Monday, April 4, 2011

Scientists unlock mystery of how the 22nd amino acid is produced

The most recently discovered amino acid, pyrrolysine, is produced by a series of just three chemical reactions with a single precursor – the amino acid lysine, according to new research.

Scientists at Ohio State University used mass spectrometry and a series of experiments to discover how cells make the amino acid, a process that until now had been unknown.

They confirmed that pyrrolysine is made from enzymatic reactions with two lysine molecules – a surprising finding, given that some portions of its structure suggested to researchers that it might have more complex origins.

The research is published in the March 31 issue of the journal Nature.

Pyrrolysine is rare and so far is known to exist in about a dozen organisms. But its discovery in 2002 as a genetically encoded amino acid in methane-producing microbes raised new questions about the evolution of the genetic code. Pyrrolysine is among 22 amino acids that are used to create proteins from the information stored in genes. Proteins are essential to all life and perform most of the work inside cells.

This information about how it is produced – its biosynthetic pathway – offers a more complete understanding of how amino acids are made. And because of its rarity, this molecule is emerging as a handy tool for manipulating proteins in biomedical research. With its production mechanism identified, scientists can use that information to devise ways to mass-produce similar or identical synthetic molecules for a variety of research purposes.

The Ohio State scientists had a genuine "ah-ha" moment over the course of the study. As part of their experimentation, they combined lysine with one other amino acid and some enzymes and expected this to produce what is called an intermediate – essentially, a piece of an amino acid that is generated in the biosynthesis process.

They had labeled the lysine so it would appear heavier than normal when observed using mass spectrometry. But one signal produced by the instrumentation had a much different mass than could be attributed to the intermediate.

"We weren't seeing this weird molecule made from two different amino acids that we were expecting. We were seeing the regular pyrrolysine molecule and all of it was coming from lysine. Every bit of it," said Joseph Krzycki, professor of microbiology at Ohio State and senior author of the study. "That was the only way we saw pyrrolysine, and all of it was labeled with lysine. That's the basic observation here. And it's a real surprise."

The finding that lysine was the only precursor was a surprise because the production process ended up being so simple – even though arriving at it was not a simple task, partly because some of the had never been observed before.

"What amazes me about the entire chemical pathway is that you need only three enzymes and two molecules of the same thing that together make one complete molecule that looks completely different from what you started with," said Marsha Gaston, first author of the paper and a doctoral student in microbiology. "You have one portion that looks exactly like the precursor, but then you have another portion that enzymes are able to re-arrange in a way that is completely unique and never seen before."

Mass spectrometry, an analytic technique that enables precision in determining the mass of particles, ended up being critical to the discoveries, Krzycki noted. Liwen Zhang and Kari Green-Church of Ohio State's Campus Chemical Instrument Center/Mass Spectrometry and Proteomics Facility are additional co-authors of the study.

Krzycki led one of the two teams of Ohio State researchers that discovered pyrrolysine in 2002. The teams have since synthesized the amino acid and shown how bacteria incorporate it into proteins.

"That left some big questions unanswered: How do you make pyrrolysine? Where does it come from? What metabolic pathways does it come off of? Because it's got to be generated within the cell that uses it," Krzycki said.

The chemical shape of pyrrolysine offered some clues. Its carbon skeleton resembles that of lysine. But it also has an unusual ring on one end, and a methyl group attached to it, which for researchers raised questions about its origin.

The researchers also knew from their previous work that three genes are required to generate the instructions for the assembly of proteins that contain pyrrolysine – pylB, pylC and pylD. So the enzymes produced by those three genes had to have a role in creation of the amino acid. Finally, previous attempts by other researchers to define its biosynthesis suggested that another amino acid, D-ornithine, was involved in pyrrolysine's production.

So Krzycki and his colleagues set out to test that theory. Conducting all of their experiments in a strain of E. coli bacteria, commonly used to test biological functions, they combined lysine and D-ornithine molecules.

