Bearing down on hydrogen Shockwaves are used to turn deuterium into a liquid metal
ILLUSTRATION: IOAN-BOGDAN MAGDAU/THE UNIVERSITY OF EDINBURGH, SCHOOL OF PHYSICS AND ASTRONOMY
U
nder high pressure, electrons can be squeezed out of the covalent bond that holds the hydrogen molecule together. Under these conditions, condensed hydrogen can become metallic, but the pressures required can be obtained only through the gravitational field of gas giant planets, or fleetingly in shock waves. On page 1455 of this issue, Knudson et al. (1) report experiments using the Z machine at Sandia National Laboratories that uses an aluminum plate propelled by giant capacitors to generate concentrated shock waves in a tiny sample (2). They observe metallic liquid hydrogen at pressures around 300 GPa and temperatures between 1000 and 2000 K created for a tenth of a microsecond. By shock wave standards, that is remarkably cold and slow. It took 50 years from prediction to discovery of the Higgs boson. By comparison, after 80 years, the most famous conjecture in condensed-matter physics remains unproven. In 1935, Hillard Huntington and Eugene Wigner calculated the properties of metallic hydrogen (3). Based on a nearly free electron picture, they calculated that a simple atomic structure (body-centered cubic) would be some 10-fold denser than cold molecular hydrogen. Ignorant of the compressibility, they stated that the required pressures would be above 25 GPa, an impossible pressure in those days. It turned out, however, that the compressibility of hydrogen is much lower than they had guessed, and to reach the needed density, a pressure of 350 GPa would be required. They also underestimated the ingenuity of experimentalists, as such “impossible” pressures are now obtained by two separate methods: shock compression and diamond anvil cells. It was previously assumed that at high pressures, hydrogen behaves like the other group I elements. When sufficient mechanical energy is applied, hydrogen would transition from a molecular insulator to an atomic metal. Curiously, it turns out that group I elements do the opposite, changing from metal to insulator under pressure. They do
this by becoming electrides: pseudo-ionic compounds in which valence electrons become localized in interstitial sites between the ions (4). Moreover, just as the idea that atomic structures need not be metallic was taking hold, calculations on hydrogen started to predict that some molecular structures may be metallic (5). Adding to this conceptual confusion is that the hydrogen atom is light enough that its quantum-mechanical wavelength approaches the interatomic spacing. The nuclei must then be treated as indistinguishable quantum particles, which has led to the prediction of exotic phases of matter such as superfluids or superconductors (6). Under such conditions, the behavior of deuterium (as used by Knudson et al.) would be different because of its higher mass and bosonic nucleus. At high temperatures, the difference between deuterium and hydrogen is likely to be smaller. Traditional shock experiments can only traverse a particular set of isentropic pressure (P), temperature (T) states, and these are different for hydrogen and deute-
Atomic liquid
Molecular liquid Mixed liquid
Temperature
By Graeme J. Ackland
rium. The shaped pulses of the Z machine allow a range of PT space to be explored. The structure of solid hydrogen up to pressures of 250 GPa is well established (7) (see the figure). Phase I is a hexagonal closepacked molecular liquid. Here, the highschool image of H2 as a dumbbell molecule is misleading. According to quantum mechanics, at low pressure, H2 behaves as a free rotor, pointing in all directions with the same probability. Phase I can therefore be thought of as the close packing of spherical molecules. As pressure increases, the molecules interact, and at low temperature, this leads to a broken symmetry phase II, where the rotation has stopped. At high temperature, the melt line shows a maximum around 900 K and 70 GPa. As the pressure is increased further, the melting temperature drops, meaning that the liquid is denser than the close-packed crystal (8, 9). Under further pressure increase, according to theory, a new motif appears—groups of three hydrogen molecules arrange themselves into hexagonal trimers. The electrons are not yet dissociated, and the structure remains nonmetallic, but the covalent bonding is much weaker. In the low-temperature phase III, all molecules are in such trimers. However, at high temperature, phase IV appears to comprise alternating layers of trimers and relatively freely rotating molecules. This is found in simulation, and evidenced
Solid I
Solid IV
Solid III
Solid II Pressure
Centre for Science at Extreme Conditions (CSEC), School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK. E-mail: [email protected]
Schematic phase diagram of hydrogen. The figure shows the four known solid phases I to IV and two observed liquid phases, together with the predicted atomic liquid. Blue rings imply rotating quantum molecules, wiggly lines imply entangled rotor state, and solid bonds are where calculation shows a covalent bond.
