Inorganic molecules achieve self-recognition

Tianbo Liu, associate professor of chemistry, and his research group have discovered a high-level molecular self-recognition in dilute aqueous solutions, something that was previously considered achievable only by biological molecules.

The group’s results were published in the March 25 issue of Science, the nation’s premier science journal. Liu was lead author on the article, which was titled "Self-Recognition Among Different Polyprotic Macroions During Assembly Processes in Dilute Solution."

“Publication of this work in Science is important recognition of the research being conducted in Tianbo’s lab,” said Robert Flowers, department chair and professor of chemistry. “His ability to succeed at such a high level shows that first-rate science is being done at Lehigh.”

Liu’s group has spent several years exploring the fascinating solutions of large, soluble ions called macroions. The behavior of these ions is completely different from the behavior of small ions, such as sodium chloride.

Despite being water-soluble and carrying the same type of charge, macroions tend to attract each other with surprising strength, says Liu, and to form very stable, uniform, single-layered hollow spheres known as “blackberry structures.” The structures are common when ions become large, and they mimic some biological processes such as the virus capsid shell formation.

Forming two distinct blackberry structures

Exciting discoveries have been generated from blackberry solutions. Liu’s group found that, when mixed into the same solution, two different types of 2.5-nm spherical macroions ({Mo72Fe30} and {Mo72Cr30}) with almost identical size, shape and molecular structures tend to form two types of individual blackberries instead of mixed ones.

The macroions—Bucky ball-shaped inorganic compounds—were synthesized by a research team led by Achim Müller, professor of chemistry at the University of Bielefeld, Germany. Müller was a coauthor on the Science article.

This result, says Liu, suggests that even in dilute solutions these two macroions can self-recognize during assembly.

This level of “intelligence,” he adds, is usually believed to be achievable only by complex biological molecules. Self-recognition by large inorganic ions could lead to more opportunities for understanding the nature of biological interactions.

Liu’s group believes the self-recognition results from the very slow formation of the dimers in the first step of the assembly. The slow speed ensures the formation of dimers with the lowest free energy, such as A-A and B-B dimers.

The differences in charge density between the two types of macroions play an important role in the recognition, says Liu, as does their surface water mobility difference.

More information: Self-Recognition Among Different Polyprotic Macroions During Assembly Processes in Dilute Solution, Science 25 March 2011: Vol. 331 no. 6024 pp. 1590-1592 DOI: 10.1126/science.1201121

ABSTRACT
We report a self-recognition phenomenon based on an assembly process in a homogeneous dilute aqueous solution of two nano-scaled, spherical polyprotic metal oxide–based macroions (neutral species in crystals), also called Keplerates of the type [(linker)30(pentagon)12]?[{M(H2O)}30{(Mo)Mo5}12] where M is FeIII or CrIII. Upon deprotonation of the neutral species, the resulting macroions assemble into hollow “blackberry”-type structures through very slow homogeneous dimer-oligomerization processes. Although the geometrical surface structures of the two macroions are practically identical, mixtures of these form homogeneous superstructures, rather than mixed species. The phase separation is based on the difference in macroionic charge densities present during the slow homogeneous dimer or oligomer formation. The surface water ligands’ residence times of CrIII and FeIII differ markedly and lead to very different interfacial water mobilities between the Keplerates.

Provided by Lehigh University (news : web)

Chemists shed new light on antibiotics and the survival of bacteria

Research in the laboratory of Shahriar Mobashery in the University of Notre Dame’s Department of Chemistry and Biochemistry has led to further understanding of how a bacterial cell wall cross-links, an event that penicillin and other antibiotics disrupt, a step in the maturation of a cell wall that is critical for the survival of bacteria.

Mobashery is the Navari Family Chair in Life Sciences at Notre Dame. His group published the findings recently in the Journal of the American Chemical Society in an article titled, “A Computational Evaluation of the Mechanism of Penicillin-Binding Protein-Catalyzed Cross-Linking of the Bacterial Cell Wall.”