They found that this didn't make pyrrolysine, but rather a molecule like pyrrolysine that was missing a key part; however, this molecule turned out not to be converted to pyrrolysine. This molecule also was formed without the involvement of pylB – a gene that could not be left out of the process that actually makes pyrrolysine.

With the mass spectrometry instead identifying lysine as the only precursor to pyrrolysine, the researchers then used genetics, of intermediates and deduction to determine the order of enzymatic reactions that converted two lysine molecules into the pyrrolysine amino acid.

They determined that the sequence of events matched the alphabetical order of the three involved enzymes: PylB uses lysine to make a D-ornithine-like intermediate, PylC joins the two lysine molecules together, and that feeds a reaction involving PylD that results in the formation of pyrrolysine. The reactions showed how the ring on pyrrolysine's end, its major identifying characteristic, is formed.

"If you splay out the pyrrolysine molecule, you can recognize that in fact it looks a lot like lysine, except that to get to this ring, you have to make the second molecule one carbon unit shorter," Krzycki said. "The lysine goes through a type of enzymatic reaction called a mutase reaction, where the carbon skeleton is rearranged to make this shorter molecule, which is like D-ornithine, but with one extra carbon now hanging off the chain in a new place. That's what one of our pyrrolysine biosynethetic enzymes, PylB, is doing."

Krzycki noted that this finding will add fuel to discussions of how the genetic code evolved. For example, the co-evolutionary theory suggests that amino acids arising from a common precursor have similar codon assignments. Codons are three-letter "words" identifying the bases that DNA uses to specify particular as building blocks of proteins. Normally, codons signal the start or end of a protein, or a particular amino acid used to construct it.

"For the scientists who are devoted to exploring how the genetic code evolved, our data provides new insights that can feed the various theories for how the code evolved; the co-evolutionary theory is just one such example," Krzycki said.

The finding that pyrrolysine derives entirely from lysine means that pyrrolysine is part of the aspartic acid family in bacteria and Archaea, a group of single-cell microorganisms that are similar to bacteria in size and shape, but with a different evolutionary history. The microbes known to contain pyrrolysine are in the Archaea domain, and are able to convert a common class of compounds – the methylamines – into methane gas.

Provided by The Ohio State University (news : web)

URI scientist discovers 54 beneficial compounds in pure maple syrup

University of Rhode Island researcher Navindra Seeram has discovered 34 new beneficial compounds in pure maple syrup and confirmed that 20 compounds discovered last year in preliminary research play a key role in human health.

Today at the 241st American Chemical Society's National Meeting in Anaheim, Calif. the URI assistant pharmacy professor is telling scientists from around the world that his URI team has now isolated and identified 54 beneficial compounds in pure maple syrup from Quebec, five of which have never been seen in nature.

"I continue to say that nature is the best chemist, and that maple syrup is becoming a champion food when it comes to the number and variety of beneficial compounds found in it," Seeram said. "It's important to note that in our laboratory research we found that several of these compounds possess anti-oxidant and anti-inflammatory properties, which have been shown to fight cancer, diabetes and bacterial illnesses."

These discoveries of new molecules from nature can also provide chemists with leads that could prompt synthesis of medications that could be used to fight fatal diseases, Seeram said.

"We know that the compounds are anti-inflammatory agents and that inflammation has been implicated in several chronic diseases, such as heart disease, diabetes, certain types of cancers and , such as Alzheimer's," Seeram said.

As part of his diabetes research, Seeram has collaborated with Chong Lee, professor of nutrition and food sciences in URI's College of the Environment and Life Sciences. The scientists have found that maple syrup phenolics, the beneficial anti-oxidant compounds, inhibit two carbohydrate hydrolyzing enzymes that are relevant to management.

The irony of finding a potential anti-diabetes compound in a sweetener is not lost on Seeram. "Not all sweeteners are created equal," he said.