SCIENCE sciencemag.org
26 JUNE 2015 • VOL 348 ISSUE 6242
Published by AAAS
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MATERIALS SCIENCE
INSIGHTS | P E R S P E C T I V E S
BIOGEOCHEMISTRY
Who can cleave DMSP? A DMSP lyase from an abundant marine eukaryote differs fundamentally from known bacterial enzymes By Andrew W. B. Johnston
M
arine organisms play a key role in the global sulfur cycle by producing dimethyl sulfide (DMS), a volatile compound that is emitted into the atmosphere. On page 1466 of this issue, Alcolombri et al. (1) report how the abundant marine phytoplankton Emiliania huxleyi (see the image) produces DMS from dimethylsulfoniopropionate (DMSP). Using a series of classical biochemical approaches, augmented by genomic and proteomic analyses, the authors isolated the enzyme and corresponding gene (termed Alma1) that cleaves DMSP into acrylate and DMS. They also found a functional Alma1-like enzyme in a dinoflagellate, a very different type of abundant single-cell marine plankton, emphasizing the widespread importance of this newly discovered DMSP lyase. DMSP is one of the most important and abundant organic molecules in the world (2), with a billion metric tons made and turned over every year. A signature molSchool of Biological Sciences, University of East Anglia, UK. E-mail: [email protected]
ecule for life at sea, it is produced by marine macroalgae as well as by single-cell phytoplankton species, such as diatoms, dinoflagellates, and—as in this case—the haptophyte E. huxleyi. It most likely serves to protect organisms to survive osmotic stress, although other functions have been suggested, ranging from defense against grazing to protection against oxidative and other stresses. When Challenger and Simpson first identified DMSP in 1948 in the red alga Polysiphonia (3), they recognized it as the chemical progenitor of the volatile DMS known to emanate from these seaweeds (4). Shortly afterwards, Cantoni and Anderson isolated an enzyme from these algae that was able to cleave DMSP; consistent with findings on Alma1, this lyase required reducing agents to maintain activity in vitro and was associated with an insoluble subcellular fraction, possibly the chloroplast (5). The cleavage products are also of interest, particularly the volatile DMS, at least 10 million metric tons of which are released into the atmosphere annually. DMS is a component of the tangy aroma of the seaside and functions as a chemical attractant
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
M. D. Knudson et al., Science 348, 1455 (2015). M. Matzen et al., Phys. Plasmas 12, 055503 (2005). E. Wigner, H. B. Huntington, J. Chem. Phys. 3, 764 (1935). M. Marqués et al., Phys. Rev. Lett. 106, 095502 (2011). C. J. Pickard, R. J. Needs, Nat. Phys. 3, 473 (2007). E. Babaev et al., Nature 431, 666 (2004). J. M. McMahon et al., Rev. Mod. Phys. 84, 1607 (2012). J. Chen et al., Nat. Commun. 4, 2064 (2013). R. T. Howie, P. Dalladay-Simpson, E. Gregoryanz, Nat. Mater. 14, 495 (2015). I. B. Magdau, G. J. Ackland, Phys. Rev. B 87, 174110 (2013). R. T. Howie et al., Phys. Rev. Lett. 108, 125501 (2012). I. Tamblyn, S. A. Bonev, Phys. Rev. Lett. 104, 065702 (2010). M. A. Morales, C. Pierleoni, E. Schwegler, D. M. Ceperley, Proc. Natl. Acad. Sci. U.S.A. 107, 12799 (2010). M. A. Morales, J. M. McMahon, C. Pierleoni, D. M. Ceperley, Phys. Rev. Lett. 110, 065702 (2013). 10.1126/science.aac6626
1430
2 µm DMS producer. Micrograph of a single cell of E. huxleyi, which cleaves DMSP to produce DMS. sciencemag.org SCIENCE
26 JUNE 2015 • VOL 348 ISSUE 6242
Published by AAAS
PHOTO: ALEX POULTON/NATIONAL OCEANOGRAPHY CENTRE, SOUTHAMPTON
experimentally by the appearance of two distinct molecular vibration frequencies (10, 11). If one treats the atoms in the trimer layer as small spheres, and the free molecules as large spheres, the average structure as seen in molecular dynamics calculation corresponds to the densest possible packing for such hard spheres. The notion that at high pressure hydrogen simply adopts the most efficient packing structure is compelling. Although there are many theoretical predictions, no metallic solid phase of hydrogen has yet been produced. Nor is it resolved whether the melting temperature continues to drop, perhaps to zero in a quantum superfluid, or rises