Universal behaviour detected in Mott metal-insulator transition
FRANKFURT. Whether water freezes to ice, iron is demagnetized or a material becomes superconducting – for physicists there is always a phase transition behind it. They endeavour to understand these different phenomena by searching for universal properties. Researchers at Goethe University Frankfurt and Technische Universität Dresden have now made a pioneering discovery during their study of a phase transition from an electrical conductor to an insulator (Mott metal-insulator transition).
According to Sir Nevill Francis Mott’s prediction in 1937, the mutual repulsion of charged electrons, which are responsible for carrying electrical current, can cause a metal-insulator transition. Yet, contrary to common textbook opinion, according to which the phase transition is determined solely by the electrons, it is the interaction of the electrons with the atomic lattice of the solid which is the determinant factor. The researchers have reported this in the latest issue of the “Science Advances” journal.
The research group, led by Professor Michael Lang of the Physics Institute at Goethe University Frankfurt, succeeded in making the discovery with the help of a homemade apparatus which is unique worldwide. It allows the measurement of length changes at low temperatures under variable external pressure with extremely high resolution. In this way, it was possible to prove experimentally for the first time that it is not just the electrons which play a significant role in the phase transition but also the atomic lattice - the solid’s scaffold.
“These experimental results will herald in a paradigm shift in our understanding of one of the key phenomena of current condensed matter research”, says Professor Lang. The Mott metal-insulator transition is namely linked to unusual phenomena, such as high-temperature superconductivity in copper oxide-based materials. These offer tremendous potential for future technical applications.
The theoretical analysis of the experimental findings is based on the fundamental notion that the many particles in a system close to a phase transition not only interact with their immediate neighbours but also “communicate” over long distances with all other particles. As a consequence, only overarching aspects are important, such as the system’s symmetry. The identification of such universal properties is thus the key to understanding phase transitions.
“These new insights open up a whole new perspective on the Mott metal-insulator transition and permit more sophisticated theoretical modelling of the phase transition”, explains Dr. Markus Garst, Senior Lecturer at the Institute of Theoretical Physics of Technische Universität Dresden.
The research work was funded by the German Research Foundation in the framework of the Collaborative Research Centre/Transregio “Condensed Matter Systems with Variable Many-Body Interactions” led by Professor Michael Lang.
Elena Gati, Markus Garst, Rudra S. Manna, Ulrich Tutsch, Bernd Wolf, Lorenz Bartosch, Harald Schubert, Takahiko Sasaki, John A. Schlueter, and Michael Lang, Breakdown of Hooke’s law of elasticity at the Mott critical endpoint in an organic conductor, Science Advances 2, e1601646 (2016).
A picture can be downloaded from: www.uni-frankfurt.de/64342454
Caption: Electrons embedded in the atomic lattice – the components of a solid. The mutual repulsion of the electrons prevents them from coming into close contact. This impedes the electron flow and the system can become an insulator (originator: Dr. Ulrich Tutsch)
Further information: Professor Dr. Michael Lang, Physics Institute, Riedberg Campus, Tel.: +49(0)69-798-47241, Michael.Lang@physik.uni-frankfurt.de.
New class of peptide from bacteria is a potential insecticide
FRANKFURT. Nature often produces a whole weaponry of active ingredients to ensure it is well prepared for any scenario that might occur. Pharmacists and medical experts have meanwhile learnt from this, since pathogens develop resistance more easily to single active drugs than to a combination therapy. The research group led by Professor Helge Bode has now discovered a whole class of new peptides with which bacteria are able to kill insect larvae.
The peptides, known as rhabdopeptide/xenortide peptides (RXPs), are produced exclusively by the bacterial genera Photorhabdus and Xenorhabdus. They live in symbiosis with nematodes, together with which they infect and kill insect larvae. Since many RXPs are toxic for eukaryotic cells (including insect cells) and are produced by many different strains of Xenorhabdus and Photorhabdus, they presumably play a very important role during infection.