Among the five new compounds is Quebecol, a compound created when a farmer boils off the water in maple sap to get maple syrup. It takes 40 liters (20.5 gallons) of sap to make 1 liter (2 pints) of syrup.

"Quebecol has a unique chemical structure or skeleton never before identified in nature," Seeram said. "I believe the process of concentrating the maple sap into maple syrup is what creates Quebecol. There is beneficial and interesting chemistry going on when the boiling process occurs. I believe the heat forms this unique compound."

Seeram said he and his team chose the common name of Quebecol for the new compound to honor the province of Quebec in Canada, which leads the worldwide production of maple syrup. Seeram's research was supported by the

Conseil pour le developpement de l'agriculture du Quebec (CDAQ) and Agriculture and Agri-Food Canada (AAFC) on behalf of the Canadian maple syrup industry.

"Producers, transformers and partners of the Canadian maple industry believe that investing in maple syrup knowledge and innovation will bring the products to another level in a few years," said Serge Beaulieu, president of the Federation of Quebec Maple Syrup Producers and member of the Canadian Maple Industry Advisory Committee.

"Quebec Maple Syrup Producers are especially proud to be leading this long-term innovative strategy on behalf of the Canadian industry and with the talented scientists of the Canadian Maple Innovation Network."

Genevieve Beland, marketing director of the Federation added, "Maple products' composition is unique and we are at the starting point of a new era. Ten years from now consumers will appreciate 100 percent pure maple products because they are delicious, natural and have a number of healthy compounds."

"As we continued our research in the past year, we were astonished when the number of beneficial compounds that we isolated is now more than double the original amount," said Seeram, who is releasing his findings today.

Seeram is the organizer of the daylong symposium on "Bioactives in Natural Sweeteners," and is joined by scientists from Canada, Japan, Mexico and the United States to discuss natural sweeteners. Seeram's collaborations with Angela Slitt, assistant professor of biomedical sciences in URI's College of Pharmacy and Professor Lee, will also be presented during the meeting.

Seeram's findings will be detailed in his article recently accepted for publication in the Journal of Functional Foods. The title of the paper is "Quebecol, a novel phenolic compound isolated from Canadian maple syrup." In addition, Seeram and Lee's work on diabetes and maple syrup will also be published in an upcoming edition of the Journal of Functional Foods.

"I can guarantee you that few, if any, other natural sweeteners have this anti-oxidant cocktail of beneficial compounds; it has some of the beneficial compounds that are found in berries, some that are found in tea and some that are found in flaxseed. People may not realize it, but while we have a wide variety of fruits and vegetables in our food chain, maple syrup is the single largest consumed food product that is entirely obtained from the sap of trees," Seeram said.

Reiterating a statement he made last year, Seeram said no one is suggesting that people consume large quantities of maple syrup, but that if they are going to use a sweetener on their pancakes, they should choose pure maple syrup and not the commercial products with high fructose corn syrup.

"Pure is not only delicious, it is so much better for you," Seeram said.

Provided by University of Rhode Island (news : web)

A surprising new vehicle for drug delivery?

Are our bodies vulnerable to some pollutants whose lack of solubility in water, or "hydrophobicity," has always been thought to protect us from them? New Tel Aviv University research has discovered that this is indeed the case.

Studies by Dr. Michael Gozin of Tel Aviv University's School of Chemistry at the Raymond and Beverly Sackler Faculty of Exact Sciences and Dr. Dan Peer of TAU's Laboratory of in the Department of Cell Research and Immunology have revealed that — the thick substance lining those internal bodily organs that come into contact with the outer environment, such as the respiratory system, the digestive system, and the female reproductive system — may instead play an active role in the penetration of hydrophobic substances, including toxins and carcinogens, into our cells.

But encouragingly, the researchers believe that their discovery will one day prove useful in enabling non-water-soluble drugs to enter cells and treat diseases such as cancer. Their most recent study was published in the American Chemical Society's Chemical Research in Toxicology journal.

