Science Concentrates: ACS Meeting News CATALYSIS
Copper dimers reduce NOx High schoolers aren’t the only ones who pair up and break up frequently. Catalytic species in the systems that clean up engine exhaust do it too, according to a study presented last week at the ACS national meeting in Washington, D.C. The work, presented in the Division of Catalysis Science & Technology, uncovered an unusual mechanism in the selective catalytic reduction (SCR) process, which reduces smog-causing nitrogen oxides (NOx) in diesel engine exhaust to nitrogen and water via reaction with ammonia.
Single copper ions in a zeolite catalyst (white geometric shapes) bind two ammonia molecules, migrate through the zeolite, and form oxygen-bridged dimer complexes, which catalyze exhaust cleanup. Cu is orange; N is blue; H is white; O is red. SCR systems, which rely on chabazite zeolite treated to incorporate copper in its lattice, come standard on many diesel vehicles, yet details of how they work remain a subject of debate. Rajamani Gounder of Purdue University, William F. Schneider of the University of Notre Dame, and coworkers found that in the presence of ammonia, copper ions form Cu(NH3)2 complexes. The NH3 moieties make the species mobile, allowing them to migrate through openings that interconnect hollow cages in the porous zeolite framework. As the copper species move about, they briefly and reversibly form dimers that are bridged by a pair of oxygen atoms. The dimers facilitate an O2-mediated Cu(I) to Cu(II) redox step that’s central to reducing NOx to nitrogen and water (Science 2017, DOI: 10.1126/science.aan5630). Fine-tuning the distribution of copper ions in the zeolite could lead to lower NOx emissions at cooler operating temperatures than is possible with current SCR systems. “Exquisite” is how Robert J. Davis described the team’s techniques for exploring this SCR system. Davis, a catalysis specialist at the University of Virginia, remarked that this exciting finding regarding the mobility of copper ions and the dynamic formation of paired copper species may also be relevant to the high selectivity of Cu-treated zeolites used for oxidizing methane to methanol.—MITCH JACOBY
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C&EN | CEN.ACS.ORG | AUGUST 28, 2017
INFECTIOUS DISEASE
Antimicrobial agents in dragons’ blood Peptides from Komodo dragons could inspire new ways to fight infections Some reptiles sport tough immune systems that help them fend off infections after suffering gnarly wounds. For example, Komodo dragons can avoid infections after bites from other Komodo dragons even though the reptiles’ mouths can harbor up to 50 strains of pathogenic bacteria. At the ACS national meeting in Washington, D.C., last week, researchers reported a method to discover antimicrobial peptides in the blood of these resilient reptiles. The peptides could inspire novel antibacterial therapies, even against multi-drug-resistant pathogens, the researchers said. This bioprospecting approach in reptiles “is a unique take on natural products isolation chemistry,” said Coran Watanabe of Texas A&M University, who was not involved in the work. All vertebrate animals, not just reptiles, rely on antimicrobial peptides as part of their immune response. These short, positively charged peptides can poke holes in the membranes of pathogenic bacteria, disrupt bacterial gene expression, and turn on other parts of animals’ immune systems to launch an attack on invaders. In the Division of Biological Chemistry, a team led by Barney M. Bishop and Monique L. van Hoek of George Mason University presented a method to prospect for antimicrobial peptides in the blood of American alligators and Komodo dragons. The method relies on custom hydrogel microparticles made from cross-linked poly(N-isopropylacrylamide). The particles contain negatively charged “bait” groups, such as acrylic acid or 2-acrylamido-2-methylpropanesulfonic acid, to attract positively charged molecules. The polymer cross-linking helps the particles preferentially capture small peptides and exclude larger proteins. The scientists incubate these particles in 100-µL samples of plasma and then recover the collected peptides using solvents. They analyze the peptides’ sequences using mass spectrometry. Using this technique, Bishop and van Hoek’s team has discovered dozens of novel antimicrobial peptides from alligators and Komodo dragons. The scientists used the sequence of one of the dragon peptides to design a synthetic peptide called DRGN-1 that accelerated healing of wounds in mice that were infected with Pseudomonas aeruginosa and Staphylococcus aureus (npj Biofilms & Microbiomes 2017, DOI: 10.1038/s41522-017-0017-2). The team is now investigating ways to incorporate DRGN-1 into possible wound dressings.—MICHAEL TORRICE
C R E D I T: MAUR E E N LI FTO N /P U R D UE ( CO P P E R ) ; S H U TT E RSTO CK ( D RAGO N )
Unusual catalytic mechanism plays key role in diesel exhaust cleanup
