Chemists Solve an 84-Year-Old Theory On How Molecules Move Energy After Light Absorption

Chemists Solve an 84-Year-Old Theory On How Molecules Move Energy After Light Absorption

  10:22:00 AM    Science Lover

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The same principle that causes figure skaters to spin faster as they draw their arms into their bodies has now been used by Michigan State University researchers to understand how molecules move energy around following the absorption of light.

MSU chemist Jim McCusker and postdoctoral researcher Dong Guo proved an 84-year-old theory. (Credit: Photo courtesy of MSU.)

Conservation of angular momentum is a fundamental property of nature, one that astronomers use to detect the presence of satellites circling distant planets. In 1927, it was proposed that this principle should apply to chemical reactions, but a clear demonstration has never been achieved.

In the current issue of Science, MSU chemist Jim McCusker demonstrates for the first time the effect is real and also suggests how scientists could use it to control and predict chemical reaction pathways in general.

"The idea has floated around for decades and has been implicitly invoked in a variety of contexts, but no one had ever come up with a chemical system that could demonstrate whether or not the underlying concept was valid," McCusker said. "Our result not only validates the idea, but it really allows us to start thinking about chemical reactions from an entirely different perspective."

The experiment involved the preparation of two closely related molecules that were specifically designed to undergo a chemical reaction known as fluorescence resonance energy transfer, or FRET. Upon absorption of light, the system is predisposed to transfer that energy from one part of the molecule to another.

McCusker's team changed the identity of one of the atoms in the molecule from chromium to cobalt. This altered the molecule's properties and shut down the reaction. The absence of any detectable energy transfer in the cobalt-containing compound confirmed the hypothesis.

"What we have successfully conducted is a proof-of-principle experiment," McCusker said. "One can easily imagine employing these ideas to other chemical processes, and we're actually exploring some of these avenues in my group right now."

The researchers believe their results could impact a variety of fields including molecular electronics, biology and energy science through the development of new types of chemical reactions.

Dong Guo, a postdoctoral researcher, and Troy Knight, former graduate student and now research scientist at Dow Chemical, were part of McCusker's team. Funding was provided by the National Science Foundation.

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New Switch Could Improve Electronics

New Switch Could Improve Electronics

  11:15:00 PM    Science Lover

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Researchers at the University of Pittsburgh have invented a new type of electronic switch that performs electronic logic functions within a single molecule. The incorporation of such single-molecule elements could enable smaller, faster, and more energy-efficient electronics.

The switch was discovered by experimenting with the rotation
of a triangular cluster of three metal atoms held together by a
nitrogen atom, which is enclosed entirely within a cage made
up entirely of carbon atoms. (Credit: Image courtesy of
University of Pittsburgh)

The research findings, supported by a $1 million grant from the W.M. Keck Foundation, were published online in the Nov. 14 issue of Nano Letters.

"This new switch is superior to existing single-molecule concepts," said Hrvoje Petek, principal investigator and professor of physics and chemistry in the Kenneth P. Dietrich School of Arts and Sciences and codirector of the Petersen Institute for NanoScience and Engineering (PINSE) at Pitt. "We are learning how to reduce electronic circuit elements to single molecules for a new generation of enhanced and more sustainable technologies."

The switch was discovered by experimenting with the rotation of a triangular cluster of three metal atoms held together by a nitrogen atom, which is enclosed entirely within a cage made up entirely of carbon atoms. Petek and his team found that the metal clusters encapsulated within a hollow carbon cage could rotate between several structures under the stimulation of electrons. This rotation changes the molecule's ability to conduct an electric current, thereby switching among multiple logic states without changing the spherical shape of the carbon cage. Petek says this concept also protects the molecule so it can function without influence from outside chemicals.

Because of their constant spherical shape, the prototype molecular switches can be integrated as atom-like building blocks the size of one nanometer (100,000 times smaller than the diameter of a human hair) into massively parallel computing architectures.

The prototype was demonstrated using an Sc3N@C80 molecule sandwiched between two electrodes consisting of an atomically flat copper oxide substrate and an atomically sharp tungsten tip. By applying a voltage pulse, the equilateral triangle-shaped Sc3N could be rotated predictably among six logic states.

