Saturday, March 17, 2012

Spider silk conducts heat as well as metals

 Xinwei Wang had a hunch that spider webs were worth a much closer look.

So he ordered eight spiders -- Nephila clavipes, golden silk orbweavers -- and put them to work eating crickets and spinning webs in the cages he set up in an Iowa State University greenhouse.

Wang, an associate professor of mechanical engineering at Iowa State, studies thermal conductivity, the ability of materials to conduct heat. He's been looking for organic materials that can effectively transfer heat. It's something diamonds, copper and aluminum are very good at; most materials from living things aren't very good at all.

But spider silk has some interesting properties: it's very strong, very stretchy, only 4 microns thick (human hair is about 60 microns) and, according to some speculation, could be a good conductor of heat. But nobody had actually tested spider silk for its thermal conductivity.

And so Wang, with partial support from the Army Research Office and the National Science Foundation, decided to try some lab experiments. Xiaopeng Huang, a post-doctoral research associate in mechanical engineering; and Guoqing Liu, a doctoral student in mechanical engineering, helped with the project.

"I think we tried the right material," Wang said of the results.

What Wang and his research team found was that spider silks -- particularly the draglines that anchor webs in place -- conduct heat better than most materials, including very good conductors such as silicon, aluminum and pure iron. Spider silk also conducts heat 1,000 times better than woven silkworm silk and 800 times better than other organic tissues.

A paper about the discovery -- "New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and its Abnormal Change under Stretching" -- has just been published online by the journal Advanced Materials.

"Our discoveries will revolutionize the conventional thought on the low thermal conductivity of biological materials," Wang wrote in the paper.

The paper reports that using laboratory techniques developed by Wang -- "this takes time and patience" -- spider silk conducts heat at the rate of 416 watts per meter Kelvin. Copper measures 401. And skin tissues measure .6.

"This is very surprising because spider silk is organic material," Wang said. "For organic material, this is the highest ever. There are only a few materials higher -- silver and diamond."

Even more surprising, he said, is when spider silk is stretched, thermal conductivity also goes up. Wang said stretching spider silk to its 20 percent limit also increases conductivity by 20 percent. Most materials lose thermal conductivity when they're stretched.

That discovery "opens a door for soft materials to be another option for thermal conductivity tuning," Wang wrote in the paper.

And that could lead to spider silk helping to create flexible, heat-dissipating parts for electronics, better clothes for hot weather, bandages that don't trap heat and many other everyday applications.

What is it about spider silk that gives it these unusual heat-carrying properties?

Wang said it's all about the defect-free molecular structure of spider silk, including proteins that contain nanocrystals and the spring-shaped structures connecting the proteins. He said more research needs to be done to fully understand spider silk's heat-conducting abilities.

Wang is also wondering if spider silk can be modified in ways that enhance its thermal conductivity. He said the researchers' preliminary results are very promising.

And then Wang marveled at what he's learning about spider webs, everything from spider care to web unraveling techniques to the different silks within a single web. All that has one colleague calling him Iowa State's Spiderman.

"I've been doing thermal transport for many years," Wang said. "This is the most exciting thing, what I'm doing right now."

Story Source:

The above story is reprinted from materials provided by Iowa State University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Xiaopeng Huang, Guoqing Liu, Xinwei Wang. New Secrets of Spider Silk: Exceptionally High Thermal Conductivity and Its Abnormal Change under Stretching. Advanced Materials, 2012 DOI: 10.1002/adma.201104668

X-rays reveal how soil bacteria carry out surprising chemistry

Researchers from Singapore, Japan, the UK and USA have discovered how soil bacteria carry out surprising chemistry, defying a longstanding set of chemical rules and thus paving the way for new synthesis of polyether drugs.

Principal investigator, Chu-Young Kim, Assistant Professor at the Department of Biological Sciences of the National University of Singapore (NUS) Faculty of Science, and his group have made use of powerful X-rays to decipher how antibiotic-producing bacteria defy a longstanding set of chemical rules.

Their result, recently reported in Nature, details how a soil bacterium, Streptomyces lasaliensis, is able to convert an epoxide into a six-membered cyclic ether during synthesis of lasalocid, a natural polyether antibiotic. The fact that bacteria can perform such chemistry has puzzled chemists and biologists for decades because this type of chemical transformation is known to be kinetically unfavorable.

According to "Baldwin's Rules for Ring Closure," which govern the way these rings form, lasalocid should contain a five-membered ring instead of the observed six-membered ring.

