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Cite this: J. Am. Chem. Soc. 2020, 142, 4, 1655–1656
Publication Date (Web):January 21, 2020
https://doi.org/10.1021/jacs.0c00642

Copyright © 2020 American Chemical Society. This publication is available under these Terms of Use.

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Shrinking Bonds: Surprising Structural Changes in Dipole-Bound Anions

Charlie Crowe
Dipole-bound anions are unique entities formed when the molecular dipole of a neutral, polar molecule is strong enough that its positive end can capture an extra electron. Initially, this excess electron was thought to have little effect on the molecule to which it was bound. However, research showed that internal bonds became shorter after the transition from neutral to anionic species. Valery Sidorkin and co-workers have investigated these unexpected changes using computational and experimental methods to examine polar silatrane cages (DOI: 10.1021/jacs.9b11694). By utilizing variable substituents, they control the molecules’ attributes and explore these “outlaw” anions.
Silatranes contain an internal dative nitrogen–silicon bond, which the authors found to be extremely sensitive. When the silatrane contained a chlorine atom, the nitrogen–silicon bond shrank by 0.15 Å after it became anionic, the most ever observed. They attribute this significant structural change to not only the strength of the molecule’s dipole moment but also its geometry before anionization. Based on this, the authors show that the primary factor behind the shrinking bond is the rate of change in the molecular dipole when the extra electron is added. This new knowledge gives scientists a better understanding of the fundamental interactions between electrons and polar molecules.

New “Mechanoacid” Polymer Releases Acid When Stretched

Christine Herman (Ph.D.)
Stress-responsive polymeric materials have a variety of potential applications, since mechanophores can be designed to release chemical cargo, catalyze reactions, and perform other useful tasks. Of particular interest to researchers is the development of force-triggered acids—also known as “mechanoacids”—which can deposit protons to prompt polymerization and cross-linking reactions, polymer degradation, and other chemical transitions. But the current mechanoacid toolkit is fairly limited, especially when it comes to thermal stability and high yield.
Now Stephen Craig and co-workers report a new mechanoacid that releases acid in response to mechanical stimulation in a manner that can also reveal how much time has passed since the mechanical event (DOI: 10.1021/jacs.9b12861). The researchers use pulsed ultrasonication to prompt a mechanochemical ring-opening step that results in elimination of either HCl or MeCl. The team demonstrates that the mechanoacid can be used in silicone elastomers to cause a strain-triggered color change warning of an impending material fracture. Since protons are highly functional chemical species, the researchers expect additional opportunities to apply the new mechanoacid in a range of mechanically adaptive contexts, such as mechanically degradable, self-healing, and stress-strengthening polymers.

Something Old, Something New: Stereoselective Bond Memories

Matthew P. McLaughlin (Ph.D.)
Symmetry and time are intimately intertwined. A structure may appear to be symmetrical based on its bonds, but if the molecule reacts fast enough, that symmetry may be illusory. At extremely short times, a newly formed bond may retain differences from preexisting bonds in ways that influence subsequent reactions. To date, these bond memories have been of mechanistic and theoretical interest, but the research groups of Oleg V. Larionov and Daniel A. Singleton have shown how they can have practical effect in synthetic chemistry (DOI: 10.1021/jacs.9b12227).
In order to investigate the high stereoselectivity of the reaction, the researchers ran parallel experiments and calculations to probe the mechanism in the (+)-limonene reaction. The authors found a temporal non-equivalence for the newly formed bonds neighboring the reaction center. For fast reaction times, this asymmetry created a preferred trajectory, yielding an inverted configuration at the reaction center. The application of the selectivity approach in this work has far-reaching implications, finding relevance in everything from pharmaceuticals to structural polymers. For the carboborative ring of (+)-limonene, the researchers ran parallel experiments and calculations to probe the mechanism. The authors found that two migration steps that are separated in time remained stereochemically interconnected by a molecular asymmetry in trajectories, resulting in a high preference for an inverted configuration at the reaction center. The application of the selectivity approach in this work has far reaching implications, finding relevance in everything from pharmaceuticals to structural polymers.

Using DNA for Catch and Release of Bone Marrow Mesenchymal Stem Cells

Devatha P. Nair (Ph.D.)
Bone marrow mesenchymal stem cells (BMSCs) are a widely studied class of stem cells that can differentiate into many cell types, giving them important applications in tissue repair and regeneration, immune response stimulation, and other cell-based therapies. However, BMSCs are extremely scarce, and existing strategies to recover stem cells entail mechanical, thermal, or electrical stress that may damage these precious cells.
Now, Chi Yao and colleagues describe a novel method to efficiently and selectively capture BMSCs from bone marrow (DOI: 10.1021/jacs.9b11001). The researchers synthesized two ultra-long, partially complementary DNA chains via rolling circle amplification. The first chain contains an aptamer motif that binds to BMCSs’ ALPL membrane protein, forming a complex with the cells. Upon addition of the second chain, the DNA hybridizes to create a fibrous network that envelops the BMSCs. The supersoft nature of the DNA network preserves the viability of the cells, and the ability to degrade the DNA with nucleases enables the BMSCs to be released in a controlled manner. The researchers used this method to successfully retrieve BMSCs from a sample of mouse bone marrow, demonstrating its utility in biological contexts. Furthermore, this work reveals the use of tunable DNA sequences that target different types of cells as a promising approach to stem cell isolation.

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