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Selectivity Control of Heterogeneous Catalysis: Designing Selective Reaction Pores on Metal Surfaces

A heterogeneous catalytic process typically involves three important steps: the adsorption of reactant molecules on the active metal surface, reaction between reactant molecules on the metal surface, and, finally, desorption of products. In Professor Frank Tsung’s design, a nanoporous material, a metal organic framework (MOF), is coated on the surface of metal nanoparticles. Because the size of the MOF pores is comparable to that of the reactant molecules, the MOF could concurrently manipulate the molecular size-selectivity, adsorption geometries and sorption energies of reactant molecules on the catalyst surface. The Tsung group has applied this catalyst to the gas-phase hydrogenation of ethylene, cyclohexene, and cyclooctene. The size of ethylene is smaller than the pore size of the MOF; the size of cyclohexene is similar to the pore size, and the size of cyclooctene is much bigger than the pore size. The MOF shell provides excellent molecular-size selectivity. The results show high activity in ethylene and cyclohexene hydrogenations but not in cyclooctene hydrogenation. Different activities for cyclohexene hydrogenation were obtained for the catalysts with and without the MOF shell. The difference could be due to the conformation of the molecules on the metal surface during the hydrogenation. The cyclohexene molecules could only interact with the active metal surface by the successive variation in their conformations. None of these enzyme-like behaviors have ever been previously observed in heterogeneous catalysis. The work has been published in J. Am. Chem. Soc. 2012, 134, 14345-14348.

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Transitions from Functionalization to Fragmentation Reactions of Secondary Organic Aerosol (SOA) Generated from OH Oxidation of Alkaline Precursors

The importance of atmospheric aerosols as climate forcing agents has become evident. The quantitative parameters required for reliable modeling aerosol climate effects, however, are not yet available. This is most notably the case for organic aerosol. To address this problem, Professor Paul Davidovits and his collaborators have been systematically studying the formation of organic aerosol via atmospheric oxidation of organic vapors due to anthropogenic and biogenic emissions. They measure cloud formation effectiveness and optical properties of organic aerosols of known shape, size, and composition. They also determine how these properties are affected by transformations in the atmosphere via oxidative reactions and coatings formed by deposition.

The experiments described in their publication in Environmental Science & Technology 2012, 46, 5430 were designed to study aerosol formation from gas phase alkanes emitted into the atmosphere as a result of oil spills such as the Deepwater Horizon accident. The experiments provided data that determine yields and chemical composition of the aerosol produced from atmospheric oxidation of the relevant gas phase species.

These studies were conducted in Professor Davidovits’s laboratories at Boston College in collaboration with colleagues from Aerodyne Research Inc., MIT, and Pennsylvania State University. The wide range of instruments and expertise required to perform complex atmospherically relevant studies make such collaborations essential. Three Boston College undergraduate students, David R. Croasdale, Justin P. Wright, and Alexander T. Martin, were important participants in these experiments and are co-authors of the publication.

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1,3,5-Triazine as a Modular Scaffold for Covalent Inhibitors with Streamlined Target Identification

Cysteine residues play key functional roles in regulating protein activity by acting as catalytic nucleophiles, sites of metal-binding, and redox-active centers. Small molecules that covalently modify these functional cysteine residues provide pharmacological tools to perturb protein activity within biological systems. In a recent publication in the Journal of the American Chemical Society, Professor Eranthie Weerapana and her coworkers reported the utility of trifunctionalized 1,3,5-triazine as an ideal modular scaffold to generate covalent inhibitors for cysteine-mediated protein activities. Triazines were derivatized with a thiol-reactive electrophile for covalent modification of target proteins, an alkyne as a click-chemistry handle for target identification, and a binding group to direct the compounds toward distinct subsets of the proteome. A library of trifunctionalized triazines was generated, and the cellular protein targets for these compounds were evaluated. These cellular screens identified two compounds, RB-2-cb and RB-11-ca, as cell permeable and highly selective covalent modifiers for Cys239 of beta-tubulin (TUBB) and Cys53 of protein disulfide isomerase (PDI), respectively. These compounds demonstrate in vitro and cellular potencies that are comparable to currently available modulators of tubulin polymerization and PDI activity. These findings demonstrate the versatility of the triazine core as a modular scaffold to generate potent and selective covalent modifiers of diverse protein families for future chemical genetics applications. Future studies will apply these bioactive small molecules to further interrogate the role of TUBB and PDI in a variety of pathological systems and expand the triazine library to target other reactive amino acids in the proteome. Postdoctoral fellow Ranjan Banerjee, graduate student Nicholas Pace, and undergraduate student Douglas Brown contributed to this study.

