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Molecular-scale phase boundaries: A 'primitive' liquid-gas transition

One of the first things taught in school science classes is that there are three states of matter - solids, liquids and gases. Bizarrely, however, at high pressures and temperatures there is a critical point above which the distinction between a liquid and a gas is lost and a single 'supercritical fluid' is formed. Here, thermodynamics says, there are no phase boundaries, creating a continuous path from liquid to gas and widely tuneable properties which make supercritical fluids promising green alternatives to chemical solvents. Widom line and the variation of molecular dipole moment between vapour-like and liquid-like domains, with corresponding electron densities Widom line and the variation of molecular dipole moment between vapour-like and liquid-like domains, with corresponding electron densities. For the first time, an international team comprising the National Physical Laboratory (NPL), the University of Edinburgh and IBM Research has discovered that a primitive form of the liquid-gas transition extends far into the supposedly featureless supercritical phase at the molecular scale in water and demonstrated that the conclusions are likely to apply more generally. Reporting in ("Molecular-Scale Remnants of the Liquid-Gas Transition in Supercritical Polar Fluids"), the team use a powerful new quantum mechanical model which they recently developed to identify the onset of intermolecular bonds (the formation of a primitive liquid from dissociated molecules) and electron redistribution in supercritical water. Abrupt changes in the molecular electronic charge distribution are found to occur at the point where the bonds between molecules are first formed marking an unambiguous separation between dissociated (gas) and associated (liquid) regimes in the supercritical fluid. The results imply that supercritical fluids exhibit far richer behaviour on the molecular scale than conventional thermodynamics predicts.
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Swinging on 'monkey bars': Motor proteins caught in the act (w/video)

The first images of motor proteins in action are published in the journal today ("Large-scale flexibility in cytoplasmic dynein stepping along the microtubule"). motor protein
These proteins are vital to complex life, forming the transport infrastructure that allows different parts of cells to specialise in particular functions. Until now, the way they move has never been directly observed.

Researchers at the University of Leeds and in Japan used electron microscopes to capture images of the largest type of motor protein, called dynein, during the act of stepping along its molecular track.

Dr Stan Burgess, at the University of Leeds' School of Molecular and Cellular Biology, who led the research team, said: "Dynein has two identical motors tied together and it moves along a molecular track called a microtubule. It drives itself along the track by alternately grabbing hold of a binding site, executing a power stroke, then letting go, like a person swinging on monkey bars.

"Previously, dynein movement had only been tracked by attaching fluorescent molecules to the proteins and observing the fluorescence using very powerful light microscopes. It was a bit like tracking vehicles from space with GPS. It told us where they were, their speed and for how long they ran, stopped and so on, but we couldn't see the molecules in action themselves. These are the first images of these vital processes."

An understanding of motor proteins is important to medical research because of their fundamental role in complex cellular life. Many viruses hijack motor proteins to hitch a ride to the nucleus for replication. Cell division is driven by motor proteins and so insights into their mechanics could be relevant to cancer research. Some motor neurone diseases are also associated with disruption of motor protein traffic. The team at Leeds, working within the world-leading Astbury Centre for Structural Molecular Biology, combined purified microtubules with purified dynein motors and added the chemical fuel ATP (adenosine triphosphate) to power the motor.