One single strain of bacteria can produce up to 40 RXP derivates. As the research group, which is led by Professor Helge B. Bode, Merck Endowed Professor of Molecular Biotechnology at Goethe University Frankfurt, reported in the latest issue of Nature Chemistry, it was surprising to see that only a maximum of four enzymes is required for their production. Bode compares them with classic chemical catalysts for the formation of polymer chains. His group has successfully solved the mechanisms responsible for the production of the unusually high diversity of RXPs.
Why do the bacteria produce a whole library of RXPs instead of single compounds? The researchers explain that the bacteria cannot control into which insect larvae they are delivered by their nematode host. However, in order to survive they must be able to kill any insect quickly and efficiently and direct the mixture of substances at perhaps completely different target sites in the insect cells at the same time. “Imagine shooting with a shotgun”, explains Bode, “even if you’re a poor marksman, there’s a good chance that the spray of bullets will ensure that at least one hits the target!”
Future work will focus on detecting the exact mode of action of the RXPs and identifying, by means of structure-activity analysis, particularly potent derivates, which can then be produced biotechnologically or chemically and perhaps used as insecticides.
Xiaofeng Cai, Sarah Nowak, Frank Wesche, Iris Bischoff, Marcel Kaiser, Robert Fürst and Helge. B. Bode: Entomopathogenic bacteria use multiple mechanisms for bioactive peptide library design, in: Nature Chemistry, DOI: 10.1038/NCHEM.2671
A picture can be downloaded from: http://www.uni-frankfurt.de/64290802
Further information: Prof. Dr. Helge Bode, Merck Endowed Professor of Molecular Biotechnology, Tel.: +49(0)69-798-29557, H.Bode@bio.uni-frankfurt.de.
Atomic physicists in Frankfurt measure bond length/precise testing of quantum-mechanical tunnel effect
FRANKFURT. Helium atoms are loners. Only if they are cooled down to an extremely low temperature do they form a very weakly bound molecule. In so doing, they can keep a tremendous distance from each other thanks to the quantum-mechanical tunnel effect. As atomic physicists in Frankfurt have now been able to confirm, over 75 percent of the time they are so far apart that their bond can be explained only by the quantum-mechanical tunnel effect.
The binding energy in the helium molecule amounts to only about a billionth of the binding energy in everyday molecules such as oxygen or nitrogen. In addition, the molecule is so huge that small viruses or soot particles could fly between the atoms. This is due, physicists explain, to the quantum-mechanical “tunnel effect”. They use a potential well to illustrate the bond in a conventional molecule. The atoms cannot move further away from each other than the “walls” of this well. However, in quantum mechanics the atoms can tunnel into the walls. “It’s as if two people each dig a tunnel on their own side with no exit”, explains Professor Reinhard Dörner of the Institute of Nuclear Physics at Goethe University Frankfurt.
Dörner’s research group has produced this helium molecule in the laboratory and studied it with the help of the COLTRIMS reaction microscope developed at the University. The researchers were able to determine the strength of the bond with a level of precision not previously achieved and measured the distance between the two atoms in the molecule. “The helium molecule is something of a touchstone for quantum-mechanical theories, as the value of the binding energy theoretically predicted is heavily dependent on how accurately all physical and quantum-mechanical effects were taken into account”, explains Dörner.
Even the theory of relativity, which is otherwise mainly required for astronomical calculations, had to be incorporated here. “If even just a small mistake occurs, the calculations produce major deviations or even indicate that a helium molecule cannot exist at all”, says Dörner. The precision measurements performed by his research group will serve as a benchmark for future experiments.
Two years spent taking measurements in the cellar
Dörner’s research group began investigating the helium molecule back in 2009, when the German Research Foundation awarded him a Reinhart Koselleck Project and funding to the tune of € 1.25 million. “This type of funding is risk capital, as it were, with which the German Research Foundation supports experiments with a long lead time”, explains Dörner. He was thus able to design and set up the first experiments with his group. Initial results were achieved by Dr. Jörg Voigtsberger in the framework of his doctoral dissertation. “In the search for atoms which ‘live in the tunnel’, Jörg Voigtsberger spent two years of his life in the cellar”, recalls Dr. Till Jahnke, senior lecturer and Voigtberger’s supervisor at the time. It is there, in the cellar, that the laser laboratory of the atomic physics group is housed.