This very process is the step in maturation of a cell wall that penicillin and other members of the ß-lactam class of antibiotics, the most commonly used antibacterial agents, interfere with. Scientists since the 1940s have worked to explain the antibiotic properties of penicillin, and research had shown that the drug interferes with the cell wall cross-linking, one of the final steps in the maturation of the cell wall. The interference by penicillin leads to points of weakness in the cell wall. Since bacteria cannot regulate their internal osmotic pressure, the action of the drug on the cell wall leads to bacterial death by bursting of the cell.

Five years ago, Mobashery’s lab determined the solution structure of the building units of the cell wall, also known as the peptidoglycan. This solution structure for the peptidoglycan was used in the present study in conjunction with a crystal structure determined by a French group for a transpeptidase, the enzyme that catalyzes the cross-linking reaction. The new research shows how the enzyme unites two fragments of the peptidoglycan in the critical cross-linking reaction of the cell wall.

“The current paper addresses the physiological function of the enzyme that penicillin inhibits,” Mobashery said. “It opens up opportunities to rethink the process of inhibition. You have the knowledge of how the cell wall cross-linking takes place and you can now mimic it. It has shed light on what I would consider to be a marvel of nature.”

Mobashery has a recent NIH grant for five years to study the maturation of the bacterial cell wall, building on the discoveries in the article and other research that is ongoing in his lab. The mature cross-linked cell wall is a single molecule, the largest molecule in bacteria, larger than the bacterial chromosome. Since it is known that a single bacterium contains tens of thousands of these cross-links and that the reaction is the step inhibited by penicillin, the present study illuminates an important aspect of bacterial physiology.

More information: A Computational Evaluation of the Mechanism of Penicillin-Binding Protein-Catalyzed Cross-Linking of the Bacterial Cell Wall, J. Am. Chem. Soc., 2011, 133 (14), pp 5274–5283. DOI: 10.1021/ja1074739

Abstract
Penicillin-binding protein 1b (PBP 1b) of the Gram-positive bacterium Streptococcus pneumoniae catalyzes the cross-linking of adjacent peptidoglycan strands, as a critical event in the biosynthesis of its cell wall. This enzyme is representative of the biosynthetic PBP structures of the ß-lactam-recognizing enzyme superfamily and is the target of the ß-lactam antibiotics. In the cross-linking reaction, the amide between the -d-Ala-d-Ala dipeptide at the terminus of a peptide stem acts as an acyl donor toward the ?-amino group of a lysine found on an adjacent stem. The mechanism of this transpeptidation was evaluated using explicit-solvent molecular dynamics simulations and ONIOM quantum mechanics/molecular mechanics calculations. Sequential acyl transfer occurs to, and then from, the active site serine. The resulting cross-link is predicted to have a cis-amide configuration. The ensuing and energetically favorable cis- to trans-amide isomerization, within the active site, may represent the key event driving product release to complete enzymatic turnover.

Provided by University of Notre Dame (news : web)

Say hello to cheaper hydrogen fuel cells: Scientists document utility of non-precious-metal catalysts

  Los Alamos National Laboratory scientists have developed a way to avoid the use of expensive platinum in hydrogen fuel cells, the environmentally friendly devices that might replace current power sources in everything from personal data devices to automobiles.

In a paper published today in Science, Los Alamos researchers Gang Wu, Christina Johnston, and Piotr Zelenay, joined by researcher Karren More of Oak Ridge National Laboratory, describe the use of a platinum-free catalyst in the cathode of a hydrogen fuel cell. Eliminating platinum—a precious metal more expensive than gold—would solve a significant economic challenge that has thwarted widespread use of large-scale hydrogen fuel cell systems.

