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Lively proteins move and shake

Wand and Lee used nuclear magnetic resonance to probe the motion of the methyl-bearing side chains of the protein calmodulin, which regulates muscle contraction. They found that a single oscillation of a side chain takes less than a nanosecond but, more surprisingly, that the degree of movement is extremely varied and falls into three separate bands. Wand emphasizes that it is the ability of the side chains to oscillate in a variety of modes – or ‘entropy’ – that is remarkable.

The researchers observed the behaviour of the three classes of movement – semi-rigid, moderately mobile and moving freely – at a range of temperatures between 278 and 346 K. They discovered that the side chains became much more active as the temperature rose for all three modes of vibration. “Most people assume the internal entropy of protein is limited and unimportant”, Wand told PhysicsWeb, “but we have shown that this is not true – it is large and potentially very important in biological processes”.

Although they studied the dynamics of only one protein, the researchers believe that all proteins will behave in a similar fashion. The pair examined existing studies of other proteins, which were carried out at a single temperature, and found that the same three classes of motion were present.

Earlier crystallographic experiments revealed that proteins undergo a structural transition – from rigid to more fluid – at a certain temperature. This was dubbed a ‘glass transition’ by physicists, but Wand and Lee now believe that the transition arises from peaks in the lower and middle bands of motion rather than ‘real’ glassy behaviour. “The beauty of this study is that motion and temperature are inextricably linked”, says Lee, “and this tells us more about the motion itself”.

Nitrogen tally tests the nebula

The present ratio of nitrogen-15 to nitrogen-14 in planets and bodies is an indication of the original ratio of the two isotopes in the early solar system. But previous analyses of nitrogen in lunar soil – which should be closely related to the proportion of isotopes in the solar wind – were ambiguous. In models of the evolution of the solar system, astronomers therefore had to assume that the ratio of nitrogen isotopes was the same throughout the solar system as it is in Earth’s atmosphere – that is, 0.00366.

According to the new data, however, the ratio of nitrogen-15 to nitrogen-14 is 0.0023. Since the relative proportions of many elements in Jupiter’s atmosphere exactly match the levels in the Sun, Owen and co-workers are confident that this figure is representative of the solar system as a whole. Together with studies of molecular and ionic reactions, the measurement supports the theory that molecular nitrogen was the dominant form of the element in the early solar system – accounting for over 90% of the total – rather than compounds such as ammonia or hydrogen cyanide.

Other studies of nitrogen-containing compounds in the Large Magellanic Cloud – a young galaxy – suggest that the amount of nitrogen-15 decreases as galaxies age. Massive short-lived stars are thought to generate the heavier isotope while smaller, longer-lived stars produce nitrogen-14. As stars reach the end of their lives and eject material into interstellar space, the relative proportion of nitrogen-14 increases. Astronomers also believe that this means the molecular nitrogen in our atmosphere was present when the Earth formed and did not accumulate at a later stage.

Comet collapse opens a window on the past

The theory that primordial comet impacts seeded life on Earth has been boosted by the unusual make-up of organic compounds discovered in C/LINEAR. Certain compounds common in other comets were scarce on C/LINEAR, but the relative proportion of hydrogen isotopes matches that in the Earth’s oceans. “For the first time, we have seen a comet with the right composition to do the job”, explains Michael Mumma of the NASA Goddard Space Flight Center. Mumma and co-workers compared the organic make-up of C/LINEAR with comets Halley, Hyakutake, Hale-Bopp and Lee, which probably formed in a cold region beyond Neptune’s orbit (M Mumma et al 2001 Science 292 1334). Frozen volatile compounds would be amalgamated into these comets, but the observed lack of these materials in C/LINEAR suggests that it coalesced in a warmer region near Jupiter and Saturn.

Meanwhile, Hal Weaver of Johns Hopkins University led a French-American collaboration that used the Hubble Space Telescope and the ground-based Very Large Telescope to examine the break-up of C/LINEAR (H Weaver et al 2001 Science 292 1329). Weaver and colleagues counted 16 large fragments up to 100 metres across, as well as a host of smaller particles. The team speculates that the fragments they observed could be those that originally fused to form the comet – and so could reveal the composition of the protoplanetary nebula. Evaporation of volatile compounds often causes comets to decompose as they approach the Sun, but the near-absence of such compounds on C/LINEAR suggests that it may instead have broken up simply because it was fragile and spinning quickly.

Emissions of gas from C/LINEAR were monitored over a four-week period by Dominique Bockelée-Morvan of the Observatoire de Paris-Meudon and colleagues (D Bockelée-Morvan et al 2001 Science 292 1339). Their radiotelescope observations of various other gases that vaporized as the comet neared the Sun provide clues about the chemical composition of the nebula from which it formed. The temperature profile of the comet’s path was also deduced from the sequence of evaporation of different gases.