Stefan Zeller, the next doctoral researcher, considerably improved the equipment with the help of Dr. Maksim Kunitski and increased measurement precision still further. To do so, one of his tasks was to shoot at the very weakly bonded helium molecule with FLASH, the free-electron laser at the DESY research centre in Hamburg and the largest "photon canon" in Germany. “Stefan Zeller’s work was remarkable. It was his untiring effort, his excellent experimental research skills and his ability not to be disheartened by temporary setbacks which made our success possible at all”, remarks Professor Dörner, Zeller’s doctoral supervisor.
Already beforehand the results have attracted considerable interest at national and international level. They will now appear in the renowned journal “Proceedings of the National Academy of Sciences of the United States of America (PNAS)” and are also part of the research work for which the group was awarded the Helmholtz Prize 2016.
S. Zeller, M. Kunitski, J. Voigtsberger, A. Kalinin, A. Schottelius, C. Schober, M. Waitz, H. Sann, A. Hartung, T. Bauer, M. Pitzer, F. Trinter, C. Goihl, C. Janke, M. Richter, G. Kastirke, M. Weller, A. Czasch, M. Kitzler, M. Braune, R. E. Grisenti, W. Schöllkopf, L. Ph. H. Schmidt, M. Schöffler, J. B. Williams, T. Jahnke, and R. Dörner:
Imaging the He2 quantum halo state using a free electron laser, in: PNAS, DOI
A caricature can be downloaded from: www.uni-frankfurt.de/64324412
“When two loners are forced to share a bed, they move well beyond its edges to get away from each other.”
Further information: Professor Dr. Reinhard Dörner, Institute of Nuclear Physics, Max-von-Laue-Str. 1, Tel.: +49(0)-798-47003, firstname.lastname@example.org;
PD Dr. Till Jahnke, Institute of Nuclear Physics, Max-von-Laue-Str. 1, Tel.: +49(0)-798-47025, email@example.com
A novel ubiquitination mechanism explains pathogenic effects of Legionella infection
FRANKFURT. The attachment of ubiquitin was long considered as giving the „kiss of death“, labelling superfluous proteins for disposal within a cell. However, by now it has been well established that ubiquitin fulfils numerous additional duties in cellular signal transduction. A team of scientists under the lead of Ivan Dikic, Director of the Institute of Biochemistry II at Goethe University Frankfurt, has now discovered a novel mechanism of ubiquitination, by which Legionella bacteria can seize control over their host cells. Legionella causes deadly pneumonia in immunocompromised patients.
According to the current understanding, the coordinated action of three enzymes is needed for attaching ubiquitin to other proteins. In April this year, U.S. scientists described an ubiquitination reaction that depends only on a single enzyme from Legionella bacteria. The Dikic team together with the group of Ivan Matic (Max Planck Institute for Biology of Ageing, Cologne, Germany) now elucidated the underlying molecular mechanism of its action and revealed a hitherto unknown type of chemical linkage between ubiquitin and target proteins.
Their discovery breaks new ground in the field. Sagar Bhogaraju, researcher in the Dikic laboratory, comments: “Most exciting is of course the question if this unusual ubiquitination also occurs in human cells independently of bacterial infection and if there are similar, so far unknown enzymes in humans, which may have a profound influence on cellular processes.”
When studying the new mechanism in more detail, the Frankfurt scientists were very surprised to find that the Legionella enzyme does not only transfer ubiquitin onto target proteins, but also chemically manipulates the remaining pool of ubiquitin molecules. Modified ubiquitin almost completely inhibits the conventional ubiquitination system, thereby revealing a new role for this enzyme during Legionella infections.
Several important cellular processes are affected by this shut-down of the ubiquitination system, which can also cause a rapid cell death. The Dikic team showed for example that modified ubiquitin prevents degradation of mitochondria (a process called mitophagy), affects transduction of inflammatory signals and constrains protein degradation.
“Most likely, Legionella is not the only bacterium using this mechanism. We hope that our results help to identify new strategies for the development of antibacterial agents, which could complement conventional antibiotics by limiting cellular damage induced by bacterial enzymes”, explains Dikic the high medical relevance of their discovery.