When mucus fails

Some of these dangerous substances, such as polycyclic aromatic hydrocarbons, are present in petroleum products and also formed through the partial combustion of fossil fuels that are used to operate power stations, planes, cars, space heaters, and stoves. In the new publication, Drs. Gozin and Peer describe their success in getting certain substances, some of them toxic, to penetrate digestive-system cell cultures and bacterial cells bathed in a mucus solution.

"Until now, mucus has been regarded as a mechanical and chemical protective membrane. We did not expect to find it actually absorbing these toxic hydrocarbons and facilitating their transport into bodily systems," explains Dr. Gozin.

Dr. Gozin, Dr. Peer and their research teams show that petroleum-based toxins can dissolve in water with the aid of mucins, the proteins that constitute the main component of mucus.

A new drug delivery system?

In their laboratory, Drs. Gozin and Peer bathed single-celled organisms in a solution of the hydrocarbon-mucin complex, and observed that the hydrocarbons penetrated the cells much more rapidly than when no mucins were present. "We do not know what mechanism enables these substances to penetrate the cell membranes. Clearly it is not a simple infiltration. Our assumption is that an endocytosis-like process is at work — substances are being absorbed into the cell through entrapment, with the cell membrane folding in on itself and creating a bubble," Dr. Gozin explains.

In an earlier study, published in 2010 in the nanotechnology journal Small, Dr. Gozin's team demonstrated that nanometer-scale substances such as carbon-based and inorganic fullerenes (ball-shaped nanoparticles) as well as carbon nanotubes can also be dispersed in physiological solutions with the aid of mucins.

"It will be possible to employ the mechanism we have discovered to facilitate the penetration of hydrophobic drugs into the body, whether via the respiratory tract — with drugs entering the body through the lungs — or by swallowing a delayed-release drug formulation to be absorbed by the digestive system beyond the stomach," Dr. Gozin notes. The next stage of the research will focus on developing systems for the transport of hydrophobic drugs.

Provided by Tel Aviv University (news : web)

Creating the perfect Bloody Mary: Good chemistry of fresh ingredients

After tackling the chemistry of coffee, tea, fruit juices, soda pop, beer, wine and other alcoholic beverages, why not take on the ultimate challenge, the Mount Everest of cocktails, what may be the most chemically complex cocktail in the world, the Bloody Mary? And in this the International Year of Chemistry (IYC), why not include its global offspring, the International Mary?

Those challenges underpin a presentation today reviewing the Bloody Mary's and the taste sensations created by those ingredients at the 241st National Meeting & Exposition of the American Chemical Society (ACS), being held here this week.

"It's a very complicated drink," said Neil C. Da Costa, Ph.D., a expert on the chemical analysis of flavors at International Flavors & Fragrances, Inc., Union Beach, N.J. "The Bloody Mary has been called the world's most complex cocktail, and from the standpoint of flavor chemistry, you've got a blend of hundreds of flavor compounds that act on the taste senses. It covers almost the entire range of human taste sensations — sweet, salty, sour and umami or savory— but not bitter."

Da Costa said those flavors originate in the basic ingredients in the traditional Bloody Mary, which by one account originated in a Paris bar in the 1930's. Stories link the name to various historical figures, especially Queen Mary I of England, noted for her bloody repression of religious dissenters. The ingredients include tomato juice, Worcestershire and Tabasco sauce, fresh lemon or lime juice, horseradish, black pepper, and celery salt. Shaken with ice or served over ice, it is often garnished with celery and a lemon wedge.

"Most of the ingredients have been analyzed for their key flavor volatiles, the chemicals that can evaporate from the glass and produce the aroma," Da Costa explained. "Similarly for the non-volatiles, which are the chemicals that stay in the liquid and contribute toward the flavor there. My presentation reviews the composition of these ingredients and highlights the key components and their sensory attributes."