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Simple Organic Molecules as Catalysts for Enantioselective Synthesis of Amines and Alcohols

Discovery of easily accessible catalysts that generate high value organic compounds by sustainable enantioselective transformations is central to advances in the life sciences. In the February 14 issue of Nature, in a paper by various members of the Hoveyda research group, graduate students Dan Silverio and Erika Vieira, postdoctoral fellows Drs. Sebastian Torker and Tanya Pilyugina, and senior research associate Dr. Fredrik Haeffner introduce a set of small organic molecules that catalyze reactions of unsaturated organoboron reagents with imines and carbonyls; products are amines and alcohols of high enantiomeric purity, intermediates used to synthesize many biologically active molecules. A distinguishing feature of the catalyst class is a proton embedded within their structure. The resulting electronic activation and structural organization play a key role in every stage of the carbon-carbon bond forming processes; this includes achieving high rates of catalyst regeneration and product release, typically obtained through rapid ligand exchange with metal-containing systems. The catalyst is derived from the abundant amino acid valine and can be prepared in large quantities in four steps with cheap chemicals. Reactions are scalable, do not demand stringent conditions, can be performed with as little as 0.25 mole % catalyst in less than six hours at room temperature, and furnish products typically in >85% yield and ≥97:3 enantiomeric ratio. The efficiency, selectivity and operational simplicity of the transformations and the range of compatible boron-based reagents render this advance vital to future progress in chemistry, biology and medicine. The research is part of a longstanding collaboration (since 1995) between the research groups of Professor Amir Hoveyda and Professor Marc Snapper in our department.

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Magnesium Fluctuations Modulate RNA Dynamics in the Sam-I Riboswitch

Experiments demonstrate that Mg2+ is crucial for structure and function of RNA systems, yet the detailed molecular mechanism of Mg2+ action on RNA is not well understood. In collaboration with the research groups of Jose Onuchic (UCSD/Rice) and Karissa Sanbonmatsu (Los Alamos), Professor Udayan Mohanty has examined the interactions between RNA and its ion atmosphere at atomic resolution in the SAM-I aptamer. Specifically, they have carried out ten 2 µs explicit solvent molecular dynamics simulations and determined the excess number of ions around RNA as a function of varying ion concentrations. This quantity, which is experimentally measurable, is called the preferential interaction coefficient. Surprisingly, the investigators found that between 80% and 85% of the excess Mg2+ ions that contribute to the preferential interaction coefficient reside in the outer-sphere layer. The electronegative RNA atoms and the hydration layers both play a role in situating the excess Mg2+ ions. Furthermore, the outer-sphere Mg2+ ions exhibit 100-fold slower kinetics than the Mg2+ ions in the diffuse layer. As the outer-sphere accounts for most of Mg2+, it changes the paradigm of the characteristics of RNA-Mg2+ interactions. The work has been published in J. Am. Chem. Soc. 2012, 134, 12043–12053.

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Efficient Syntheses of 6,5'-(S)- and 6,5'-(R)- Cyclouridine

Professor Larry W. McLaughlin and graduate student Christopher S. Theile have successfully synthesized both the 5’-R and the 5’-S diastereomers of 6,5’-cyclouridine. Cyclonucleosides feature an extra bond between the backbone/sugar portion of the nucleoside and the base. These compounds are products formed in vivo from oxidative damage induced from radiation exposure or reactive oxygen species. The work has been published in Chem. Commun. 2012, 48, 5587.

Two diastereomers can be formed in the cyclization reaction. The 5’-S isomer has its 5’ hydroxyl group positioned over the sugar portion of the nucleoside, whereas the 5’-R compound has its hydroxyl group pointed away. These compounds have been very difficult to synthesize, especially the 5’-R isomer. The McLaughlin lab synthesis features an efficient oxidation step that generates both isomers at once, but favors the 5’-R compound over the 5’-S in a nearly 2:1 ratio. X-ray crystal structures of the compounds were also obtained, which show that the nucleobase is “pulled” back from its normal position, preventing it from forming hydrogen bonds in its native fashion.

Now that these compounds have finally been synthesized in the laboratory in an efficient manner, other scientists can begin using them to probe enzymes and biological systems, in order to understand better the damage that these cyclonucleosides can cause.

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Dual Absorber Hematite/Si Nanowire System for Photoelectrochemical Water Splitting at Low Applied Potentials

Using the sun to split water into oxygen and hydrogen, which could be used for fuel, remains elusive. However, a new study by Professor Dunwei Wang and co-workers brings this goal tantalizingly closer by crafting silicon nanowires coated with hematite (alpha-Fe2O3) that work synergistically to collect photons from different areas of the solar spectrum for conversion into electricity.

Such sun-powered water splitting, also known as photoelectrochemical (PEC) water splitting, would be an efficient way to collect solar energy and use it in the form of hydrogen fuel. The choice of the photoelectrode, the component that collects solar energy and converts it into electricity, is especially important because it determines the water-splitting device’s performance over time. The ideal photoelectrode would absorb light broadly, be inexpensive, and resist photocorrosion. Hematite fits some of these characteristics, but alone, it doesn’t perform well enough for consideration.

To improve hematite’s performance, Professor Wang’s research team grew crystals of this iron oxide on silicon nanowires, a material that absorbs light that is transparent to hematite. Together, the two materials absorb light from a much larger portion of the solar spectrum and efficiently convert that energy into electricity. The fact that the photoelectrode’s active materials are “primarily composed of three of the four most abundant elements in Earth’s crust (O, Si, and Fe) offers promise that renewable energy harvesting by PEC water splitting remains an achievable goal”, Professor Wang concludes.

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