"Our study has broad implications because the six-membered cyclic ether is a common structural feature found in hundreds of drug molecules produced by nature," said Dr Kim. "We have analysed the genes of six other organisms that produce similar polyether drugs and we are now confident that the biosynthetic strategy we have uncovered is also used by those organisms."

The solution to the molecular mystery depended in large part on a deeper understanding of the unique enzyme Lsd19 that catalyses the formation of two cyclic ether moieties that are part of the lasalocid structure. To determine the protein's atomic structure, researchers hit frozen crystals of Lsd19 with X-rays at the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, and analysed how the crystals diffracted the X-rays. "You need atomic-level detail of the protein's structure to understand what's really happening," said co-author Irimpan Mathews, a staff scientist at SLAC.

Lessons from the bugs

"The bugs have taught us a valuable chemistry lesson," Dr Kim said.

"With a new understanding of how nature synthesises the six-membered rings, chemists may be able to develop new methods to produce polyether drugs with ease in the laboratory. Alternatively, protein engineers may be able to use our results to develop a biofactory, where polyether drugs are mass produced using fermentation. Either method will make more effective and more affordable drugs available to the public."

Next challenge: Elucidating how nature synthesises an anti-cancer compound

Dr Kim's group has moved on to their next challenge: investigating how nature synthesises echinomycin, an anti-cancer compound produced, again, by soil bacteria. "We still have much chemistry to learn from the bugs."

Additional authors included Kinya Hotta, Xi Chen, Hao Li and Kunchithapadam Swaminathan of the National University of Singapore, Robert S. Paton of Oxford University, Atsushi Minami and Hideaki Oikawa of Hokkaido University, Kenji Watanabe of the University of Shizuoka and Kendall N. Houk of the University of California at Los Angeles.

Story Source:

The above story is reprinted from materials provided by National University of Singapore, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Kinya Hotta, Xi Chen, Robert S. Paton, Atsushi Minami, Hao Li, Kunchithapadam Swaminathan, Irimpan I. Mathews, Kenji Watanabe, Hideaki Oikawa, Kendall N. Houk, Chu-Young Kim. Enzymatic catalysis of anti-Baldwin ring closure in polyether biosynthesis. Nature, 2012; DOI: 10.1038/nature10865

Light-emitting nanocrystal diodes go ultraviolet

A multinational team of scientists has developed a process for creating glass-based, inorganic light-emitting diodes (LEDs) that produce light in the ultraviolet range. The work, reported this week in the online Nature Communications, is a step toward biomedical devices with active components made from nanostructured systems.

LEDs based on solution-processed inorganic nanocrystals have promise for use in environmental and biomedical diagnostics, because they are cheap to produce, robust, and chemically stable. But development has been hampered by the difficulty of achieving ultraviolet emission. In their paper, Los Alamos National Laboratory's Sergio Brovelli in collaboration with the research team lead by Alberto Paleari at the University of Milano-Bicocca in Italy describe a fabrication process that overcomes this problem and opens the way for integration in a variety of applications.

The world needs light-emitting devices that can be applied in biomedical diagnostics and medicine, Brovelli said, either as active lab-on-chip diagnostic platforms or as light sources that can be implanted into the body to trigger some photochemical reactions. Such devices could, for example, selectively activate light-sensitive drugs for better medical treatment or probe for the presence of fluorescent markers in medical diagnostics. These materials would need to be fabricated cheaply, on a large scale, and integrated into existing technology.

The paper describes a new glass-based material, able to emit light in the ultraviolet spectrum, and be integrated onto silicon chips that are the principal components of current electronic technologies.

The new devices are inorganic and combine the chemical inertness and mechanical stability of glass with the property of electric conductivity and electroluminescence (i.e. the ability of a material to emit light in response to the passage of an electric current). As a result, they can be used in harsh environments, such as for immersion into physiologic solutions, or by implantation directly into the body. This was made possible by designing a new synthesis strategy that allows fabrication of all inorganic LEDs via a wet-chemistry approach, i.e. a series of simple chemical reactions in a beaker. Importantly, this approach is scalable to industrial quantities with a very low start-up cost. Finally, they emit in the ultraviolet region thanks to careful design of the nanocrystals embedded in the glass.