Polymer-electrolyte hydrogen fuel cells convert hydrogen and oxygen into electricity. The cells can be enlarged and combined in series for high-power applications, including automobiles. Under optimal conditions, the hydrogen fuel cell produces water as a "waste" product and does not emit greenhouse gasses. However, because the use of platinum in catalysts is necessary to facilitate the reactions that produce electricity within a fuel cell, widespread use of fuel cells in common applications has been cost prohibitive. An increase in the demand for platinum-based catalysts could drive up the cost of platinum even higher than its current value of nearly $1,800 an ounce.

The Los Alamos researchers developed non-precious-metal catalysts for the part of the fuel cell that reacts with oxygen. The catalysts—which use carbon (partially derived from polyaniline in a high-temperature process), and inexpensive iron and cobalt instead of platinum—yielded high power output, good efficiency, and promising longevity. The researchers found that fuel cells containing the carbon-iron-cobalt catalyst synthesized by Wu not only generated currents comparable to the output of precious-metal-catalyst fuel cells, but held up favorably when cycled on and off—a condition that can damage inferior catalysts relatively quickly.

Moreover, the carbon-iron-cobalt catalyst fuel cells effectively completed the conversion of hydrogen and oxygen into water, rather than producing large amounts of undesirable hydrogen peroxide. Inefficient conversion of the fuels, which generates hydrogen peroxide, can reduce power output by up to 50 percent, and also has the potential to destroy fuel cell membranes. Fortunately, the carbon- iron-cobalt catalysts synthesized at Los Alamos create extremely small amounts of hydrogen peroxide, even when compared with state-of-the-art platinum-based oxygen-reduction catalysts.

Because of the successful performance of the new catalyst, the Los Alamos researchers have filed a patent for it.

"The encouraging point is that we have found a catalyst with a good durability and life cycle relative to platinum-based catalysts," said Zelenay, corresponding author for the paper. "For all intents and purposes, this is a zero-cost catalyst in comparison to platinum, so it directly addresses one of the main barriers to hydrogen fuel cells."

The next step in the team's research will be to better understand the mechanism underlying the carbon-iron-cobalt catalyst. Micrographic images of portions of the catalyst by researcher More have provided some insight into how it functions, but further work must be done to confirm theories by the research team. Such an understanding could lead to improvements in non-precious-metal catalysts, further increasing their efficiency and lifespan.

Provided by Los Alamos National Laboratory (news : web)

Building a better battery

“What we are trying to do is put different pieces of a puzzle together,” said Argonne National Laboratory scientist Daniel P. Abraham. The puzzle is a lithium-rich compound material, Li1.2Co0.4Mn0.4O2, that holds key insights into the development of more powerful and robust batteries for electric cars. Using a suite of advanced techniques, including the resources of the U.S. Department of Energy’s Advanced Photon Source at Argonne, Abraham and his colleagues have pieced together both the long-range and local structure of this compound, devising a model that could explain how such materials operate on the electrochemical level — and how to use them to build a better battery.

Argonne is at the cutting edge of the effort to create advanced battery technologies, focusing especially on the lithium-ion (Li-ion) battery, which promises more energy and longer life than the nickel-metal hydride battery currently used in electric and hybrid vehicles. For instance, the new Chevy Volt, the first mass-produced plug-in hybrid electric car, is powered by a battery whose chemistry is based in part on important breakthroughs developed by scientists at Argonne. Like all batteries, Li-ion batteries work by moving electric charge (in this case, lithium ions) between an anode and cathode. But vital properties — such as the amount of electricity a battery can deliver before recharging is necessary and how many times it can be recharged before its microstructure breaks down under the repeated cycling stress — can vary greatly depending on the composition of the cathode and anode. Most research has focused on compounds using transition metals such as cobalt and manganese, but the precise structural formulation of the compounds and what might make one work better than another have largely been a matter of trial and error and speculation. The new Argonne work, published in Chemistry of Materials, is an important step toward resolving the confusion.