Photometric measurements made before and after the comet split apart also suggest that comet C/LINEAR may have been more massive than originally thought. Tony Farnham of the University of Texas, US, headed a team that estimated the diameter of the comet’s nucleus to be just under 500 metres (T Farnham et al 2001 Science 292 1348). But a calculation based on this figure predicts that the comet was originally a thousand times more massive than the group thought, based on its estimate of the mass of the comet’s tail – 300 million kilograms. This is probably because many medium-sized fragments go undetected: they are too small to observe directly and too large to contribute to the light reflected by the comet’s tail.

Teemu Mäkinen of the Finnish Meteorological Institute and co-workers believe that 300 million kilograms of water vaporized from comet C/LINEAR in the period leading up to its disintegration (J Mäkinen et al 2001 Science 292 1326). Ice is widely thought to be the ‘glue’ that holds comets together, and the periodic bursts of water vapour ejected from C/LINEAR are thought to indicate the spacing of ice pockets within.

Although dissimilar from other comets in many respects, C/LINEAR is typical in terms of its X-ray profile. Carey Lisse of the University of Maryland, US, and colleagues monitored the X-ray output of the comet at the Chandra X-ray Observatory (C Lisse et al 2001 Science 292 1343). They believe that the emissions arose from collisions between highly charged ions in the solar wind with gas molecules and atoms emitted from the comet. Although the combination of gases seeping from the comet was unusual, its X-ray profile was remarkably similar to those of other well-known comets. Lisse and colleagues add that a powerful solar flare produced a strong peak in the X-ray output of C/LINEAR, which makes such X-rays emissions a useful way to probe the solar wind.

Baby star blows a bubble

Cepheus A is shrouded in gas and dust, but Ho and colleagues were able to observe the clouds because water vapour is a ‘maser’ – microwave amplification by stimulated emission of radiation – and an energy transition between different rotational states of the water molecules amplifies radiation with a frequency of 22 GHz.

The team used the Very Long Baseline Array of the National Radio Astronomy Observatory to make extremely accurate measurements of the velocities of the arcs of water vapour, which make up a sphere perfect to one part in a thousand. “We tracked these masers over a period of weeks, and the arc of water molecules is travelling at nearly 20 000 miles per hour”, says Ho. Calculations show that the shell of material has taken around 33 years to reach its present size.

The shell of water vapour is very thin, which together with its regular shape, strongly suggests that it originates from a short sharp event on the central star. Ho and co-workers also believe this means that the material was ejected from the star, rather than swept up from gas and dust in the region. There is no detectable star at the centre of the bubble, although the astronomical archives reveal that a dim source was spotted there in 1991.

It is well known that powerful jets of gas emanate from the poles of some young stars. Current models of star formation propose that this process carries away excess angular momentum, allowing interstellar material to drift towards the star and form an accretion disk that may eventually give rise to planets. “We were quite surprised to see evidence that this object may be ejecting not jets but spheres of gas”, says Ho.

Elastic laser spans the spectrum

All of the molecules in one plane of a so-called cholesteric liquid crystal are aligned, and the molecules in each successive plane are rotated slightly with respect to those in the previous plane. This creates a series of parallel helices within the liquid crystal. Over a certain length of the helix, the molecules gradually turn through a full circle – this distance is known as the pitch.

When light shines into the liquid crystal along the axis of the helixes, the light in a short frequency range – with a central wavelength exactly equal to the pitch length – is strongly reflected. This effect can be exploited to make a laser by adding a fluorescent dye to the liquid crystal. The dye is chosen such that the peak in its emission coincides with the wavelength reflected by the liquid crystal. When a laser stimulates this structure, the emission profile is modified and the dye emits laser light at the wavelengths that correspond to the edges of the ‘reflection band’.

Finkelmann’s team realised that this effect could be controlled by adding the dye to a type of highly uniform elastic liquid crystal known as a cholesteric liquid single crystal elastomer, and exciting the system with a laser. When elongated, they found that the laser light shifted from green to red, with a wavelength spread of just 0.3 nanometres. Most lasers have mirrors that form an optical cavity in which the light is amplified, but in the elastic laser the liquid crystal itself behaves as a ‘distributed’ cavity in which the laser light is continually internally reflected.

The magnetic mysteries of atom clusters

In order to make very sensitive measurements, Jamet and colleagues embedded cobalt clusters into the junctions of a micro-SQUID – a miniature superconducting quantum interference device – that exploits quantum effects to measure extremely weak magnetic signals. The clusters were widely spaced within the junctions so that the signals from individual clusters could be distinguished. The team imposed a strong magnetic field to align the clusters in the same direction, and then applied an opposing magnetic field. The micro-SQUID measured the final state to confirm that the direction of magnetization had switched. The discovery is a success for the new micro-SQUID technique because the signals it measured were a thousand times weaker than any previous measurements.