In traditional light-emitting diodes, light emission occurs at the sharp interface between two semiconductors. The oxide-in-oxide design used here is different, as it allows production of a material that behaves as an ensemble of semiconductor junctions distributed in the glass. This new concept is based on a collection of the most advanced strategies in nanocrystal science, combining the advantages of nanometric materials consisting of more than one component. In this case the active part of the device consists of tin dioxide nanocrystals covered with a shell of tin monoxide embedded in standard glass: by tuning the shell thickness is it possible to control the electrical response of the whole material.

The paper was produced with the financial support of Cariplo Foundation, Italy, under Project 20060656, the Russian Federation under grant 11.G34.31.0027, the Silvio Tronchetti Provera Foundation, and Los Alamos National Laboratory's Directed Research and Development Program.

Story Source:

The above story is reprinted from materials provided by DOE/Los Alamos National Laboratory.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Sergio Brovelli, Norberto Chiodini, Roberto Lorenzi, Alessandro Lauria, Marco Romagnoli, Alberto Paleari. Fully inorganic oxide-in-oxide ultraviolet nanocrystal light emitting devices. Nature Communications, 2012; 3: 690 DOI: 10.1038/ncomms1683

Responding to the radiation threat

 The New York Times recently reported that in the darkest moments of the triple meltdown last year of the Fukushima Daiichi nuclear power plant, Japanese officials considered the evacuation of the nearly 36 million residents of the Tokyo metropolitan area. The consideration of so drastic an action reflects the harsh fact that in the aftermath of a major radiation exposure event, such as a nuclear reactor accident or a "dirty bomb" terrorist attack, treatments for mass contamination are antiquated and very limited.

The only chemical agent now available for decontamination -- a compound known as DTPA -- is a Cold War relic that must be administered intravenously and only partially removes some of the deadly actinides -- the radioactive chemical elements spanning from actinium to lawrencium on the periodic table -- that pose the greatest health threats.

Scientists at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) are developing a much more effective alternative that decontaminates a large number of the actinides likely to be part of the radiation exposure from a nuclear plant or weapon, including plutonium, americium, curium, uranium and neptunium. Furthermore, the Berkeley Lab treatment can be administered orally in the form of a pill, a necessity for prompt treatment in the event of mass contamination. Depending on the level of radiation exposure and how soon treatment can start, one of these pills would result in the excretion of approximately 90-percent of the actinide contaminants within 24 hours. Taking one pill daily for two weeks should be enough to remove virtually all of the actinide contaminants.

"With the expanding use of nuclear power and unfortunate possibility of nuclear weapon use, there is an urgent need to develop and implement an improved therapy for actinide contamination of a large population," says Rebecca Abergel, a chemist who leads the Bioactinide Group at Berkeley Lab's Glenn T. Seaborg Center. "We are now in the process of demonstrating that our actinide-specific decontaminating agents are ready for clinical development."

Once actinides are ingested or inhaled, their radioactivity and cancerous interactions with cells and tissue demand they be immobilized and removed from the body as soon as possible. Abergel and her group are part of an effort at Berkeley Lab that began more than two decades ago under the leadership of Ken Raymond, a chemist who holds joint appointments with Berkeley Lab and the University of California (UC) Berkeley, where he is the Chancellor's Professor of Chemistry, in collaboration with the late Patricia Durbin. The primary goal of this project has been to identify sequestering agents that can encapsulate actinides into tightly bound cage-like chemical complexes for transport out of the body. The early focus of this research was on plutonium, the alpha particle-emitting actinide discovered by Berkeley Lab Nobel laureate Glenn Seaborg, and natural chelators, the crablike molecules that specifically bind with iron and other metal ions.

"Since the biochemical properties of plutonium(IV) and iron(III) are similar, we modeled our sequestering agents after the chelating unit found in siderophores," Raymond says. Siderophores are small molecules secreted by bacteria to extract and solubilize iron. "This biomimetic approach enabled us to design multidentate hydroxypyridonate ligands that are unrivaled in terms of actinide-affinity, selectivity and efficiency."

The two best candidate hydroxypyridonate ligands -- nicknamed HOPO -- developed by Abergel and her colleagues are a tetradentate, which has four chelating arms, and an octadentate, which has eight chelating arms. The "arms" in this case are atoms with pairs of electrons available for covalent bonding with an actinide.

"We've advanced our two candidate ligands through the initial phases of pre-clinical development by successfully scaling up synthesis to the 5-kilograms level and establishing baseline preparation and analytical methods suitable for manufacturing larger amounts under good manufacturing practice guidelines," Abergel says.

The team has also carried out extensive studies in animal models and human cell lines that established the two HOPO candidates as being highly effective and non-toxic at the tested doses. As for comparisons between the two, each has its own merits.

