The Brookhart Group carries out research in the general area of synthetic and mechanistic organometallic chemistry, with particular emphasis on the application of late metal organometallic complexes in catalysis. Perhaps the most well-known systems are the Ni(II) and Pd(II) α-diimine catalysts, coined as the “Brookhart Catalysts”, used for the polymerization of ethylene. Additionally, new "ligand-free" catalysts have been developed and are under investigation for 1,3-diene polymerization to create highly cis-enchained polymers with rubbery characteristics. Mechanistic investigations include various NMR techniques, low temperature, multi-nuclear, and multi-dimensional, kinetics, and deuterium labeling studies to provide a detailed understanding of catalyst resting states and relative intermediates.
A second major focus concerns studies of C-C bond metathesis with Ir and Rh pincer complexes in efforts to combat the current energy crisis. Alkane metathesis has potentially tremendous applicability via converting low-value alkanes (C5-C8) from the Fisher-Tropsch process into more usable, linear alkanes in the diesel fuel range (C9-C19). The Brookhart Group has recently developed a well-defined and highly efficient tandem catalytic system for the metathesis of n-alkanes, which first effects alkane dehydrogenation and olefin hydrogenation followed by olefin metathesis to selectively afford linear alkane products.
One focus in the Ashby Group is the synthesis of functionalized dienes with groups ranging from ethers to amines. We have recently reported the use of tertiary amine functionalized poly(isoprene) materials as efficient gene transfection agents at low concentrations and current work is focused on developing guidelines for material design. Group members are also investigating new methods for fabricating poly(thiophene) based organic solar cells, and have developed a method for enhanced processing of poly(thiophene) by functionalizing the aromatic ring to maintain solubility after polymer formation. Films are then able to be solvent cast followed by thermolysis of the functional group to form insoluble films.
As reported in the October 23, 2009, issue of the journal Science, Carolina chemists in collaboration with colleagues at the University of Washington have taken an important step in converting methane gas to a liquid, potentially making it more useful as a fuel and as a source for making other chemicals.
The carbon-hydrogen bonds of alkanes are weak ligands and thus reports of isolation or spectroscopic observation of alkane complexes in solution are extremely rare. Nevertheless, such complexes are postulated as intermediates that form prior to C-H bond scission in most oxidative addition reactions of alkanes. The Shilov system for catalytic conversion of methane to methanol is thought to involve a Pt(II) methane complex as a key intermediate. While a postdoctoral fellow in the Brookhart Group, Wes Bernskoetter, now on the faculty at Brown, succeeded in preparing the first solution-stable, NMR-observable transition metal complex of the simplest alkane, methane (CH4).
The methane complex was obtained by low temperature protonation of a pincer rhodium methyl complex and fully characterized by 1H, 13C and 31P NMR spectroscopy. Cindy Schauer, co-author of the study, carried out DFT calculations that suggest one C-H bond interacts preferentially with the Rh center to form a three-center, two-electron bond, as per the above figure. The Brookhart Group hopes that investigation of the properties of this and other methane complexes may lead to more efficient catalysts for functionalization of alkanes.
The chromatin folding problem is an exciting and rich field for modern research. On the most basic level, chromatin fiber consists of a collection of protein-nucleic acid complexes, known as nucleosomes, joined together by segments of linker DNA. Understanding how the cell successfully compacts meters of highly charged DNA into a micrometer size nucleus while still enabling rapid access to the genetic code for transcriptional processes is a challenging goal.
In an article published in JACS, the Papoian Group, discusses the way mobile ions condense around the nucleosome core particle, as revealed by an extensive all-atom molecular dynamics simulation. Overall, this research facilitates a better understanding of the way ionic and hydration interactions within a nucleosome may affect internucleosomal interactions in higher order chromatin fibers.
Cellular RNA molecules undergo complex folding transitions to form specific, biologically active, three-dimensional structures. A persistent and poorly explained observation is that many RNAs fold very slowly, on timescales requiring minutes or longer. Slow folding ultimately governs the rate at which an RNA can perform its biological function.
In work reported in PNAS, Stefanie Mortimer in the Weeks Lab used time-resolved SHAPE chemistry to show that slow folding at a single nucleotide in the unusual C2'-endo conformation constitutes the rate-determining step for folding a large 50 kDa RNA. Nucleotides in the C2'-endo conformation are relatively rare but are highly overrepresented in functionally critical RNA motifs. This work thus identifies a surprisingly simple, but likely ubiquitous, mechanism for controlling biological processes involving RNA.
Serotonin, also known as 5-HT is an important molecule in the brain that is implicated in mood and emotional processes. Although there is a heavy pharmaceutical emphasis on serotonin's involvement in many neurological disorders, in vivo, its dynamic release and uptake kinetics are poorly understood. This is due to a lack of analytical techniques for its rapid measurement. Whereas fast-scan cyclic voltammetry with carbon fiber microelectrodes is used frequently to monitor subsecond dopamine release in freely moving and anesthetized rats, the electrooxidation of serotonin forms products that quickly polymerize and irreversibly coat the carbon electrode surface.
In a paper published in Analytical Chemistry, the Wightman Group identifies the root of this fouling to not only be due to serotonin, but also to the negatively charged extracellular metabolites of serotonin, present in 200−1000 times the concentration of serotonin in vivo. To impede access of these negatively charged species, a thin layer of Nafion, a cation exchange polymer, was electrodeposited onto cylindrical carbon-fiber microelectrodes. The team visually confirmed the presence of the Nafion film using scanning electron microscopy and showed that the signals for negatively charged species were diminished. Interestingly, the properties of the Nafion also increased sensitivity to serotonin, providing an electrochemical signature of serotonin that could be verified in vitro. In vivo, the team used physiological, anatomical, and pharmacological evidence to validate the signal as serotonin. Using Nafion-modified microelectrodes, the Wightman Group presents the first endogenous recording of serotonin in the mammalian brain.
Darren Zhu, a graduate of the North Carolina School of Science and Mathematics, has been selected as a 2009 Davidson Fellow. Darren earned this very prestigious recognition for research he conducted with the You Group during the summer of 2008.
Mentored by Jeremy Niskala, a graduate student in the You Group, Darren worked to develop more efficient data storage technologies by exploring nanofabrication methods for spintronics. Spintronics, or spin-based electronics, are inherently more powerful than electronics, as they exploit electron spin and subsequently are more sensitive than integrated circuit technology. He incorporated molecular self-assembled monolayers (SAMs) into spintronics and performed surface analyses to find that isocyanide-based SAMs are a viable candidate for implementation in nanoscale spintronics fabrication. Darren’s work has strong applications in nanotechnology, specifically in the field of nanolithography.
Researchers in the Berkowitz Group performed molecular dynamics simulations on systems containing phosphatidycholine headgroups attached to graphene plates (PC−headgroup plates) immersed in water to study the interaction between phosphatidylcholine bilayers in water. The potential of mean force (PMF) between PC−headgroup plates shows that the interaction is repulsive.
As described in The Journal of Physical Chemistry B, the investigators observed three distinct regimes in the PMF depending on the interplate distances: the small distance regime, intermediate distance regime, and large distance regime. The researchers believe that the repulsive interaction in the intermediate interplate distance regime is associated with the hydration force due to the removal of water molecules adjacent to the headgroups.