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Dr Hiroshi Imai, now Assistant Professor in the Department of Biological Sciences at Chuo University, Japan, carried out the experiments while working at the University of Leeds. He explained: "We set the dyneins running along their tracks and then we froze them in 'mid-stride' by cooling them at about a million degrees a second, fast enough to prevent the water from forming ice crystals as it solidified. Then using a cryo-electron microscope we took many thousands of images of the motors caught during the act of stepping. By combining many images of individual motors, we were able to sharpen up our picture of the dynein and build up a dynamic idea of how it moved. It is a bit like figuring out how to swing along monkey bars by studying photographs of many people swinging on them." Dr Burgess said: "Our most striking discovery was the existence of a hinge between the long, thin stalk and the 'grappling hook', like the wrist between a human arm and hand. This allows a lot of variation in the angle of attachment of the motor to its track. "Each of the two arms of a dynein motor protein is about 25 nanometres (0.000025 millimetre) long, while the binding sites it attaches to are only 8 nanometres apart. That means dynein can reach not only the next rung but the one after that and the one after that and appears to give it flexibility in how it moves along the 'track'." Dynein is not only the biggest but also the most versatile of the motor proteins in living cells and, like all motor proteins, is vital to life. Motor proteins transport cargoes and hold many cellular components in position within the cell. For instance, dynein is responsible for carrying messages from the tips of active nerve cells back to the nucleus and these messages keep the nerve cells alive. Co-author Peter Knight, Professor of Molecular Contractility in the University of Leeds' School of Molecular and Cellular Biology, said: "If a cell is like a city, these are like the truckers on its road and rail networks. If you didn't have a transport system, you couldn't have specialised regions. Every part of the cell would be doing the same thing and that would mean you could not have complex life." "Dynein is the multi-purpose vehicle of cellular transport. Other motor proteins, called kinesins and myosins, are much smaller and have specific functions, but dynein can turn its hand to a lot of different of functions," Professor Knight said. For instance, in the motor neurone connecting the central nervous system to the big toe--which is a single cell a metre long-- dynein provides the transport from the toe back to the nucleus. Another vital role is in the movement of cells. Dr Burgess said: "During brain development, neurones must crawl into their correct position and dynein molecules in this instance grab hold of the nucleus and pull it along with the moving mass of the cell. If they didn't, the nucleus would be left behind and the cytoplasm would crawl away." The study involved researchers from the University of Leeds and Japan's Waseda and Osaka universities, as well as the Quantitative Biology Center at Japan's Riken research institute and the Japan Science and Technology Agency (JST). The research was funded by the Human Frontiers Science Program and the Biotechnology and Biological Sciences Research Council (BBSRC). The study used powerful electron microscopes at the University of Leeds' Astbury Centre for Structural Molecular Biology. The University has since announced a £17 million investment in state-of-the-art facilities that will allow even closer observation of life within cells. New equipment includes two 300 kilovolt (kV) electron microscopes (EM) and a 950 megahertz (MHz) nuclear magnetic resonance spectrometer alongside existing 120kV and 200kV EMs, and 500, 600 and 750 MHz NMR machines.
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An atomic laser capable of operating at a wavelength of 0.15 nanometers

The creation of lasers that can generate a coherent stream of X-ray radiation at short wavelengths has long been a goal for scientists. The primary aim of such ‘X-ray lasers’ is to produce high quality, high resolution images of tiny targets, such as molecules. However, the wavelengths of lasers developed to date are still too large to result in clear, detailed images of such targets. Now, Hitoki Yoneda and co-workers at the University of Electro-Communications in Tokyo, together with scientists across Japan, have built an atomic X-ray laser with the shortest wavelength yet, producing a stable beam with a wavelength of 1.5 Ångström, or 0.15 nanometers ("Atomic inner-shell laser at 1.5-Ångström wavelength pumped by an X-ray free-electron laser"). This tiny wavelength is nearly ten times shorter than that of previously-reported atomic lasers. High coherent X-ray laser generated with pump ad seed XFEL pulses High coherent X-ray Kα laser generated with pump ad seed XFEL pulses. Using copper foil as a medium bombarded by two X-ray pulses of different energies produced by an X-ray free-electron laser, researchers at the University of Electro-Communications in Tokyo have successfully created a powerful and highly coherent atomic X-ray laser with the shortest wavelength to date. (click on image to enlarge) Lasers work by using an energy source to excite a laser medium which then release a concentrated stream of photons. This stimulated emission process can increase the coherence of laser light under proper conditions. This work demonstrated that this is true even in the X-ray region. The team built their atomic laser based on copper atoms. The researchers exposed the foil to two X-ray pulses of different energies, generated by an X-ray free-electron laser; one pulse was tuned as a pump source and the other as a seed for the main laser beam. Yoneda and his team found that using the pumped copper medium in combination with seeding greatly enhanced the coherence and energy extraction efficiency, short-wavelength beam. The researchers hope their ‘hard X-ray inner-shell atomic laser’ will eventually produce ultrastable, high quality X-ray images, and could feasibly transform the fields of medicine, quantum optics and particle physics. Background The advances in laser technology The ability to harness X-rays in an ultra-short, coherent and highly focused way will allow X-ray scientists to image tiny objects – as small as an individual molecule – clearly and precisely. Until now, X-ray lasers have been limited to the larger wavelengths on the margins between UV and X-ray radiation on the electromagnetic spectrum. These larger wavelengths mean the laser beam can bypass smaller molecules without bouncing off them. Existing lasers also produce less detailed images of the target objects as a result. Now, the long-standing goal of X-ray science appears to be edging ever closer, with the new ‘hard X-ray inner-shell atomic laser’ developed by Yoneda and his team. The researchers conducted initial tests on a design incorporating a copper foil medium bombarded by one laser pulse from an X-ray free-electron laser acting as a pump for the main laser. However, the team were aware of the need to generate as coherent and as high efficiency of extraction as possible, and so decided to incorporate both pumping and seeding X-ray pulses from the X-ray free-electron laser into their design. The resulting X-ray beam is far superior to that of the free-electron laser on its own, with a significantly reduced wavelength of just 1.5 Ångström. Future work The new laser represents a significant step forward in the future of ultrafast X-ray spectroscopy and X-ray quantum optics. The team hope it will soon be possible to commercialise the product as an high coherent X-ray laser capable of producing high resolution, highly detailed images.
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