again when metallic phases occur. Returning to the liquid phase, the diagnostic technique used by Knudson et al. essentially looks for the reflection of visible light from the interface between the deuterium sample and its aluminum holder. The first strong signal is the drop in reflectivity around 120 GPa. This is not a structural transition; the bandgap is small enough that visible light is absorbed. Then, at a pressure between 280 to 300 GPa, the reflected light reappears, implying that hydrogen has turned into a shiny metal in a transformation that is primarily driven by compression rather than heating. Simulations in these conditions suggest the onset of molecular dissociation; however, the calculated metallization pressure depends sensitively on the approximations made in the calculation (1, 8, 12–14). Thus, it appears that metallization occurs at lower pressures in the liquid than in the solid. The high temperature probably helps: At very high temperatures like those obtained in a nuclear fusion device, deuterium forms a plasma in which the electrons are boiled off rather than squeezed out. Despite static compression beyond values of Knudson et al., no solid hydrogen metal has yet been made, and so Wigner and Huntington’s prediction still awaits its proof. ■
Bearing down on hydrogen Graeme J. Ackland Science 348, 1429 (2015); DOI: 10.1126/science.aac6626
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/348/6242/1429.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/348/6242/1429.full.html#related This article cites 14 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/348/6242/1429.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on July 1, 2015
The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 30, 2015 ):
ILLUSTRATION: IOAN-BOGDAN MAGDAU/THE UNIVERSITY OF EDINBURGH, SCHOOL OF PHYSICS AND ASTRONOMY
U
nder high pressure, electrons can be squeezed out of the covalent bond that holds the hydrogen molecule together. Under these conditions, condensed hydrogen can become metallic, but the pressures required can be obtained only through the gravitational field of gas giant planets, or fleetingly in shock waves. On page 1455 of this issue, Knudson et al. (1) report experiments using the Z machine at Sandia National Laboratories that uses an aluminum plate propelled by giant capacitors to generate concentrated shock waves in a tiny sample (2). They observe metallic liquid hydrogen at pressures around 300 GPa and temperatures between 1000 and 2000 K created for a tenth of a microsecond. By shock wave standards, that is remarkably cold and slow. It took 50 years from prediction to discovery of the Higgs boson. By comparison, after 80 years, the most famous conjecture in condensed-matter physics remains unproven. In 1935, Hillard Huntington and Eugene Wigner calculated the properties of metallic hydrogen (3). Based on a nearly free electron picture, they calculated that a simple atomic structure (body-centered cubic) would be some 10-fold denser than cold molecular hydrogen. Ignorant of the compressibility, they stated that the required pressures would be above 25 GPa, an impossible pressure in those days. It turned out, however, that the compressibility of hydrogen is much lower than they had guessed, and to reach the needed density, a pressure of 350 GPa would be required. They also underestimated the ingenuity of experimentalists, as such “impossible” pressures are now obtained by two separate methods: shock compression and diamond anvil cells. It was previously assumed that at high pressures, hydrogen behaves like the other group I elements. When sufficient mechanical energy is applied, hydrogen would transition from a molecular insulator to an atomic metal. Curiously, it turns out that group I elements do the opposite, changing from metal to insulator under pressure. They do