The clusters are formed using a method known as low-energy cluster beam deposition. The atoms assemble themselves into truncated octahedrons because this shape offers the lowest surface energy. Jamet’s team predicted the overall magnetic anisotropy of a single cluster by calculating the interactions of all the atoms within the cluster. But their predictions fell dramatically short of the micro-SQUID measurements of magnetic anisotropy. Using a three-dimensional analysis technique, the team successfully separated the contributions from different regions of the cluster and found that the atoms at the surface dominated this magnetic effect.

Carbon clock could show the wrong time

Living organisms and some geological features absorb stable carbon-12 and radioactive carbon-14, which are present in the air in a well-known ratio. This is part of the carbon cycle – the recirculation of carbon through the oceans, atmosphere, plants and animals. Scientists use carbon dating to determine when objects ceased to absorb carbon by measuring how much of the carbon-14 – which has a half-life of 5730 years – has decayed. But Beck and colleagues believe that the ratio of stable and radioactive carbon in the atmosphere may have changed considerably over the last 50 thousand years. This raises questions about the accuracy of carbon dating for very old objects.

Beck and colleagues tested slices of a half-metre long stalagmite that grew between 45 000 and 11 000 years ago in a cave in the Bahamas. Stalagmites are calcium carbonate deposits left behind when carbon dioxide evaporates out of cave seepage water. They found that carbon-14 concentrations were twice their modern level during that period. Current records of the levels of carbon-14 in the atmosphere only cover the last 16 thousand years, and this discovery extends those records a further 30 thousand years.

Galactic cosmic rays create most of the carbon-14 in our atmosphere, while solar cosmic rays generate a smaller fraction. The Earth is partially shielded from galactic cosmic rays by its own magnetic field and the solar magnetic field, which fluctuates as the solar cycle proceeds. But these effects are predictable and are thought to have changed little in the last million years – which means they cannot explain the glut of carbon-14. Evidence from North Atlantic sediments suggests that the Earth’s magnetic field may have dipped around 40 thousand years ago, but this would still only account for – at best – half of the observed peak in carbon-14 concentrations.

Beck’s team concludes that either a jump in the cosmic ray flux or a fundamental change in the carbon cycle must have produced the sudden increase of carbon-14. The team speculates that a supernova shock wave could have produced a flurry of cosmic rays. “Weaker circulation of the oceans – which are the biggest reservoirs of carbon on Earth – would explain the excess of carbon-14”, David Richards, joint team leader, told PhysicsWeb. If carbon-14 is carried more slowly from the surface to the depths of the ocean, he explains, the carbon-14 content of the atmosphere will rise.

The discovery also has implications for our understanding of the environment as a whole. “We should take this as a warning that climate change may affect the carbon cycle in previously unexpected way”, says Beck.

Cannibal star ate planet

The star – known as HD82943 – is a ‘metal-rich’ star. These mature stars have an abundance of heavy elements, and current theories of stellar evolution propose that they burn off the rare isotope lithium-6 early in their lives. Lithium-6 is, however, plentiful in the atmospheres of giant gas planets like Jupiter.

Astronomers studying extra-solar planets know that some of these systems contain giant gas planets in very tight orbits around their parent stars. Israelian and colleagues believe that the gravitational pull of a central star on such a nearby planet is strong enough to draw matter from the planet – or even engulf it entirely. They propose that this kind of gravitational interaction caused HD82943 to swallow an orbiting planet.

Israelian’s group used models of the relative abundance of elements in the primordial universe to estimate the amount of lithium-6 in massive gas planets. To explain the quantity of lithium-6 detected in its spectrum, the researchers calculated that HD82943 absorbed one giant planet – or several smaller ones – equivalent in mass to two Jupiters. The whole process took between 10 and 20 million years.

Small quantities of lithium-6 are produced by flares erupting from the surface of the Sun, but the team calculated that such explosions on HD82943 could only account for a thousandth of the lithium-6 detected in its spectrum. Israelian and colleagues also mention that theories of nucleosynthesis are uncertain and the primordial universe – and therefore the gas planets – could have contained more lithium. This would suggest that HD82943 ingested a smaller planet or that processes within the star have depleted the levels of lithium-6.

Novel nitrogen is a semiconductor

Ordinary nitrogen gas – an insulator – is abundant in the atmosphere and the robust triple bonds that pair the atoms in each molecule are well understood. But in 1985 theorists predicted that, at a pressure between 50 and 94 GPa, this arrangement would be replaced by a non-molecular structure in which each nitrogen atom forges three single bonds with neighbouring atoms.