The group of Ivan Dikic is located at both the Institute of Biochemistry II and the Buchmann Institute for Molecular Life Sciences at Goethe University Frankfurt and has previously contributed significantly to a paradigm change in the ubiquitin field. Ivan hypothesized early on that ubiquitin signals are recognized and translated by specialized domains in other proteins. He identified ubiquitin-binding domains in more than 200 ubiquitin receptors and was able to prove their role in diseases like cancer, amyotrophic lateral sclerosis and Parkinson’s.
Publication: Bhogaraju S, Kalayil S, Liu Y, Bonn F, Colby T, Matic I, Dikic I. Phosphoribosylation of ubiquitin promotes serine ubiquitination and impairs conventional ubiquitination. Cell. 2016 Dec;167(6). DOI10.1016/j.cell.2016.11.019
Information: Dr. Kerstin Koch, Institute of Biochemistry II, University Hospital Frankfurt, Phone: +49 69 6301 84250, firstname.lastname@example.org
Study by PwC and Goethe University Frankfurt reveals: In Dax and MDax-listed companies, bonuses are decoupled from corporate success after payout
Frankfurt. To date, the bonuses paid out to executive board members in Germany have not been sufficiently linked to long-term corporate success. This is the conclusion reached in the new “Remuneration Report” by consultants PwC in cooperation with Goethe University Frankfurt. Whilst about 60 percent of overall remuneration for Dax and MDax board members is indeed made up of performance-related payments – of which in turn more than half are long-term – most of these payments, which are known as Long Term Incentives (LTIs), are in cash rather than based on share plans. That means: Bonuses are automatically decoupled from business development the moment they are paid out.
“When compared at international level, Germany can thus be classed as a special case. In our view, it would be more advantageous if long-term bonuses were awarded in the shape of shares in the respective company and the board members were also bound by means of corresponding clauses actually to keep these securities for a longer period of time, if necessary even beyond their term of office. This would ensure that it is in board members’ vested interest really to work towards the company’s long-term success”, says Remo Schmid, co-author of the report and partner at PwC in Germany responsible for remuneration issues.
CEOs earn almost twice as much as their board colleagues
For the report, PwC and Goethe University Frankfurt examined remuneration structures in Dax and MDax companies. The analysis shows that developments in executive board remuneration in 2015 differed in comparison to the previous year: On average, a Dax CEO earned € 5.6 million, a plus of a good 3 percent. Other board members earned € 2.9 million, which equates to a minus of 2 percent. CEOs in MDax companies earned € 2.5 million on average, whilst their board colleagues were paid € 1.3 million. In other words: On the Dax, the barometer for blue chip companies, top managers are paid more than twice as much as their counterparts in medium-sized enterprises on the MDax. What’s more: The average CEO earns on average almost double the salary of a “normal” board member.
Women are still greatly underrepresented
According to the analysis, the percentage of women amongst top-level executives is rising at an astoundingly slow pace. At the end of 2015, a good 91 percent of all board positions in Dax companies were still occupied by men – whilst in MDax firms it was even 96 percent. The picture is far better, though still not good, in regulatory bodies: In Dax companies, 26 percent of all supervisory board members were female compared to 18 percent in MDax firms. “In many enterprises, cultural change in this respect would be highly desirable”, says Professor Dr. Hans-Joachim Böcking, co-author of the report and professor at Goethe University Frankfurt. Still: “At least as far as the basic salary of executive and supervisory board members is concerned, no gender-specific differences were detected either in Dax or MDax companies”, says Böcking.
A chairman on the supervisory board of a Dax company received an average total remuneration in 2015 of € 311,000. Deputies earned € 230,000, other supervisory board members € 127,500. Pay scales in MDax firms were far lower. Here companies considered their main overseer to be worth an average of € 183,500, the deputy was paid € 121,500 and the other members received € 75,000. Worth noting: Whilst the fixed remuneration for supervisory board members increased on average by about 12 percent in Dax and MDax companies, variable remuneration is still on the decline: in 2014, four out of ten Dax companies still made some form of variable payment. In 2015 it was only three out of ten.
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