Some of the ingredients have been linked with beneficial health effects, Da Costa, noted, citing the rich source of lycopene, for instance, in the tomato juice; horseradish with its allyl isothiocyanate, which can be effective at lower concentrations; other phytochemicals in lemon; and even the alcohol in vodka, which some studies suggest can be beneficial when taken occasionally in small amounts.

Does Da Costa's research provide any insights for making a good Bloody Mary? He cites several:
Make it fresh. Chemically, the Bloody Mary is a "highly unstable" concoction, and the quality tends to deteriorate quickly.
Ice it up. Serving Bloody Marys on ice helps to slow down the chemical reactions involving acids in tomato juice and other ingredients that degrade the taste.
Mind your mixes. If you use a cocktail mix, add some fresh to enhance the flavor and aroma.
Splurge on the juice. Tomato juice makes up most of the Bloody Mary's volume, so use high quality juice that has a deep, rich flavor.
Economize on the vodka. The intense, spicy flavor of a Bloody Mary masks the vodka, and using premium vodka makes little sense.In the spirit of the IYC, Da Costa discussed the variations on the Bloody Mary consumed in other parts of the world. These "International Marys" include Denmark's Danish Mary; the Highland Mary (a.k.a. the Bloody Scotsman); the Russian Mary; the Bloody Geisha (yes, that's sake instead of vodka); the Bloody Maureen (replace vodka with Guinness); and the Bloody Molly (Irish whiskey replaces vodka).

Provided by American Chemical Society (news : web)

Researchers electrify polymerization

Scientists led by Carnegie Mellon University chemist Krzysztof Matyjaszewski are using electricity from a battery to drive atom transfer radical polymerization (ATRP), a widely used method of creating industrial plastics. The environmentally friendly approach, reported in the April 1 issue of Science, represents a breakthrough in the level of control scientists can achieve over the ATRP process, which will allow for the creation of even more complex and specialized materials.

ATRP, first developed by Matyjaszewski in 1995, allows scientists to easily form polymers by putting together component parts, called , in a controlled piece-by-piece fashion. Assembling polymers in such a manner has allowed scientists to create a wide range of polymers with highly specific, tailored functionalities. ATRP has been used to develop , coatings, and drug delivery systems, and is used to develop "smart" materials — those that respond to environmental changes, such as changes in temperature, light, pressure or pH.

The current study represents the latest in a series of advances Matyjaszewski's research group has made since ATRP's inception that make the technique more precise and more environmentally friendly. In a process they are calling electrochemically mediated ATRP, or eATRP, the researchers used a computer-controlled battery to apply an electrochemical potential across the ATRP reaction.

"This marks the first time that we've paired electrochemistry with ATRP, and the results were startlingly successful," said Matyjaszewski, the J.C. Warner Professor of Natural Sciences at CMU. "We found that by adjusting the current and voltage we could slow and speed up, or even start and stop the reaction on-demand. This gives us a great deal more flexibility in conducting our reactions that should lead to the development of precisely engineered materials."

In traditional ATRP reactions scientists use a copper catalyst to grow a complex polymer structure by adding a few monomeric units at a time to the chain. The process relies on paired reduction-oxidation (redox) reactions between two species of copper — the activator CuI and deactivator CuII — where the two catalysts exchange electrons back and forth. Occasionally, one of the exchanges will spontaneously stop, called a radical termination, resulting in the accumulation of CuII. To keep the polymerization going, researchers must rebalance the system by compensating for the excess CuII.

In the early ATRP experiments, scientists addressed this problem by adding more CuI to the system. This generated materials with high, sometimes toxic, levels of copper, reaching around 5,000 parts-per-million (ppm). Such levels of copper are hard to remove using current industrial equipment. As an alternative, Matyjaszewski and colleagues developed novel methods for using activators and reducing agents to reactivate the CuII. Most notably, they found that environmentally friendly reducing agents like sugars or vitamin C were highly effective in reducing the amount of copper catalyst used in ATRP reactions.

