction proceeds smoothly to yield extended graphene-like networks.

Formation of graphene-like precursors via acetyl cyclotrimerization

In contrast to the analogous alkyne cyclotrimerization reaction, acetyl cyclotrimerization is highly regioselective; it forms the 1,3,5-substituted benzene ring as the sole structural motif.

This method for forming 2-D π-conjugated porous polymers not only contributes to materials science, but it is also a valuable addition to the on-surface synthesis toolbox. The authors believe that by judiciously designing acetyl monomers, chemists will be able to make large-scale, surface-covalent organic frameworks. (J. Am. Chem. Soc. DOI: 10.1021/jacs.5b00774; Ben Zhong Tang)

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Detect trace nitrogen dioxide with phosphorus “black magic”. White and red phosphorus are the two best-known phosphorus allotropes, but researchers recently “rediscovered” black phosphorus as a single-element two-dimensional layered material. Obtained by heating white phosphorus under high pressure, black phosphorus consists of interconnected six-membered rings (similar to graphene’s configuration) and has potentially useful optical and electronic properties.

C. Zhou and coauthors at the University of Southern California (Los Angeles), the University of Jeddah (Saudi Arabia), and the Technical University of Munich (Germany) used multilayer black phosphorus to prepare field-effect transistor (FET)–based sensors for highly sensitive chemical sensing of nitrogen dioxide (NO2) at concentrations as low as 5 ppb.

The authors synthesized black phosphorus by heating red phosphorus over Sn/SnI4. The black phosphorus was delaminated with a commercial tape on a P2+ Si/SiO2 substrate. They used electron beam lithography to define and evaporation to deposit electrodes, thereby completing the preparation of the multilayer black phosphorus FET (see figure).

Scheme of a multilayer black phosphorus field-effect transistor

When the authors exposed the black phosphorus–based sensor to NO2, the conductance of the sensor increased. By varying the NO2 concentration in the range 5–40 ppb, they found that the relative conductance change followed the Langmuir adsorption isotherm, which indicates that adsorbed NO2 molecules modify the electronic properties of black phosphorus.

This first example of black phosphorus–based gas sensor shows excellent sensitivity and fast response times toward trace amounts of NO2. This work gives insights into the electronic responsiveness of black phosphorus; and it may inspire the development of other black phosphorus–based devices for related sensing applications. (ACS Nano DOI: 10.1021/acsnano.5b01961; Xin Su)

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Macadamia is a hard nut to crack. Getting at the prized macadamia kernels, or nuts as they are commonly called, requires an extensive drying process to loosen the kernel within the shell by reducing the moisture level from 30% to 1.5%, followed by cracking the extremely hard shells while preserving the kernels. The strength of the shells exceeds that of concrete, making them notoriously difficult to crack mechanically. The processing is energy intensive, and the shells yield significant waste.

L. Aldous and co-workers at the University of New South Wales (Kensington, Australia) tested ionic liquids (ILs) to chemically facilitate the cracking process. ILs are universally recognized solvents for lignocellulosic biomass, including hard and soft woods, rice husks, and grasses.

Working with macadamia nuts that required 2240 ± 430 N of force to crack, the authors tested several ILs to determine their ability to dissolve the macadamia nut shell flour. 1-Ethyl-3-methylimidazolium acetate ([Emim][OAc]) was the most effective. They then proceeded to test heated [Emim][OAc] on whole shells. Heating the shells at 110 ºC for 72 h reduced the required force from ≈2240 N to 760 ± 240 N, about 34% of the original force.

This research shows that ILs can be used to pretreat the lignocellulosic biomass of the macadamia shell to improve the cracking process and to reduce the need for extensive drying. Many questions must be answered about how this chemical process affects food safety and appearance and how it can progress from lab to mass production. But it looks like chemistry can crack the hardest nuts. (ACS Sustainable Chem. Eng. DOI: 10.1021/acssuschemeng.5b00126; Beth Ashby Mitchell)

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Use β-cyclodextrin to solubilize the DPPH radical. 2,2-Diphenyl-1-picrylhydrazyl (DPPH, 1 in the figure) is a stable free radical that is usually used as a model for evaluating the radical-scavenging activity of antioxidants by observing the disappearance of a characteristic absorption band. But because of DPPH’s low solubility in water, experiments must be conducted in alcohol solvents, and buffer solutions cannot be used.

I. Nakanishi at the National Institute of Radiological Sciences (Chiba, Japan), S. Fukuzumi at Osaka University, and coauthors in Japan and Korea report that b-cyclodextrin assists the solubilization of DPPH in water. Cyclodextrins (CDs) are cyclic oligosaccharides that have hydrophobic cavities and hydrophilic surfaces. They can form water-soluble inclusion complexes with organic molecules. α-, β-, and γ-CD have six, seven, and eight D-glucopyranoside units, respectively.

The method for solubilizing DPPH consists of adding boiling water or a phosphate buffer (pH 7.4) to a mixture of DPPH and β-CD, which is then cooled and filtered to yield a violet solution that is stable for several days. The use of α- or γ-CD or the similar radical DOPPH [2,2-di(4-tert-octylphenyl)-1-picrylhydrazyl] does not result in solubilization.

Molecular modelling indicated that the picryl group in DPPH is encapsulated in the hydrophobic cavity, whereas the hydrazyl radical is on the exterior. The authors attribute the insolubility of DOPPH to the hydrophobicity of its tert-octyl substituents.

The researchers exposed the DPPH–β-CD solution to the antioxidants ascorbic acid and Trolox (a water-soluble analogue of α-tocopherol). In both cases, the DPPH absorption band disappears, which suggests antioxidant activity. The presence of CD does not inhibit the reaction of DPPH with the antioxidants and allows DPPH–β-CD to be used for detecting antioxidants in buffered aqueous solutions. (Chem. Commun. DOI: 10.1039/C5CC02236C; José C. Barros)

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The stable free radical DPPH