The research was led by Petek in collaboration with chemists at the Leibnitz Institute for Solid State Research in Dresden, Germany, and theoreticians at the University of Science and Technology of China in Hefei, People's Republic of China. The experiments were performed by postdoctoral researcher Tian Huang and research assistant professor Min Feng, both in Pitt's Department of Physics and Astronomy.

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Scientists a step closer to understanding 'natural antifreeze' molecules

Scientists a step closer to understanding 'natural antifreeze' molecules

  8:17:00 PM    Science Lover

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Scientists have made an important step forward in their understanding of cryoprotectants – compounds that act as natural 'antifreeze' to protect drugs, food and tissues stored at sub-zero temperatures.

Researchers from the Universities of Leeds and Illinois, and Columbia University in New York, studied a particular type of cryoprotectants known as osmolytes. They found that small osmolyte molecules are better at protecting proteins than larger ones.

The findings, published in Proceedings of the National Academy of Sciences, could help scientists develop better storage techniques for a range of materials, including human reproductive tissue used in IVF.

Biological systems can usually only operate within a small range of temperatures. If they get too hot or too cold, the molecules within the system can become damaged (denatured), which affects their structure and stops them from functioning.

But certain species of fish, reptiles and amphibians can survive for months below freezing by entering into a kind of suspended animation. They are able to survive these extreme conditions thanks to osmolytes – small molecules within their blood that act like antifreeze –preventing damage to their vital organs.

These properties have made osmolytes attractive to scientists. They are used widely in the storage and testing of drugs and other pharmaceuticals; in food production; and to store human tissue like egg and sperm cells at very low temperatures (below ��ºC) for a long period of time.

"If you put something like human tissue straight in the freezer, ice crystals start to grow in the freezing water and solutes – solid particles dissolved in the water – get forced out into the remaining liquid.

This can result in unwanted high concentrations of solutes, such as salt, which can be very damaging to the tissue," said Dr Lorna Dougan from the University of Leeds, who led the study. "The addition of cryoprotectants, such as glycerol, lowers the freezing temperature of water and prevents crystallisation by producing a 'syrupy' semi-solid state. The challenge is to know which cryoprotectant molecule to use and how much of it is necessary.



"We want to get this right so that we recover as much of the biological material as possible after re-thawing. This has massive cost implications, particularly for the pharmaceutical industry because at present they lose a large proportion of their viable drug every time they freeze it."

Dr Dougan and her team tested a range of different osmolytes to find out which ones are most effective at protecting the 3D structure of a protein. They used an atomic force microscope to unravel a test protein in a range of different osmolyte environments to find out which ones were most protective. They discovered that smaller molecules, such as glycerol, are more effective than larger ones like sorbitol and sucrose.

Dr Dougan said: "We've been able to show that if you want to really stabilise a protein, it makes sense to use small protecting osmolytes. We hope to use this discovery and future research to develop a simple set of rules that will allow scientists and industry to use the best process parameters for their system and in doing so dramatically increase the amount of material they recover from the freeze-thaw cycle."

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Brain Cell Communication: Why It's So Fast

Brain Cell Communication: Why It's So Fast

  11:55:00 PM    Science Lover

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Billions of brain cells are communicating at any given moment. Like an organic supercomputer they keep everything going, from breathing to solving riddles, and "programming errors" can lead to serious conditions such as schizophrenia, Parkinson's Disease and attention-deficit hyperactivity disorder.

Vesicle with three "linking bridges". (Credit: Image courtesy of University of Copenhagen)

The brain uses biochemical signal molecules

Today, the biochemical language of the nerve cells is the subject of intensive research right down at the molecular level, and for the first time researchers, some from the University of Copenhagen, have described just how nerve cells are capable of transmitting signals practically simultaneously.

The cells of the nervous system communicate using small molecule neurotransmitters such as dopamine, serotonin and noradrenalin. Dopamine is associated with cognitive functions such as memory, serotonin with mood control, and noradrenaline with attention and arousal.

The brain cell communication network, the synapses, transmit messages via chemical neurotransmitters packaged in small containers (vesicles) waiting at the nerve ends of the synapses. An electrical signal causes the containers and membrane to fuse and the neurotransmitters flow from the nerve ending to be captured by other nerve cells. This occurs with immense rapidity in a faction of a millisecond.