In the current study, Matyjaszewski and Visiting Assistant Professor of Chemistry Andrew Magenau looked to electrochemistry as a means for maintaining balance in ATRP reactions. They found that adding electricity capitalized on the redox reaction by moderating the transfer of electrons. This allowed them to compensate for the radical terminations and reduce the amount of copper needed to run ATRP. As a result the amount of copper in the system was reduced to 50 ppm, a 100-fold decrease. In terms of creating a greener, less toxic form of ATRP, this amount rivaled Matyjaszewski's previous studies that used vitamin C and sugars as reducing agents, but has the added benefit of not requiring the addition of any additional organic or inorganic reducing agents.

The researchers found that applying electricity to the system also gave them more precise control over the reaction. The computer-controlled battery allowed them to manipulate the ATRP process in real-time by changing the current or voltage.

The researchers have used this process to create the standard types of polymers made with ATRP: star, brush and block copolymers. They believe that the meticulous control eATRP gives them over the rate of polymerization will allow for the creation of polymers with even more complex architectures.

Provided by Carnegie Mellon University (news : web)

Next-generation chemical mapping on the nanoscale

A pixel is worth a thousand words? Not exactly how the saying goes, but in this case, it holds true: scientists at Berkeley Lab's Molecular Foundry have pioneered a new chemical mapping method that provides unprecedented insight into materials at the nanoscale. Moving beyond traditional static imaging techniques, which provide a snapshot in time, these new maps will guide researchers in deciphering molecular chemistry and interactions at the nanoscale -- critical for artificial photosynthesis, biofuels production and light-harvesting applications such as solar cells.

"This new technique allows us to capture very high-resolution images of nanomaterials with a huge amount of physical and chemical information at each pixel," says Alexander Weber-Bargioni, a postdoctoral scholar in the Imaging and Manipulation of Nanostructures Facility at the Foundry. "Usually when you take an image, you just get a picture of what this material looks like, but nothing more. With our method, we can now gain information about the functionality of a nanostructure with rich detail."

The Molecular Foundry is a U.S. Department of Energy (DOE) Office of Science nanoscience center and national user facility. With the Foundry's state-of-the-art focused ion beam tool at their disposal, Weber-Bargioni and his team designed and fabricated a coaxial antenna capable of focusing light at the nanoscale, -- a harnessing of light akin to wielding a sharp knife in a thunderstorm, Weber-Bargioni says.

Consisting of gold wrapped around a silicon nitride atomic force microscope tip, this coaxial antenna serves as an optical probe for structures with nanometer resolution for several hours at a time. What's more, unlike other scanning probe tips, it provides enough enhancement, or light intensity, to report the chemical fingerprint at each pixel while collecting an image (typically 256 x 256 pixels). This data is then used to generate multiple composition-related "maps," each with a wealth of chemical information at every pixel, at a resolution of just twenty nanometers. The maps provide information that is critical for examining nanomaterials, in which local surface chemistry and interfaces dominate behavior.

"Fabricating reproducible near-field optical microscopy probes has always been a challenge," says Frank Ogletree, acting Facility Director of the Imaging and Manipulation of Nanostructures Facility at the Foundry. "We now have a high-yield method to make engineered plasmonic probes for spectroscopy on a variety of surfaces."

To test out the capability of their new probe, the team examined carbon nanotubes, sheets of carbon atoms rolled tightly into tubes just a few nanometers in diameter. Carbon nanotubes are ideal for this type of interactive investigation as their unmatched electronic and structural properties are sensitive to localized chemical changes.

Users coming to the Molecular Foundry to seek information about light-harvesting materials or any dynamic system should benefit from this imaging system, Weber-Bargioni says.