"A single octadentate HOPO can form a full actinide complex and results in more total actinide excretion," Abergel says. "However, it is easier for the smaller tetradentate HOPO to pass through biological membranes and access desired target sites in the body. Both warrant further development for emergency use in the case of a radiological event."

Abergel says the basic research and development phase of these two candidates has been completed and she and her group have started the process with the U.S. Food and Drug Administration (FDA) to determine what further data is needed to move into clinical trials. Typically at this stage of development a private pharmaceutical company would step in but it is difficult to attract private investors for a drug that will hopefully never be needed.

"As we move further along with the FDA process it should be easier to convince private pharmaceutical companies to get involved," Abergel says.

In addition to Abergel, Raymond and Durbin, other researchers who are or have been involved in this project include Dahlia An, Kathleen Bjornstad, Eleanor Blakely, Deborah Bunin, Polly Chang, Shirley Ebbe, Erin Jarvis, Birgitta Kullgren, Chris Rosen, David Shuh, Manuel Sturzbecher-Hoehne and Jide Xu.

There have been several scientific papers published about this work with the most recent being "Multidentate terephthalamidate and hydroxypyridonate ligands: towards new orally active chelators," in the journal Hemoglobin. It was written by Abergel and Raymond.

This research was primarily supported by the National Institutes of Health through the National Institute of Allergy and Infectious Diseases and the Rapid Access to Interventional Development Program. Support also came from the DOE Office of Science.

Story Source:

The above story is reprinted from materials provided by DOE/Lawrence Berkeley National Laboratory.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Rebecca J. Abergel, Kenneth N. Raymond. Multidentate Terephthalamidate and Hydroxypyridonate Ligands: Towards New Orally Active Chelators. Hemoglobin, 2011; 35 (3): 276 DOI: 10.3109/03630269.2011.560771

Nanomaterials: A coating protocol

A robust approach for preparing polymer-coated quantum dots may find use in a wide range of applications.

Quantum dots (QDs) are tiny crystals of semiconducting material that produce fluorescence. The color or the wavelength of the fluorescence is dependent on the size, shape and composition of QDs. Larger QDs tend to emit light at the red end (longer wavelengths) of the electromagnetic spectrum. As the size of the QDs decrease, so does the wavelength of emitted light. This tunability of emission wavelength is one reason why QDs have become popular for use as fluorescent markers in biological research. For example, scientists can attach QDs to single molecules and cells and track their movements over time using fluorescence microscopy.

Dominik Jańczewski, Nikodem Tomczak and Ming-Yong Han at the A*STAR Institute of Materials Research and Engineering and co-workers1 have now described a protocol for the preparation of quantum dots coated with an amphiphilic polymer -- a polymer that contains both water-attracting and -repelling components. "Our aim is to develop a robust approach for the preparation of QD for use as fluorescent tags for bioimaging, sensing and therapeutics," says Han. "The method we have developed is applicable to any nanoparticles, not just QDs."

Most biological applications require the use of QDs that disperse and remain stable in an aqueous solution. Conventional approaches for synthesizing QDs typically endow the QDs with a coating of hydrophobic ligands, which are repelled by water. Although it is possible to exchange the ligands after synthesis, a ligand shell that is exchangeable is, by its very nature, unstable and might result in the release of toxic materials, such as cadmium, into solution.

Instead of exchanging the ligands, an alternative method to make the QDs disperse in water is to coat them with a polymer that has both hydrophilic and hydrophobic parts. This works on the simple principle that like attracts like -- or in other words, hydrophobic parts of the polymer attract hydrophobic ligands that stabilize the QDs, and hydrophilic parts of the polymer attract water molecules in solution.

The new protocol describes the procedure in detail and aims to provide the benefits of the research team's experience in QD synthesis to others whose interests might be focused more on applications rather than the development of synthetic methods. The synthesis of the polymer coating allows the incorporation of a wide variety of functional groups. "In the future we hope to work towards image guided therapy," says Han. "QDs could be prepared that not only produce an image of cancer cells, but also release drugs at such a target."

Story Source:

The above story is reprinted from materials provided by The Agency for Science, Technology and Research (A*STAR), via ResearchSEA.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Dominik Jańczewski, Nikodem Tomczak, Ming-Yong Han, G Julius Vancso. Synthesis of functionalized amphiphilic polymers for coating quantum dots. Nature Protocols, 2011; 6 (10): 1546 DOI: 10.1038/nprot.2011.381