The vesicle uses three copies of the "linking bridge"

Researchers from the Universities of Copenhagen, Göttingen and Amsterdam have been studying the complex organic protein complexes that link vesicles and membrane prior to fusion, in order to find an explanation for the rapidity of these transmissions. They have discovered that the vesicle contains no fewer than three copies of the linking bridge or "SNARE complex."

With only one SNARE complex the vesicle takes longer to fuse with the membrane and the neurotransmitter is therefore secreted more slowly.

"The precursors for the SNARE complexes are present in the vesicles before they reach the target membrane," said Professor Jakob Balsev Sørensen from the Department of Neuroscience and Pharmacology at the University of Copenhagen. "Fast (synchronous) fusion is enabled when at least three of them work in tandem. If the vesicle only has one SNARE complex it can still fuse with the target membrane, but it takes much longer."

The discovery has just been published in Science.

"Our next step will be to investigate the factors that influence and regulate the number of SNARE complexes in the vesicles. Is this a way for the nerve cells to choose to communicate more or less rapidly, and is this regulation altered when the brain is diseased?", professor Sørensen says.

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Powerful New Science Tools

Powerful New Science Tools

  11:05:00 PM    Science Lover

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With controlled stretching of molecules, Cornell researchers have demonstrated that single-molecule devices can serve as powerful new tools for fundamental science experiments. Their work has resulted in detailed tests of long-existing theories on how electrons interact at the nanoscale.

A scanning electron micrograph of a gold bridge 
suspended 40 nanometers above a silicon substrate.
In the experiment, the bridge is severed in the middle, 
a single molecule is suspended across the gap, and 
the substrate is bent to stretch the molecule while 
simultaneously measuring the electron current 
through the molecule. (Credit: J.J. Parks)

The work, led by professor of physics Dan Ralph, is published in the June 10 online edition of the journal Science. First author is J.J. Parks, a former graduate student in Ralph's lab.

The scientists studied particular cobalt-based molecules with so-called intrinsic spin -- a quantized amount of angular momentum.

Theories first postulated in the 1980s predicted that molecular spin would alter the interaction between electrons in the molecule and conduction electrons surrounding it, and that this interaction would determine how easily electrons flow through the molecule. Before now, these theories had not been tested in detail because of the difficulties involved in making devices with controlled spins.

Understanding single-molecule electronics requires expertise in both chemistry and physics, and Cornell's team has specialists in both.

"People know about high-spin molecules, but no one has been able to bring together the chemistry and physics to make controlled contact with these high-spin molecules," Ralph said.

The researchers made their observations by stretching individual spin-containing molecules between two electrodes and analyzing their electrical properties. They watched electrons flow through the cobalt complex, cooled to extremely low temperatures, while slowly pulling on the ends to stretch it. At a particular point, it became more difficult to pass current through the molecule. The researchers had subtly changed the magnetic properties of the molecule by making it less symmetric.

After releasing the tension, the molecule returned to its original shape and began passing current more easily -- thus showing the molecule had not been harmed. Measurements as a function of temperature, magnetic field and the extent of stretching gave the team new insights into exactly what is the influence of molecular spin on the electron interactions and electron flow.

The effects of high spin on the electrical properties of nanoscale devices were entirely theoretical issues before the Cornell work, Ralph said. By making devices containing individual high-spin molecules and using stretching to control the spin, the Cornell team proved that such devices can serve as a powerful laboratory for addressing these fundamental scientific questions.

The study was funded primarily by the National Science Foundation.

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New Hydrogen-Storage Method Discovered

New Hydrogen-Storage Method Discovered

  1:11:00 AM    Science Lover

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Scientists at the Carnegie Institution have found for the first time that high pressure can be used to make a unique hydrogen-storage material. The discovery paves the way for an entirely new way to approach the hydrogen-storage problem.

This schematic shows the structure of the new material, Xe(H2)7. 

Freely rotating hydrogen molecules (red dumbbells) surround xenon atoms (yellow). 

(Credit: Image courtesy of Nature Chemistry)

The researchers found that the normally unreactive, noble gas xenon combines with molecular hydrogen (H2) under pressure to form a previously unknown solid with unusual bonding chemistry. The experiments are the first time these elements have been combined to form a stable compound. The discovery debuts a new family of materials, which could boost new hydrogen technologies.

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