Co-principal investigator Mahalingam Balasubramanian (also from Argonne), Abraham, and colleagues from Argonne and the Frederick Seitz Materials Research Laboratory of the University of Illinois examined the layered lithium oxide Li1.2Co0.4Mn0.4O2 , which is a model compound for many of the advanced battery materials currently under development for electric vehicles. The experimenters subjected samples to a variety of techniques including x-ray diffraction (XRD), scanning-transmission electron microscopy (STEM), and x-ray absorption spectroscopy (XAS). The combination of methods yielded a more comprehensive picture than previously achieved. “It’s sort of like the story of the blind men and the elephant,” Abraham explained. “When different people use different techniques, they obtain data that are only one part of the puzzle, but often the results are presented in literature as the final answer. The most important thing that we did was to put together information from the various experimental techniques to obtain a composite picture.”

When examined by XRD, Li1.2Co0.4Mn0.4O2 reveals an average long-range crystal structure similar to that of layered LiCoO2. STEM results, however, revealed the presence of lithium atoms, in sites typically occupied by cobalt and manganese atoms, ordered in a manner similar to that in layered Li2MnO3. But the STEM data could not establish the structural relationship between the ordered lithium atoms and the neighboring cobalt and manganese atoms. The XAS experiments conducted at the X-ray Science Division 20-BM bending magnet beamline of the Advanced Photon Source, on the other hand, were successful in distinguishing the local atomic environments around the cobalt and manganese. Balasubramanian and his collaborators determined that while the crystalline structure of the Li1.2Co0.4Mn0.4O2 compound is largely homogeneous over the long range, the local structure contains Li2MnO3 and LiCoO2 nanoclusters. These local clusters might present connectivity over long distances as a dendritic network. “Having continuous regions allows for each unit cell to ‘talk’ to the next unit cell over very long distances,” Balasubramanian said. This connectivity enhances the transport of lithium ions and hence, the electrochemical performance of the material for battery applications. “It’s the local structure that determines the properties of the compound,” Abraham added.

The team’s broad-based approach of bringing together a wide palette of experimental methods, rather than relying on a single technique, is an important step towards the custom design of battery materials based on results from advanced diagnostic techniques. “In the past, most advances in battery materials development arose from intuitive experimentation,” explained Balasubramanian. “We want to complement this intuitive Edisonian approach with a rigorous basic approach to tailoring new materials.”

The next step is to observe the structural changes that occur in these lithium-rich compounds as they are cycled through various voltage windows, which is critical for determining battery life and durability. “Tools and techniques like those available at the APS can provide us unique insights to develop structure-property correlations, which are very important to predict long-term battery performance,” Abraham said.

The work lays a solid foundation for the design and development of new compounds. “We want to be able to tailor these materials with desired properties for specific applications," Balasubramanian said. "We need transformational materials with high capacities, high power, and stable crystal structures for the successful commercialization of lithium-ion batteries for a myriad of demanding applications.”

More information: J. Bareno1, M. et al, “Long-Range and Local Structure in the Layered Oxide 2 Li1.2Co0.4Mn0.4O2,” Chem. Mater. 23(8), 2039 (2011). DOI:10.1021/cm200250a

Provided by Argonne National Laboratory (news : web)

2D beats 3D: Ceria in platelet form stores more oxygen than nanocrystalline form

Three dimensions are not necessarily better than two. Not where ceria is concerned, in any case. Ceria is an important catalyst. Because of its outstanding ability to store oxygen and release it, ceria is primarily used in oxidation reactions. Christopher B. Murray and a team at the University of Pennsylvania have now developed a simple synthetic technique to produce ceria in the form of nanoplates. As the researchers report in the journal Angewandte Chemie, these have proven to be better at storing oxygen than conventional three-dimensional nanoparticles.

In automotive catalytic converters, ceria helps to level out hydrocarbon spikes. It can also be used in the removal of soot from diesel exhaust and organic compounds from wastewater, for example. In fuel cells, ceria is used as a solid electrolyte. Cerium, a rare-earth metal, can easily switch between two different oxidation states (+IV and +III), so it undergoes a smooth transition between CeO2 and materials with a lower oxygen content. This makes ceria an ideal material for oxygen storage.