this by becoming electrides: pseudo-ionic compounds in which valence electrons become localized in interstitial sites between the ions (4). Moreover, just as the idea that atomic structures need not be metallic was taking hold, calculations on hydrogen started to predict that some molecular structures may be metallic (5). Adding to this conceptual confusion is that the hydrogen atom is light enough that its quantum-mechanical wavelength approaches the interatomic spacing. The nuclei must then be treated as indistinguishable quantum particles, which has led to the prediction of exotic phases of matter such as superfluids or superconductors (6). Under such conditions, the behavior of deuterium (as used by Knudson et al.) would be different because of its higher mass and bosonic nucleus. At high temperatures, the difference between deuterium and hydrogen is likely to be smaller. Traditional shock experiments can only traverse a particular set of isentropic pressure (P), temperature (T) states, and these are different for hydrogen and deute-
Atomic liquid
Molecular liquid Mixed liquid
Temperature
By Graeme J. Ackland
rium. The shaped pulses of the Z machine allow a range of PT space to be explored. The structure of solid hydrogen up to pressures of 250 GPa is well established (7) (see the figure). Phase I is a hexagonal closepacked molecular liquid. Here, the highschool image of H2 as a dumbbell molecule is misleading. According to quantum mechanics, at low pressure, H2 behaves as a free rotor, pointing in all directions with the same probability. Phase I can therefore be thought of as the close packing of spherical molecules. As pressure increases, the molecules interact, and at low temperature, this leads to a broken symmetry phase II, where the rotation has stopped. At high temperature, the melt line shows a maximum around 900 K and 70 GPa. As the pressure is increased further, the melting temperature drops, meaning that the liquid is denser than the close-packed crystal (8, 9). Under further pressure increase, according to theory, a new motif appears—groups of three hydrogen molecules arrange themselves into hexagonal trimers. The electrons are not yet dissociated, and the structure remains nonmetallic, but the covalent bonding is much weaker. In the low-temperature phase III, all molecules are in such trimers. However, at high temperature, phase IV appears to comprise alternating layers of trimers and relatively freely rotating molecules. This is found in simulation, and evidenced
Solid I
Solid IV
Solid III
Solid II Pressure
Centre for Science at Extreme Conditions (CSEC), School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK. E-mail: [email protected]
Schematic phase diagram of hydrogen. The figure shows the four known solid phases I to IV and two observed liquid phases, together with the predicted atomic liquid. Blue rings imply rotating quantum molecules, wiggly lines imply entangled rotor state, and solid bonds are where calculation shows a covalent bond.
SCIENCE sciencemag.org
26 JUNE 2015 • VOL 348 ISSUE 6242
Published by AAAS
1429
Downloaded from www.sciencemag.org on July 1, 2015
MATERIALS SCIENCE
INSIGHTS | P E R S P E C T I V E S
BIOGEOCHEMISTRY
Who can cleave DMSP? A DMSP lyase from an abundant marine eukaryote differs fundamentally from known bacterial enzymes By Andrew W. B. Johnston
M
arine organisms play a key role in the global sulfur cycle by producing dimethyl sulfide (DMS), a volatile compound that is emitted into the atmosphere. On page 1466 of this issue, Alcolombri et al. (1) report how the abundant marine phytoplankton Emiliania huxleyi (see the image) produces DMS from dimethylsulfoniopropionate (DMSP). Using a series of classical biochemical approaches, augmented by genomic and proteomic analyses, the authors isolated the enzyme and corresponding gene (termed Alma1) that cleaves DMSP into acrylate and DMS. They also found a functional Alma1-like enzyme in a dinoflagellate, a very different type of abundant single-cell marine plankton, emphasizing the widespread importance of this newly discovered DMSP lyase. DMSP is one of the most important and abundant organic molecules in the world (2), with a billion metric tons made and turned over every year. A signature molSchool of Biological Sciences, University of East Anglia, UK. E-mail: [email protected]
