Spotlights pubs.acs.org/JACS
Spotlights on Recent JACS Publications
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COVALENT AND NONCOVALENT CHEMISTRIES UNITE IN MACROCYCLE SYNTHESIS Historically, investigations into covalent and noncovalent chemistries have been performed largely independent from each other. Yet in nature, the two processes often occur simultaneously and in a complementary fashion. The noncovalent assembly of a protein complex that changes its ability to catalyze a covalent chemical reaction is one example of this type of interdependence. Researchers led by Sijbren Otto present a new study that demonstrates how the nature of noncovalent interactions can be controlled to drive a covalent chemical reaction down one of two pathways: toward molecular diversity or specificity (DOI: 10.1021/jacs.7b01814). The team employs a method known as dynamic covalent chemistry. By controlling the self-assembly pathway of the precursor molecules, the researchers are able to control the type of covalent bond formation that takes place. Under certain circumstances, they create a highly diverse mixture of large macrocycles of unprecedented size. When the same building blocks are used under a different set of reaction conditions, the reaction is driven toward the autocatalytic formation of a very specific ring size. The results open the way to “multifaceted dynamic self-assembling systems”, the researchers write, which may find applications in materials science and artificial life. Christine Herman, Ph.D.
HYDROPERSULFIDES EXCEL IN H-ATOM TRANSFER Hydropersulfides (RSSH) occur naturally at significant levels in mammalian tissue, where they are formed endogenously through the reaction of hydrogen sulfide (H2S) with disulfides or sulfenic acid. Although the physiological effects of H2S have been actively investigated, the underlying chemistry remains poorly understood. For example, can RSSH store H2S in vivo as a line of defense against oxidative stress? If so, does the H-atom transfer chemistry of RSSH contribute to this activity? How does this reactivity compare with the well-known H-atom transfer chemistry of the thiols from which they are derived? Derek Pratt and colleagues set out to answer these questions and present several key findings about RSSH and their H-atom transfer reactivity (DOI: 10.1021/jacs.7b02571). The team observes that RSSH are superior H-atom donors over thiols by as much as 4 orders of magnitude when it comes to alkyl, alkoxyl, peroxyl, and thiyl radicals. The researchers note that RSSH compete with “Nature’s premier radical-trapping antioxidant” αtocopherol when it comes to reactivity toward peroxyl radicals. They recommend that future areas of research consider the potential of RSSH reactivity to be exploited in the design of smallmolecule agents with therapeutic potential against diseases where lipid peroxidation has been implicated. Christine Herman, Ph.D.
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STOPPING ELECTRON SPIN COUPLING IN SOLID-STATE NMR Nuclear magnetic resonance spectroscopists have long employed spin decoupling to narrow lines, boost signal-to-noise, and simplify spectra. Alexander Barnes and colleagues now demonstrate that such benefits can be enjoyed in solid-state NMR through electron spin decoupling using dynamic nuclear polarization (DNP) (DOI: 10.1021/jacs.7b02714). DNP can increase NMR sensitivity by orders of magnitude by transferring spin polarization from electron paramagnetic resonance to nuclei. However, the use of paramagnetic DNP agents can come at the cost of broadening NMR lines. To mitigate these unwanted effects, the researchers have developed a novel electron spin decoupling DNP NMR pulse sequence. The sequence begins with a DNP and spin diffusion period, quickly followed by microwave irradiation tuned to the resonance frequency of the electrons on the DNP polarizing agent. Compared to NMR experiments without decoupling, magic angle spinning DNP NMR experiments with electron decoupling have narrower line widths, longer transverse relaxation times, and higher intensity resonances. The approach is tested by interrogating 13C nuclei in biomolecules frozen in a glassy matrix. Electron decoupling reduces line widths by 11% and increases intensity by 14%. The results suggest the possibility of manipulating electron spins in multi-dimensional NMR. Erika Gebel Berg, Ph.D.
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FUELING TOMORROW: MOLYBDENUM COMPLEXES DERIVE H2 FROM NH3 Synthesizing ammonia (NH3) and then breaking it down to produce hydrogen gas (H2) are critical steps to positioning NH3 as a practical carbon-free fuel. While many efforts have focused on improving the production of NH3, considerably less research has concentrated on deriving H2 after NH3 activation. Paul Chirik and co-workers detail three molybdenum-based complexes capable of promoting the production of H2 from NH3 while also reversibly activating the N−H bond (DOI: 10.1021/jacs.7b03070). The three complexes are bis(imino)pyridine molybdenum bis(ammine)s with coordinated benzenes, ethylenes, or cyclohexenes. When treated with NH3, all three of these compounds lose a coordinated arene followed by N−H bond activation and bis(imino)pyridine modification. The metal products are missing one hydrogen atom; subsequent analysis of the volatile byproducts generated by the reaction shows the formation of hydrogen gas. Computational studies suggest that reversible metal−ligand cooperativity and coordination-induced bond weakening contribute to H2 formation. In particular, in the ethylene-coordinated complex, the weakened N−H bonds enable the synthesis of a terminal nitride from coordinated ammonia, an important step in NH3 oxidation. The authors write that their current efforts include finding ways to encourage nitride coupling and N2 evolution, completing the oxidation cycle. Christen Brownlee © 2017 American Chemical Society
Published: May 17, 2017 6513
DOI: 10.1021/jacs.7b04636 J. Am. Chem. Soc. 2017, 139, 6513−6513