Ceria can be produced as a nanomaterial in various different forms. Almost all of the previously described forms were three-dimensional. Murray’s team has now developed a handy method for the synthesis of two-dimensional nanoplates. Their synthetic technique is based on the thermal decomposition of cerium acetate at 320 to 330 °C. Critical to their success is the presence of a mineralization agent, which speeds up the crystallization process and controls the morphology. Depending on the reaction conditions, the researchers obtained either roughly square plates with a thickness of 2 nm and edges about 12 nm in length, or elongated plates with dimensions of about 14 x 152 nm.

To test the oxygen storage capacities of the various forms of ceria, the researchers established a very simple thermogravimetric test: They alternately exposed the samples to oxygen and hydrogen and recorded the change in mass due to oxygen absorption/emission. The nanoplates proved to be superior to the conventional particulate systems and displayed an oxygen capacity three to four times as high as that of conventional three-dimensional nanoparticles. The plates do have a higher surface-to-volume ratio than the three-dimensional particles but the uptake of oxygen in the body of the nanoplates is required to explain this magnitude of enhancement. Furthermore, not all surfaces of a ceria crystal are equally good for the absorption and emission of oxygen. It turns out that the platelet surfaces were of the right type.

More information: Christopher B. Murray, Synthesis and Oxygen Storage Capacity of 2-D Ceria Nanocrystals, Angewandte Chemie International Edition 2011, 50, No. 19, 4378–4381, Permalink to the article: http://dx.doi.org/ … ie.201101043

Provided by Wiley (news : web)

Carbohydrate adhesion gives stainless steel implants beneficial new functions

A new chemical bonding process can add new functions to stainless steel and make it a more useful material for implanted biomedical devices. Developed by an interdisciplinary team at the University of Alberta and Canada's National Institute for Nanotechnology, this new process was developed to address some of the problems associated with the introduction of stainless steel into the human body.

Implanted biomedical devices, such as cardiac stents, are implanted in over 2 million people every year, with the majority made from stainless steel. Stainless steel has many benefits -- strength, generally stability, and the ability to maintain the required shape long after it has been implanted. But, it can also cause severe problems, including blood clotting if implanted in an artery, or an allergenic response due to release of metal ions such as nickel ions.

The University of Alberta campus is home to a highly multidisciplinary group of researchers, the CIHR Team in for Glyconanotechnology in Transplantation, that is looking to develop new synthetic nanomaterials that modify the body's immune response before an organ transplant. The ultimate goal is to allow cross-blood type organ transplants, meaning that blood types would not necessarily need to be matched between donor and recipient when an organ becomes available for transplantation. Developing new nanomaterials that engage and interact with the body's immune system are an important step in the process. In order to overcome the complex range of requirements and issues, the project team drew on expertise from three major areas: surface science chemistry and engineering, carbohydrate chemistry, and immunology and medicine.

For the transplantation goals of the project, sophisticated carbohydrate (sugar) molecules needed to be attached to the stainless steel surface to bring about the necessary interaction with the body's immune system. Its inherent stainless characteristic makes stainless steel a difficult material to augment with new functions, particularly with the controlled and close-to-perfect coverage needed for biomedical implants. The Edmonton-based team found that by first coating the surface of the stainless steel with a very thin layer (60 atoms deep) of glass silica using a technique available at the National Institute for Nanotechnology, called Atomic Layer Deposition (ALD), they could overcome the inherent non-reactivity of the stainless steel. The silica provide a well-defined "chemical handle" through which the carbohydrate molecules, prepared in the Alberta Ingenuity Centre for Carbohydrate Science, could be attached. Once the stainless steel had been controlled, the researchers demonstrated that the carbohydrate molecules covered the stainless steel in a highly controlled way, and in the correct orientation to interact with the immune system.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by National Institute for Nanotechnology, via EurekAlert!, a service of AAAS.