After creating this polymer-like substance, Hemley’s team measured its electrical resistance under various temperatures and pressures. When the pressure rose to 140 GPa – around a million times atmospheric pressure – and the temperature reached 300 K, the material became a solid semiconductor.

The team then reduced the pressure on different samples at a variety of temperatures. Remarkably, they found that the semiconducting state persisted at normal atmospheric pressure at temperatures below 100 K. A pronounced hysteresis effect arises because the change in phase lags behind the change in pressure.

When non-molecular nitrogen transforms into ordinary nitrogen gas, an enormous amount of energy is released from its chemical bonds. The triple bond in molecular nitrogen contains around six times more energy than a single bond in non-molecular nitrogen, but the high density of the new material makes it an extremely dense storage medium for energy. Since the new material is stable at normal pressures, Hemley’s team is optimistic that this feature could be exploited.

“We have only created very small amounts of the material so far, and only at low temperatures”, Hemley told PhysicsWeb. “But the material has a very high energy density and could be used as a fuel eventually.”

Into the Antiworld

Anne Gaud McKee and Markus Schmid established the Mimescope theatre company in 1997 to convey the wonder and excitement of scientific discovery to non-scientists and scientists alike. A molecular biologist by training, McKee’s passion for drama inspired her to join forces with her college friend Schmid, who learned the art of mime from Marcel Marceau.

Paul Dirac is played by the hyper-expressive Schmid. He wrestles with the equations of quantum mechanics and relativity, and his succession of triumphs and deadlocks are played out as a narrated – but fictional – letter to Werner Heisenberg. The rollercoaster ride of Dirac’s train of thought is reflected by the elaborate acrobatics of Yasmina Krim, entwined in a swathe of fabric high above the stage.

When he finally accepts that the equations point to the existence of antimatter, Dirac, in a half-dreaming state, sees in his mind’s eye the parallel states of antimatter and matter. Excerpts of the letter to Heisenberg keep us up to date with Dirac’s musings, and the contrasting worlds are interpreted by a dance sequence in which two dancers – depicted as darkness and light – mirror each other on opposite sides of the stage. Dirac’s galloping imagination conjures up the explosive recombination of matter and antimatter that we now know as annihilation. Choreographed whirlpools of coloured torches mimic the trails created by clashing particles inside beam colliders.

Dirac realises that the signature of antimatter should be apparent in astronomical observations if his theory is correct. His futile search of the astronomical archives is illustrated by enchanting aerial gymnastics against a backdrop of the night sky. Dirac’s despondence is lifted when he stumbles upon a quirk of the equations that allows for a small imbalance between the quantities of matter and antimatter, explaining the lack of celestial evidence. Dirac concludes the make-believe letter to Heisenberg by proclaiming that the quest for knowledge brings meaning to his life.

Into the Antiworld was originally staged at CERN inside the underground cavern that houses the Delphi experiment, in which collisions between electrons and their antiparticles – positrons – are studied. That setting must have been awe-inspiring, particularly as the show closed. The audience would have been whisked from the wonder and novelty of Dirac’s theory over 70 years ago to the sophisticated particle physics experiments of today that the discovery inspired. At CERN, the curtain behind the stage ripped apart to reveal the Delphi detector the performance ended – but the gigantic photograph of the Delphi experiment that concluded the show at the Bloomsbury worked surprisingly well.

McKee says that the reaction of the audience is very important to her, and she hopes that Mimescope’s productions trigger an interest in science by imparting the thrill of scientific discovery. The company’s previous works have explored genetic engineering and the possibility that life on Earth originates from comets. “We can’t explain everything about a subject”, McKee told PhysicsWeb, “but we do hope that we inspire people to go away and find out about it for themselves”. Into the Antiworld doesn’t attempt a rigorous explanation of antimatter and its implications, but the areas it does include are covered well. James Gillies, a science writer who trained as a particle physicist, advised the company on the scientific content of the production.

The show included phenomena like the annihilation of particles and antiparticles in bursts of energy, but I am not sure how meaningful this would have been to people without a physics background. Two teachers I met in the audience said they would have brought a party of A-level students had the show not been so close to their exams. They thought that the students would have enjoyed it although some of the concepts might have been beyond their experience. I also wonder how many non-scientists would be attracted to a physics-inspired performance in the first place. It would be a shame for Into the Antiworld to ‘preach only to the converted’.

Into the Antiworld runs at the Bloomsbury Theatre until Saturday 5 May, but you may get another chance to see the show next year as part of the centenary celebrations of Paul Dirac’s birth.

Bloomsbury Theatre, London WC1H 0AH Box Office 020 7388 8822 blooms.theatre@ucl.ac.uk

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