Adds Jim Schuck, staff scientist in the Imaging and Manipulation of Nanostructures Facility at the Foundry, "We're very excited -- this new nano-optics capability enables us to explore previously inaccessible properties within nanosystems. The work reflects a major strength of the Molecular Foundry, where collaboration between scientists with complementary expertise leads to real nanoscience breakthroughs."

This work at the Molecular Foundry was supported by DOE's Office of Science.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

Alexander Weber-Bargioni, Adam Schwartzberg, Matteo Cornaglia, Ariel Ismach, Jeffrey J. Urban, YuanJie Pang, Reuven Gordon, Jeffrey Bokor, Miquel B. Salmeron, D. Frank Ogletree, Paul Ashby, Stefano Cabrini, P. James Schuck. Hyperspectral Nanoscale Imaging on Dielectric Substrates with Coaxial Optical Antenna Scan Probes.. Nano Letters, 2011; 11 (3): 1201 DOI: 10.1021/nl104163m

First non-trivial atom circuit: Progress toward an atom SQUID

Researchers from the National Institute of Standards and Technology (NIST) and the University of Maryland (UM) have created the first nontrivial "atom circuit," a donut-shaped loop of ultracold gas atoms circulating in a current analogous to a ring of electrons in a superconducting wire. The circuit is "nontrivial" because it includes a circuit element -- an adjustable barrier that controls the flow of atom current to specific allowed values.

The newly published work was done at the Joint Quantum Institute, a NIST/UM collaboration.

Ultracold gases, such as the Bose-Einstein condensate of sodium atoms in this experiment, are fluids that exhibit the unusual rules of the quantum world. Atomic quantum fluids show promise for constructing ultraprecise versions of sensors and other devices such as gyroscopes (which stabilize objects and aid in navigation). Superfuid helium circuits have already been used to detect rotation. Superconducting quantum interference devices (SQUIDs) use superconducting electrons in a loop to make highly sensitive measurements of magnetic fields. Researchers are striving to create an ultracold-gas version of a SQUID, which could detect rotation. Combined with ultracold atomic-gas analogs of other electronic devices and circuits, or "atomtronics" that have been envisioned, such as diodes and transistors, this work could set the stage for a new generation of ultracold-gas-based precision sensors.

To make their atom circuit, researchers created a long-lived persistent current -- a frictionless flow of particles -- in a Bose-Einstein condensate of sodium atoms held by an arrangement of lasers in a so-called optical trap that confines them to a toroidal, or donut, shape. Persistent flow -- occurring for a record-high 40 seconds in this experiment -- is a hallmark of superfluidity, the fluid analog of superconductivity.

The atom current does not circle the ring at just any velocity, but only at specified values, corresponding in this experiment to just a single quantum of angular momentum. A focused laser beam creates the circuit element -- a barrier across one side of the ring. The barrier constitutes a tunable "weak link" that can turn off the current around the loop.

Superflow stops abruptly when the strength of the barrier is sufficiently high. Like water in a pinched garden hose, the atoms speed up in the vicinity of the barrier. But when the velocity reaches a critical value, the atoms encounter resistance to flow (viscosity) and the circulation stops, as there are no external forces to sustain it.

In atomic Bose-Einstein condensates, researchers have previously created Josephson junctions, a thin barrier separating two superfluid regions, in a single atomic trap. SQUIDs require a Josephson junction in a circuit. This present work represents the implementation of a complete atom circuit, containing a superfluid ring of current and a tunable weak link barrier. This is an important step toward realizing an atomic SQUID analog.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by National Institute of Standards and Technology (NIST).

Journal Reference:

A. Ramanathan, K. Wright, S. Muniz, M. Zelan, W. Hill, C. Lobb, K. Helmerson, W. Phillips, G. Campbell. Superflow in a Toroidal Bose-Einstein Condensate: An Atom Circuit with a Tunable Weak Link. Physical Review Letters, 2011; 106 (13) DOI: 10.1103/PhysRevLett.106.130401