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The Jeff Johnson Group focuses on the development of new synthetic methods for the assembly of stereochemically complex small molecules. We are particularly interested in the design and synthesis of tailor-made reagents and catalysts for multicomponent reactions. In our recent investigations of several interesting problems, our continuing interest in the exploitation of ring strain as a source of novel reactivity led us to some mechanistically unusual heterocycle-forming cycloadditions of cyclopropanes and aldehydes. On another front, we are interested in the development of dipolar synthons for the coupling of complementary nucleophilic and electrophilic reaction partners. Our development of silyl glyoxylate reagents is an example of work in this area.
An article titled "Catalytic Hydrotrifluoromethylation of Styrenes and Unactivated Aliphatic Alkenes via an Organic Photoredox System," published in the journal Chemical Science by Professor David Nicewicz, his postdoctoral assistant Dale Wilger, and graduate student Nathan Gesmundo, has been listed as one of the 25 most read articles of 2013.
Chemical Science is the Royal Society of Chemistry's flagship journal, publishing research articles of exceptional significance and high-impact reviews from across the chemical sciences. Research in Chemical Science is not only of the highest quality but also has excellent visibility.
Assistant Professor Scott Warren, a joint faculty member with the Department of Chemistry and the Applied Physical Sciences Department here at UNC, is one of the co-authors of an article, published in Angewandte Chemie, describing how single crystals of a cyclodextrin-based metalâ€“organic framework, MOF, infused with an ionic electrolyte and flanked by silver electrodes act as memristors. The article is highlighted in ChemistryWorld, where the invention is referred to as "Computer Memory Made From Sugar Cube."
Tony Kenyon, an electronic engineer at University College London, says the sugar-cube memoryâ€™s performance would not be compatible with existing complementary metal oxide semiconductor, CMOS, technology, the staple of modern computing. But he also points out that other applications could be very interesting. The authors comment that commercial RRAMs have faster read and write times, but state they believe they can make this type of memory cheaper and most definitely greener. The researchers are thinking along the lines of "biodegradable memory."
Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using scaling theory, researchers in the Rubinstein Group, as published in Macromolecules, studied a simple model of autonomic self-healing of unentangled polymer networks. In this model one of the two end monomers of each polymer chain is fixed in space mimicking dangling chains attachment to a polymer network, while the sticky monomer at the other end of each chain can form pairwise reversible bond with the sticky end of another chain. The group studied the reaction kinetics of reversible bonds in this simple model and analyzed the different stages in the self-repair process.
The team observed the slowest formation of bridges for self-adhesion after bringing into contact two bare surfaces with equilibrium, very low, density of open stickers in comparison with self-healing. The primary role of anomalous diffusion in material self-repair for short waiting times is established, while at long waiting times the recovery of bonds across fractured interface is due to hopping diffusion of stickers between different bonded partners. Acceleration in bridge formation for self-healing compared to self-adhesion is due to excess nonequilibrium concentration of open stickers. Full recovery of reversible bonds across fractured interface, formation of bridges, occurs after appreciably longer time than the equilibration time of the concentration of reversible bonds in the bulk.
New research from the You Group, in collaboration with researchers at NCSU, reveals that energy is transferred more efficiently inside of complex, three-dimensional organic solar cells when the donor molecules align face-on, rather than edge-on, relative to the acceptor. This finding may aid in the design and manufacture of more efficient and economically viable organic solar cell technology.
The paper appears online in Nature Photonics. Fellow NC State collaborators were John Tumbleston, Brian Collins, Eliot Gann, and Wei Ma. Liqiang Yang and Andrew Stuart from UNC-Chapel Hill also contributed to the work. The work was funded by the U.S. Department of Energy, Office of Science, Basic Energy Science, the Office of Naval Research, and the National Science Foundation.
Presently, there are few estimates of the number of molecules occupying membrane domains. In a collaborative work published in the journal Traffic, researchers in the Thompson Group describe how they, using a total internal reflection fluorescence microscopy (TIRFM) imaging approach, based on comparing the intensities of fluorescently labeled microdomains with those of single fluorophores, measured the occupancy of DC-SIGN, a C-type lectin, in membrane microdomains.
DC-SIGN or its mutants were labeled with primary monoclonal antibodies (mAbs) in either dendritic cells (DCs) or NIH3T3 cells, or expressed as GFP fusions in NIH3T3 cells. The number of DC-SIGN molecules per microdomain ranges from only a few to over 20, while microdomain dimensions range from the diffraction limit toâ€‰>â€‰1â€‰Âµm. The largest fraction of microdomains, appearing at the diffraction limit, in either immature DCs or 3â€‰T3 cells contains only 4â€“8 molecules of DC-SIGN, consistent with the group's preliminary super-resolution Blink microscopy estimates. The article further discusses how these small assemblies are sufficient to bind and efficiently internalize a small (âˆ¼50â€‰nm) pathogen, dengue virus, leading to infection of host cells.
In vivo optical imaging must contend with the limitations imposed by the optical window of tissue, 600â€“1000 nm. Although a wide array of fluorophores are available that are visualized in the red and near-IR region of the spectrum, with the exception of proteases, there are few long wavelength probes for enzymes. This situation poses a particular challenge for studying the intracellular biochemistry of erythrocytes, the high hemoglobin content of which optically obscures subcellular monitoring at wavelengths less than 600 nm.
To address this, researchers in the Lawrence Group, published in Angewandte Chemie, developed tunable fluorescent reporters for protein kinase activity. The probing wavelength is preprogrammed by using readily available fluorophores, thereby enabling detection within the optical window of tissue, specifically in the far-red and near-IR region. These agents were used to monitor endogenous cAMP-dependent protein kinase activity in erythrocyte lysates and in intact erythrocytes when using a light-activatable reporter.
At the Department of Chemistry, we feel strongly that diversity is crucial to our pursuit of academic excellence, and we are deeply committed to creating a diverse and inclusive community. We support UNC's policy, which states that "the University of North Carolina at Chapel Hill is committed to equality of opportunity and pledges that it will not practice or permit discrimination in employment on the basis of race, color, gender, national origin, age, religion, creed, disability, veteran's status, sexual orientation, gender identity or gender expression."