ecule for life at sea, it is produced by marine macroalgae as well as by single-cell phytoplankton species, such as diatoms, dinoflagellates, and—as in this case—the haptophyte E. huxleyi. It most likely serves to protect organisms to survive osmotic stress, although other functions have been suggested, ranging from defense against grazing to protection against oxidative and other stresses. When Challenger and Simpson first identified DMSP in 1948 in the red alga Polysiphonia (3), they recognized it as the chemical progenitor of the volatile DMS known to emanate from these seaweeds (4). Shortly afterwards, Cantoni and Anderson isolated an enzyme from these algae that was able to cleave DMSP; consistent with findings on Alma1, this lyase required reducing agents to maintain activity in vitro and was associated with an insoluble subcellular fraction, possibly the chloroplast (5). The cleavage products are also of interest, particularly the volatile DMS, at least 10 million metric tons of which are released into the atmosphere annually. DMS is a component of the tangy aroma of the seaside and functions as a chemical attractant
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
M. D. Knudson et al., Science 348, 1455 (2015). M. Matzen et al., Phys. Plasmas 12, 055503 (2005). E. Wigner, H. B. Huntington, J. Chem. Phys. 3, 764 (1935). M. Marqués et al., Phys. Rev. Lett. 106, 095502 (2011). C. J. Pickard, R. J. Needs, Nat. Phys. 3, 473 (2007). E. Babaev et al., Nature 431, 666 (2004). J. M. McMahon et al., Rev. Mod. Phys. 84, 1607 (2012). J. Chen et al., Nat. Commun. 4, 2064 (2013). R. T. Howie, P. Dalladay-Simpson, E. Gregoryanz, Nat. Mater. 14, 495 (2015). I. B. Magdau, G. J. Ackland, Phys. Rev. B 87, 174110 (2013). R. T. Howie et al., Phys. Rev. Lett. 108, 125501 (2012). I. Tamblyn, S. A. Bonev, Phys. Rev. Lett. 104, 065702 (2010). M. A. Morales, C. Pierleoni, E. Schwegler, D. M. Ceperley, Proc. Natl. Acad. Sci. U.S.A. 107, 12799 (2010). M. A. Morales, J. M. McMahon, C. Pierleoni, D. M. Ceperley, Phys. Rev. Lett. 110, 065702 (2013). 10.1126/science.aac6626
1430
2 µm DMS producer. Micrograph of a single cell of E. huxleyi, which cleaves DMSP to produce DMS. sciencemag.org SCIENCE
26 JUNE 2015 • VOL 348 ISSUE 6242
Published by AAAS
PHOTO: ALEX POULTON/NATIONAL OCEANOGRAPHY CENTRE, SOUTHAMPTON
experimentally by the appearance of two distinct molecular vibration frequencies (10, 11). If one treats the atoms in the trimer layer as small spheres, and the free molecules as large spheres, the average structure as seen in molecular dynamics calculation corresponds to the densest possible packing for such hard spheres. The notion that at high pressure hydrogen simply adopts the most efficient packing structure is compelling. Although there are many theoretical predictions, no metallic solid phase of hydrogen has yet been produced. Nor is it resolved whether the melting temperature continues to drop, perhaps to zero in a quantum superfluid, or rises again when metallic phases occur. Returning to the liquid phase, the diagnostic technique used by Knudson et al. essentially looks for the reflection of visible light from the interface between the deuterium sample and its aluminum holder. The first strong signal is the drop in reflectivity around 120 GPa. This is not a structural transition; the bandgap is small enough that visible light is absorbed. Then, at a pressure between 280 to 300 GPa, the reflected light reappears, implying that hydrogen has turned into a shiny metal in a transformation that is primarily driven by compression rather than heating. Simulations in these conditions suggest the onset of molecular dissociation; however, the calculated metallization pressure depends sensitively on the approximations made in the calculation (1, 8, 12–14). Thus, it appears that metallization occurs at lower pressures in the liquid than in the solid. The high temperature probably helps: At very high temperatures like those obtained in a nuclear fusion device, deuterium forms a plasma in which the electrons are boiled off rather than squeezed out. Despite static compression beyond values of Knudson et al., no solid hydrogen metal has yet been made, and so Wigner and Huntington’s prediction still awaits its proof. ■
Bearing down on hydrogen Graeme J. Ackland Science 348, 1429 (2015); DOI: 10.1126/science.aac6626
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/348/6242/1429.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/348/6242/1429.full.html#related This article cites 14 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/348/6242/1429.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on July 1, 2015
The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 30, 2015 ):
