Research in the Pielak Group focuses on In-cell NMR, a new method which allows us to obtain high-resolution NMR data from proteins in living cells. Much of this work involves quantifying the effects of macromolecular crowding on protein chemistry. Additionally, we study the oxidative aggregation of the key protein involved in Parkinson's disease, α-synuclein.
Members of the Ashby Group focus on the synthesis of functionalized materials with applications ranging from biology to alternative energy. We synthesize degradable polyester based material to take advantage of their enhanced biocompatibility and high degree of physical and chemical control. The materials we have synthesized range in applications from cell scaffolds to degradable shape memory devices.
A need in current biomaterials is the integration of functional groups into degradable polymers to impart properties for specialized applications. Two methods we employ utilize cyclization chemistry through "click" type reactions and Diels-Alder chemistry to integrate polar groups into a polyester backbone. Group members are also investigating functionalization chemistry based on aminooxy coupling reactions.
As described in Chemical Science, members of the Dempsey Group, in collaboration with the Meyer Group, used a layer-by-layer procedure to prepare chromophore–catalyst assemblies consisting of phosphonate-derivatized porphyrin chromophores and a phosphonate-derivatized ruthenium water oxidation catalyst on the surfaces of tin oxide and titanium dioxide mesoporous, nanoparticle films. In the procedure, initial surface binding of the phosphonate-derivatized porphyrin is followed in sequence by reaction with a zirconium salt and then with the phosphonate-derivatized water oxidation catalyst.
Fluorescence from both the free base and zinc porphyrin derivatives on tin oxide is quenched; substantial emission quenching of the zinc porphyrin occurs on titanium dioxide. Transient absorption difference spectra provide direct evidence for appearance of the porphyrin radical cation on tin oxide via excited-state electron injection. For the chromophore–catalyst assembly on tin oxide, transient absorption difference spectra demonstrate rapid intra-assembly electron transfer oxidation of the catalyst following excitation and injection by the porphyrin chromophore.
Chemists have long sought new ways to create energy-rich fuels - ideally via reactions powered by a renewable resource such as the sun. But scientists still have a lot to learn about solar-powered reactions, and a new study by Thomas Eisenhart and Jillian Dempsey sheds light on how they occur. The proton-coupled electron transfer reaction, PCET, is a key light-driven step in the conversion of small molecules into energy-rich fuels. Although prior research has provided a basic understanding of PCET reactions between molecules in their ground states, much less is known about the reactions between electronically excited molecules.
In the article, which made the cover of JACS, and was also featured in JACS Spotlights, the team reports results from a mechanistic study of excited-state PCET reactions between two small molecules, acridine orange and tri-tert-butylphenol. The step-by-step process by which the reaction occurs has not been determined previously, but since each of the reaction components has a unique spectroscopic signature, the researchers can monitor each step with transient absorption spectroscopy. The results help explain the intimate coupling of light absorption with both proton and electron transfer, which the authors say will help pave the way for new avenues in solar fuel production.
Christine Herman, Ph.D., JACS
Assistant Professor Brian Hogan has been honored for his recent graduation from the Carolina Center for Public Service's Thorp Faculty Engaged Scholars program. Brian was one of nine members of the Thorp Faculty Engaged Scholars, FES, Class IV who worked over the past two years to strengthen partnerships between the University of North Carolina at Chapel Hill and the surrounding community.
The program, an initiative of the Carolina Center for Public Service, brings together selected faculty from across campus to engage in a two-year experiential, competency-based curriculum designed to advance their scholarship. Scholars participate in sessions in community settings to learn from Carolina faculty and their community partners and build relationships through work such as training teachers to integrate experiential learning into their classrooms. Brian is the academic director for the Scholars' Latino Initiative, a program dedicated to increasing college access for Latino high school students. He helped build "SLIence," a collaboration between McDougle Middle School and the Scholars’ Latino Initiative.
Caitlin McMahon, a fourth year graduate student in the Alexanian Group, has been selected by the ACS Division of Organic Chemistry to receive a 2014-2015 Graduate Fellowship. Awardees for this highly competitive award are selected by an independent committee, and evidence of research accomplishments is an important factor in the selection process. Caitlin will travel to the 2015 National Organic Symposium to present a poster of her research.
Caitlin's research focuses on the development of metal-catalyzed organic reactions, with the goal of discovering new ways to form carbon-carbon bonds and expanding the methodology available to synthesize organic building blocks. More specifically, she has developed a palladium-catalyzed, intermolecular Heck-type reaction using alkyl electrophiles - significantly expanding the scope of the widely-utilized Heck reaction. She is currently studying carbonylative metal-catalyzed reactions, building functionalized organic molecules by forming two carbon-carbon bonds in one step under mild conditions.
Published in Macromolecules, Professor Michael Rubinstein, in collaboration with Ekaterina Zhulina with the Institute of Macromolecular Compounds, Russian Academy of Sciences in Saint Petersburg, describe the development of a scaling model relating the friction forces between two polyelectrolyte brushes sliding over each other to the separation between grafted surfaces, number of monomers and charges per chain, grafting density of chains, and solvent quality. They demonstrate that the lateral force between brushes increases upon compression, but to a lesser extent than the normal force.
The shear stress at larger separations is due to solvent slip layer friction. The thickness of this slip layer sharply decreases at distances on the order of undeformed brush thickness. The corresponding effective viscosity of the layer sharply increases from the solvent viscosity to a much higher value, but this increase is smaller than the jump of the normal force resulting in the drop of the friction coefficient. At stronger compression the group members predict the second sharp increase of the shear stress corresponding to interpenetration of the chains from the opposite brushes. In this regime the velocity-dependent friction coefficient between two partially interpenetrating polyelectrolyte brushes does not depend on the distance between substrates because both normal and shear forces are reciprocally proportional to the plate separation. Although lateral forces between polyelectrolyte brushes are larger than between bare surfaces, the enhancement of normal forces between opposing polyelectrolyte brushes with respect to normal forces between bare charged surfaces is much stronger resulting in lower friction coefficient. The model quantitatively demonstrates how polyelectrolyte brushes provide more effective lubrication than bare charged surfaces or neutral brushes.
A new small molecule receptor, A2N, has been identified that binds specifically to trimethyllysine, Kme3, with sub-micromolar affinity. This receptor, as published in Organic & Biomolecular Chemistry was discovered by Nicholas Pinkin and Marcey Waters in the Waters Group, through the iterative redesign of a monomer known to incorporate through dynamic combinatorial chemistry, DCC, into a previously reported receptor for Kme3, A2B. In place of monomer B, the newly designed monomer N introduces an additional cation–Π interaction into the binding pocket, resulting in more favorable binding to Kme3 amounting to a ten-fold improvement in affinity and a five-fold improvement in selectivity over Kme2.
This receptor exhibits the tightest affinity and greatest selectivity for Kme3-containing peptides reported to date. Comparative studies of A2B and A2N provide mechanistic insight into the driving force for both the higher affinity and higher selectivity of A2N, indicating that the binding of Kme3 to A2N is both enthalpically and entropically more favorable. This work demonstrates the ability of iterative redesign coupled with DCC to develop novel selective receptors with the necessary affinity and selectivity required for biological applications.
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."